sports medicine Volume 40 Issue 10 October 2010

Sports Med 2010; 40 (10): 809-815 0112-1642/10/0010-0809/$49.95/0

CURRENT OPINION

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Is the ‘Crunch Factor’ an Important Consideration in the Aetiology of Lumbar Spine Pathology in Cricket Fast Bowlers? Paul S. Glazier Centre for Sports Engineering Research, Sheffield Hallam University, Collegiate Campus, Sheffield, UK

Abstract

The ‘crunch factor’ is defined as the instantaneous product of lateral flexion and axial rotational velocity of the lumbar spine. It was originally implicated in the development of lumbar spine pathology and lower back pain in golfers and, although empirical evidence supporting or refuting the crunch factor is inconclusive, it remains an intuitively appealing concept that requires further investigation, not only in golf, but also in other sports involving hitting and throwing motions. This article considers whether the crunch factor might be instrumental in the aetiology of contralateral lumbar spine injuries sustained by cricket fast bowlers. Based on recent empirical research, it is argued that the crunch factor could be important in cricket fast bowling especially considering that peak crunch factor appears to occur just after front foot impact when ground reaction forces are known to be at their highest. The crunch factor may also occupy an integral role in lower back injuries sustained in other sports involving unilateral overhead throwing (e.g. javelin throwing) and hitting (e.g. tennis serving) actions where the spatial orientation of the arm at release or impact is largely determined by lateral flexion of the trunk and where the transfer of energy and momentum along the kinetic chain is initiated by a rapid rotation of the pelvis. Further research is required to empirically verify the role of the crunch factor in the development of lower back injuries in cricket fast bowling and sports that involve similar lower trunk mechanics. This research programme should ideally be supported by modelling work examining the stresses imposed on bony, disc and joint structures by lateral flexion and axial rotation motions so that their respective contribution to injury can be identified.

1. Background Epidemiological studies undertaken in Australia,[1] South Africa,[2] England[3] and the West Indies[4] have repeatedly demonstrated that fast bowlers have the highest risk of injury in cricket with the lower back being most susceptible to

both traumatic and overuse injuries.[5] Although the aetiology of stress-related injuries to the lumbar spine is generally considered to be multifactorial,[6] mechanical variables have consistently shown to be statistically linked to the appearance of abnormal radiological features and lower back pain (see Elliott[7] for a review). Specifically, the

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‘mixed’ bowling technique, characterized by a counter-rotation of the shoulder axis relative to the hip axis in the transverse plane between back foot impact and front foot impact, has repeatedly been shown to be associated with the development of spondylolysis, spondylolisthesis, pedicle sclerosis, intervertebral disc degeneration and protruding annulus fibrosus.[8-11] Of all these abnormal radiological features, spondylolysis (a unilateral or bilateral fracture of the pars interarticularis[9]) is the diagnosis that results in the greatest loss of cricket playing time[1] and most lesions occur on the contralateral (non-bowling arm) side of the lower lumbar spine.[12] Recently, however, Ranson et al.[13] suggested that counter-rotation of the shoulder axis between back foot impact and front foot impact, in itself, may not be wholly responsible for lower back injuries sustained by fast bowlers. They argued that as the lower trunk is in a relatively neutral position between back foot impact and front foot impact and because ground reaction forces are typically low during this period, the contralateral pars interarticularis remains relatively stress free. Instead, they proposed that excessive contralateral flexion and ipsilateral rotation of the lumbar spine at front foot impact through to ball release and beyond are likely to be the main predisposing factors in the development of contralateral stress fractures to the pars interarticularis and intervertebral disc degeneration. In this article, these ideas are expanded upon and the concept of the ‘crunch factor’ is introduced. This concept has been implicated in the aetiology of lower back injuries in golf but may prove to have greater significance to cricket fast bowling. The relevance of the crunch factor to other sports involving unilateral throwing and hitting motions is also briefly explored. 2. The Crunch Factor: Definition and Empirical Evidence The term ‘crunch factor’ was originally introduced by Sugaya et al.[14] to describe the instantaneous product of lateral trunk flexion and axial trunk rotational velocity during the golf swing. Morgan et al.[15] subsequently revised this definiª 2010 Adis Data Information BV. All rights reserved.

tion to target more specifically the motion of the lumbar spine as they demonstrated that focusing on the whole back can introduce ambiguities when attempting to relate loading patterns with the onset of pain. Based on a combination of epidemiological, radiographical and biomechanical data, Sugaya et al.[16] and Morgan et al.[17] postulated that a high crunch factor, particularly during the impact and early follow-through phases of the golf swing, is likely to increase the magnitude of asymmetric compression and shear forces leading to the onset of lower back pain and the development of lumbar spine pathology in golfers. Recently, several empirical studies comparing differences in spine motion between golfers with and without lower back pain have produced results that appear to question the usefulness of the crunch factor. Both Lindsay and Horton[18] and Cole and Grimshaw[19] reported lower, albeit not significantly so, crunch factors in golfers experiencing pain compared with those who did not. However, a number of issues need to be taken into account when interpreting these results. First, neither study examined the crunch factor of the lumbar spine, as advocated by Morgan et al.[15] but rather, calculated the crunch factor for the whole back. Lindsay and Horton[18] used a lightweight triaxial electrogoniometer that did not permit lumbar spine motion from being separated from thoracic spine motion. Similarly, Cole and Grimshaw[19] simply used the angle formed by the intersection of the spine axis (defined as the vector between the mid-hip and mid-shoulder markers) and the hip axis (defined as the vector between the right and left hip markers) in their calculation of the crunch factor. Second, both of these studies adopted retrospective rather than prospective research designs, which makes it difficult to establish whether some variable other than the crunch factor was instrumental in the onset of lower back pain or whether the lumbar spine in those golfers experiencing pain was constrained to work within a much narrower range of motion. Although the evidence supporting and refuting the crunch factor is inconclusive and further research is clearly necessary to establish its full role in the aetiology of lumbar spine pathology in Sports Med 2010; 40 (10)

The Crunch Factor

golf, it remains an intuitively appealing concept that could have even greater relevance in other sports. In the next section, the potential role of the crunch factor in the aetiology of lower back injuries in cricket fast bowling is explored in light of recent research findings. 3. The Crunch Factor and its Potential Application to Cricket Fast Bowling As of yet, the crunch factor has not been investigated in cricket fast bowlers, possibly because of a lack of awareness of its potential importance and the inherent difficulties with accurately measuring lumbar spine kinematics. In the past, sport biomechanists have typically used image-based motion analysis systems to coordinate digitize hip and shoulder joint centres from which hip and shoulder axis alignment data could be obtained.[20,21] More recently, however, the use of optoelectronic motion analysis systems has enabled additional markers to be placed on prominent landmarks pertaining to pelvis and thorax motions and more sophisticated reconstruction models have been implemented to obtain kinematic data related to lumbar spine motion.[13,22] An electromagnetic tracking device has also been used to measure the position and orientation of the lumbar spine during fast bowling.[23] Although these studies did not directly examine the crunch factor, they produced some interesting data that are related to this potentially important parameter. In their investigation of the bowling techniques of 50 healthy professional fast bowlers from English County cricket clubs, Ranson et al.[13] showed that, although no statistically significant differences existed between ‘mixed’ and ‘non-mixed’ bowling action groups for percentage of total range of motion for contralateral flexion and ipsilateral axial rotation, the differences were only marginally non-significant (p-values were 0.07 and 0.08, respectively). Furthermore, effect size statistics revealed medium effects between groups for percentage of contralateral flexion (d = 0.62) and ipsilateral axial rotation (d = 0.57). These results were similar to those of Burnett et al.[23] who found no statistically significant differences in lower trunk kinematics for mixed and nonª 2010 Adis Data Information BV. All rights reserved.

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mixed bowlers but reported a medium sized effect (d = 0.75) for contralateral flexion. When considered together, the results of Ranson et al.[13] and Burnett et al.[23] appear to indicate that bowlers adopting the potentially injurious mixed technique may use a greater proportion of lower trunk range compared with those bowlers adopting the non-mixed (front-on, midway and side-on) techniques. Given that the total range of lateral flexion is effectively reduced when the lower trunk is extended and rotated to the ipsilateral side,[24] as is the case in fast bowling, it is highly plausible that the excessive contralateral flexion between front foot impact and ball release may predispose to contralateral stress-related injuries to the pars interarticularis. Although Ranson et al.[13] did not directly measure the crunch factor, they produced data that suggest it might be an important consideration in the aetiology of lower back injuries. As shown in figure 1, peak crunch factor (the instantaneous product of contralateral flexion and ipsilateral axial rotation velocity) occurs approximately 0.05 seconds after front foot impact when ground reaction forces are known to be high, typically between 3.8–6.4 times bodyweight.[25] Although these compressive forces are somewhat less than the peak forces of 8.57 bodyweight and 8.13 bodyweight calculated by Hosea et al.[26] for

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Fig. 1. A graph showing typical flexion-extension, lateral flexion and axial rotation of the lower trunk between back foot impact (BFI) and ball release (BR). Visual inspection indicates that peak contralateral flexion and peak axial rotation velocity (indicated by the gradient of the slope of the axial rotation time series) coincide at approximately 0.05 sec after front foot impact (FFI) [reproduced from Ranson et al.,[13] with permission from Taylor & Francis Group, http://www. informaworld.com].

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groups of professional and amateur golfers respectively, they are arguably more significant given that peak compressive forces during the golf swing do not appear to coincide with peak crunch factor. Indeed, Hosea et al.[26] demonstrated that peak compressive forces in the lumbar spine occurred during the downswing phase of the golf swing, whereas Morgan et al.[15] showed that peak crunch factor does not occur until the impact and early follow-through phases. It could be argued, therefore, that the coincidence of peak ground reaction force and peak crunch factor during the delivery stride of fast bowling may make the crunch factor a far more dangerous entity in fast bowling. The suggestion that the crunch factor might be an important consideration certainly corresponds with the supposition of Ranson et al.[13] that the period between front foot impact and early follow-through is likely to be when the lower back is at its greatest risk of injury. Unfortunately, only data pertaining to the crunch factor for a ‘typical’ participant in their study was provided, thus differences between mixed and non-mixed bowlers cannot be established. Further evidence of the stress imposed on the lumbar spine during the latter part of the delivery stride and the potential role of the crunch factor, or variant of, in the aetiology of lumbar spine injuries, was provided by Ferdinands et al.[22] In their analysis of 21 New Zealand premier club grade and above fast bowlers, they found that between front foot impact and ball release, peak axial rotation and lateral flexion kinetics (torque and power) coincided at approximately 22–23% of the phase duration (see figure 2). Compressive forces were also shown to be high during this period, with the vertical ground reaction force peaking at 27% of the phase duration. As the focus of this study was predominantly on the kinetics of lumbar spine motion during fast bowling, no corresponding kinematic time series data were provided, so making any direct observations regarding the crunch factor is impossible. However, as power is the scalar product of torque and angular velocity, it can be deduced from figure 2 that high axial rotation and lateral flexion velocities were being produced almost concurrently just after front foot impact, when ground reac-

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Fig. 2. Time normalized ensemble averages –95% CI for (a) axial rotation kinetics; and (b) lateral flexion kinetics of the lumbar spine between front foot impact (FFI) and ball release (BR). Left rotation and right lateral flexion torques were defined as positive. Positive power indicates that motion was active (i.e. segment torque and angular velocity acted in the same direction and corresponded to power generation or net concentric muscle action), whereas negative power indicates that motion was controlled (i.e. segment torque and angular velocity acted in opposite directions and corresponded to power absorption or net eccentric muscle action) [reproduced from Ferdinands et al.,[22] with permission from Elsevier].

tion forces were reported to be high. Although, until now, the crunch factor has been defined as the instantaneous product of lateral flexion and axial rotational velocity of the lumbar spine, it might be the combination of lateral flexion velocity and axial rotational velocity of the lumbar spine that is a more potentially injurious combination. Again, as Ferdinands et al.[22] only presented their results as ensemble averages, differences in lumbar spine kinetics and kinematics between mixed and non-mixed bowlers cannot be established. Sports Med 2010; 40 (10)

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4. The Crunch Factor and its Potential Application to Other Sports Having established that the crunch factor, or variant of, could occupy a role in the development of lumbar spine pathology in cricket fast bowling, it may also prove to be applicable to lower back injuries sustained by athletes in other sports, particularly those involving unilateral throwing and striking motions, where lower back injuries are known to be prevalent.[27,28] Atwater[29] showed that the spatial orientation of the arm at release or impact in these motor skills is determined primarily by lateral flexion of the trunk (see figure 3). Bartlett[30] and Elliott[31] also dem-

onstrated that high end-point velocity in more distal limb segments in unilateral throwing and hitting motions is generated by a precisely timed sequence of segmental rotations originating from more proximal segments. As the flow of energy and momentum along the kinetic chain in these actions is typically initiated by a rapid rotation of the pelvis before transferring to the upper torso and limbs,[32,33] lateral trunk flexion is likely to be accompanied by high axial rotational velocity of the lower trunk, as has already been demonstrated in cricket fast bowling. Furthermore, in activities involving an approach run and subsequent front leg plant (e.g. javelin throwing), ground reaction forces at front foot impact are likely to generate

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Fig. 3. The spatial orientation of the arm at release or impact in throwing and striking skills is determined primarily by lateral flexion of the trunk (reproduced from Atwater,[29] with permission from Lippincott Williams & Wilkins).

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additional compression forces in the lower back at a time when compressive and shearing forces from the crunch factor are already likely to be high. 5. Conclusions The information provided in this article suggests that the crunch factor might be instrumental in the aetiology of contralateral lumbar spine injuries and intervertebral disc degeneration in cricket fast bowlers. Further research is clearly necessary to empirically verify the role of the crunch factor in the development of lower back injuries, not only in cricket fast bowling, but also in other sports actions that involve similar lower trunk mechanics. This research would ideally be supported by modelling work, similar to that undertaken recently by de Visser et al.[34] examining the stresses imposed on bony, disc and joint structures by each direction of lower trunk motion (flexion/extension, lateral flexion and axial rotation) so that their respective contribution to injury could be identified. As motion tracking technology and modelling methods advance, the accuracy and precision of measurement of the lumbar spine will improve as will the capacity of sport biomechanists to identify injury mechanisms. The information emerging from this research could be used to direct coaching interventions, devise strength and conditioning programmes and define rehabilitation strategies. Acknowledgements No sources of funding were used to assist in the preparation of this article. The author has no conflicts of interest that are directly relevant to the content of this article.

References 1. Orchard JW, James T, Portus MR. Injuries to elite male cricketers in Australia over a 10-year period. J Sci Med Sport 2006; 9: 459-67 2. Stretch RA. Cricket injuries: a longitudinal study of the nature of injuries to South African cricketers. Br J Sports Med 2003; 37: 250-3 3. Leary T, White JA. Acute injury incidence in professional county club cricket players (1985-1995). Br J Sports Med 2000; 34: 145-7

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4. Mansingh A, Harper L, Headley S, et al. Injuries in West Indies cricket 2003-2004. Br J Sports Med 2006; 40: 119-23 5. Weatherley CR, Hardcastle PH, Foster DH, et al. Cricket. In: Watkins R, editor. The spine in sports. St Louis (MO): Mosby, 1996: 414-29 6. Bell PA. Spondylolysis in fast bowlers: principles of prevention and a survey of awareness among cricket coaches. Br J Sports Med 1992; 26: 273-5 7. Elliott BC. Back injuries and the fast bowler in cricket. J Sports Sci 2000; 18: 983-91 8. Foster D, John D, Elliott B, et al. Back injuries to fast bowlers in cricket: a prospective study. Br J Sports Med 1989; 23: 150-4 9. Elliott BC, Hardcastle PH, Burnett AF, et al. The influence of fast bowling and physical factors on radiologic features in high performance young fast bowlers. Sports Med Train Rehab 1992; 3: 113-30 10. Burnett AF, Khangure MS, Elliott BC, et al. Thoracolumbar disc degeneration in young fast bowlers in cricket: a follow-up study. Clin Biomech 1996; 11: 305-10 11. Portus MR, Mason BR, Elliott BC, et al. Technique factors related to ball release speed and trunk injuries in high performance cricket fast bowlers. Sports Biomech 2004; 3: 263-84 12. Gregory PL, Batt ME, Kerslake RW. Comparing spondylolysis in cricketers and soccer players. Br J Sports Med 2004; 38: 737-42 13. Ranson CA, Burnett AF, King M, et al. The relationship between bowling action classification and three-dimensional lower trunk motion in fast bowlers in cricket. J Sports Sci 2008; 26: 267-76 14. Sugaya H, Morgan DA, Banks SA, et al. Golf and low back injury: defining the crunch factor. 22nd Annual Meeting of the American Orthopaedic Society for Sports Medicine; 1996; Sun Valley (ID) 15. Morgan D, Sugaya H, Banks S, et al. A new ‘twist’ on golf kinematics and low back injuries: the crunch factor. Proceedings of the 21st Annual Meeting of the American Society of Biomechanics; 1997 Sept 24-27 Clemson (SC). Clemson (SC): Clemson University, 1997 16. Sugaya H, Tsuchiya A, Moriya H, et al. Low back injury in elite and professional golfers: an epidemiologic and radiographic study. In: Farrally MR, Cochran AJ, editors. Science and golf III. Proceedings of the World Scientific Congress on Golf; 1998 Jul 20-24; St Andrews. Champaign (IL): Human Kinetics, 1999: 83-91 17. Morgan D, Cook F, Banks S, et al. The influence of age on lumbar mechanics during the golf swing. In: Farrally MR, Cochran AJ, editors. Science and golf III. Proceedings of the World Scientific Congress on Golf; 1998 Jul 20-24; St Andrews. Champaign (IL): Human Kinetics, 1999: 120-6 18. Lindsay D, Horton J. Comparison of spine motion in elite golfers with and without low back pain. J Sports Sci 2002; 20: 599-605 19. Cole MH, Grimshaw PN. Low back pain in golf: does the crunch factor contribute to low back injuries in golfers? In: Harrison AJ, Anderson R, Kenny I, editors. Scientific proceedings of the XXVII International Conference on Biomechanics in Sports; 2009 Aug 17-21; Limerick. Limerick: University of Limerick, 2009: 442

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20. Elliott BC, Foster DH. A biomechanical analysis of the front-on and side-on fast bowling techniques. J Hum Mov Stud 1984; 10: 83-94 21. Elliott BC, Foster DH, Gray S. Biomechanical and physical factors influencing fast bowling. Aus J Sci Med Sport 1986; 18: 16-21 22. Ferdinands RED, Kersting U, Marshall RN. Three-dimensional lumbar segment kinetics of fast bowling in cricket. J Biomech 2009; 42: 1616-21 23. Burnett AF, Barrett CJ, Marshall RN, et al. Three-dimensional measurement of lumbar spine kinematics for fast bowlers in cricket. Clin Biomech 1998; 13: 574-83 24. Burnett A, O’Sullivan P, Ankarberg L, et al. Lower lumbar spine axial rotation is reduced in end range sagittal postures when compared to a neutral spine posture. Man Ther 2008; 13: 300-6 25. Bartlett RM, Stockill NP, Elliott BC, et al. The biomechanics of fast bowling in men’s cricket: a review. J Sports Sci 1996; 14: 403-24 26. Hosea TM, Gatt CJ, Galli KM, et al. Biomechanical analysis of the golfer’s back. In: Cochran AJ, editor. Science and golf. Proceedings of the First World Scientific Congress on Golf; 1990 Jul 9-13; St Andrews. London: E & FN Spon, 1990: 43-8 27. Soler T, Caldero´n C. The prevalence of spondylolysis in the Spanish elite athlete. Am J Sports Med 2000; 28: 57-62 28. Ruiz-Cotorro A, Balius-Matas R, Estruch-Massana AE, et al. Spondylolysis in young tennis players. Br J Sports Med 2006; 40: 441-6

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29. Atwater AE. Biomechanics of overarm throwing movements and of throwing injuries. Exerc Sport Sci Rev 1979; 7: 43-85 30. Bartlett RM. Principles of throwing. In: Zatsiorsky VM, editor. Biomechanics in sport: performance enhancement and injury prevention. Oxford: Blackwell Science, 2000: 365-80 31. Elliott BC. Hitting and kicking. In: Zatsiorsky VM, editor. Biomechanics in sport: performance enhancement and injury prevention. Oxford: Blackwell Science, 2000: 487-504 32. Mero A, Komi PV, Korjus T, et al. Body segment contributions to javelin throwing during final thrust phases. J Appl Biomech 1994; 10: 166-77 33. Jo¨ris HJJ, Edwards van Muyen AJ, van Ingen Schenau GJ, et al. Force, velocity and energy flow during the overarm throw in female handball players. J Biomech 1985; 18: 409-14 34. de Visser H, Adam CJ, Crozier S, et al. The role of quadratus lumborum asymmetry in the occurrence of lesions in the lumbar vertebrae of cricket fast bowlers. Med Eng Phys 2007; 29: 877-85

Correspondence: Paul S. Glazier, Centre for Sports Engineering Research, Sheffield Hallam University, Collegiate Campus, Sheffield, S10 2BP, UK. E-mail: [email protected]

Sports Med 2010; 40 (10)

Sports Med 2010; 40 (10): 817-839 0112-1642/10/0010-0817/$49.95/0

REVIEW ARTICLE

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The Rodeo Athlete Injuries – Part II Michael C. Meyers1 and C. Matthew Laurent Jr2 1 Department of Health and Human Development, Montana State University, Bozeman, Montana, USA 2 Department of Kinesiology, St Ambrose University, Davenport, Iowa, USA

Contents Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Limitations in Rodeo Injury Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Injury Awareness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Data Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 No Uniform Definition of Injury or Reporting System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Predisposing Factors Prior to Injury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Incidence of Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Head and Neck Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Thoracoabdominal and Spine Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Upper Extremity Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Lower Extremity Injuries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Fatal Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Aetiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Fatigue and Overuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Age and Skill Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Inadequate Medical Care . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Environmental Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Equipment Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Lack of Fitness and Training. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Behaviour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Education and Instruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Fitness/Conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Abstract

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A previous instalment to this review focused on the sport science for rodeo, the history behind the sport and what is currently known about the physical and physiological status, coronary risk profile, strength and power levels, event-specific kinesiological and biomechanical aspects, nutritional habits and psychological indices associated with the rodeo athlete. In regards to injury, rodeo is well known for its high-velocity, high-impact atmosphere where athletes compete against the clock and uncooperative livestock. Considered by many to be a dangerous sport with high vulnerability towards trauma and frequent injuries, animal/human contact events comprise ~80% of reported injuries. Severe trauma includes fractures, dislocations, subluxations,

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concussions, ligament ruptures, pneumothorax and various neurapraxias. Head and neck trauma account for 10–29% of total trauma and up to 63% of upper body injuries, with concussion incidence rates of 3.4 per 1000 competitive exposures. The incidence of thoracic, back and abdominal injuries comprise 11–84% of trauma, while shoulder injuries, involving anterior/ posterior arthralgia, inflammation, instability and increasing weakness, account for 8–15% of upper extremity cases. Lower extremity trauma accounts for 26–34% of cases, with the majority involving the knee. Many believe that the incidence of trauma is underestimated, with studies hampered by numerous limitations such as a lack of injury awareness, missing data, poor injury recall, an array of reporting sources, delays in subject response and treatment, no uniform definition of injury or reporting system and predisposing factors prior to injury. Primary mechanisms of injuries are attributed to physical immaturity, fatigue, age and experience, behaviour, the violent nature of the sport and lack of adequate medical intervention. Although there is limited adherence to organized conditioning programmes, when properly planned, sport-specific conditioning may enhance athletic potential, minimize predisposition to injury and enhance recovery. Education in care and rehabilitation should be spearheaded by the medical community to reduce injury, as several studies have linked trauma to poor technique, inexperience and poor judgement. Medical services should encompass emergency medical oversight for trauma at all levels and press toward preventive care. Competitors should also be cognizant of the signs and symptoms of overtraining, a condition exacerbated by overuse and minimal recovery. The use of helmets, taping, bracing, protective vests, cervical collars and mouthpieces is gaining popularity but has not been thoroughly studied. Guidelines requiring padding of chutes, gates or equipment essential for performance may also avert trauma. Whether increases in knowledge, education and technology are able to reduce predisposition to injury among this population, remains to be seen. As with all high-risk sports, the answer may lie in increased wisdom and responsibility of coaches and athletes to ensure an adequate level of ability, self-control and common sense as they compete in this sport.

One of the fastest growing sports in the US, rodeo is well known for its high-velocity, highimpact atmosphere where athletes compete against the clock and uncooperative livestock on a daily basis.[1-4] Driven by tradition, history and independence, the rodeo athlete’s lifestyle encompasses a gruelling year-round schedule of competition, participating in over 100 events annually, often competing in several rodeos per week without an off-season.[1-4] These athletes compete in an unforgiving environment where protection is limited, conventional medical care is sparse and the potential for severe injury is imminent.[5-8] Rodeo is comparable in skill level to traditional sports and often has an inherently higher ª 2010 Adis Data Information BV. All rights reserved.

risk of injury.[7,9-13] Also, there is limited treatment, rehabilitation, education and research in this area. Certainly, the sport of rodeo elicits a unique atmosphere involving typical human competition, human and animal together in competitive team situations, as well as disproportionate confrontation between human and animal, leading to challenges that transcend those of traditional athletic events.[9,14] Rodeo has experienced a tremendous increase in media attention, corporate sponsorship, product licensing and prize money. The popularity, however, has led to an increase in the number of athletes entering the sport, and a subsequent increase in the number of trauma cases and deaths.[10,15] Sports Med 2010; 40 (10)

Injuries in Rodeo

This stark rise in rodeo injuries is especially evident among young, inexperienced athletes, continuing to draw concern among the medical community to address the injury potential in this sport as early as possible in an athlete’s career.[10,16,17] 1. Limitations in Rodeo Injury Research Unfortunately for the rodeo athlete, the sports medicine knowledge and facilities conducive for optimal performance that have existed for traditional sports over the years still remain elusive for many in this sport. In addition, extensive efforts to document the incidence and aetiology of injury among rodeo athletes have not been without challenges. Many studies have been hampered by numerous limitations ranging from a lack of injury awareness or underreporting, missing data and poor injury recall to a multiplicative array of reporting sources, delays in subject response and subsequent treatment, no uniform definition of injury or reporting system, and predisposing factors prior to injury also impede accurate documentation. In many cases, youth and adolescent injury data are not delineated from adult cases.[18,19] 1.1 Injury Awareness

Although there is a unique pain perception and dissociation prominent among competitors in this sport,[7,18] the awareness of trauma among athletes participating at all levels of rodeo competition is paramount to acquiring a comprehensive understanding of the environment, and minimizing predisposition of injury.[20-23] Many rodeo injuries are not attended to by medical personnel,[7,8,19,24] and injury awareness is often hampered by an individual’s behaviour and/or pain threshold. The athlete’s perception and understanding of exactly what an injury encompasses, the degree of severity, and the necessary triage required for successful prognosis can also impede optimal recovery.[7,22,25-27] Non-perceptualization of inherent injury among younger competitors has been reported in many sports, and rodeo may be no exception.[12,28,29] Among younger ages, and especially among males, social desirability and/or impression management may come into play, ª 2010 Adis Data Information BV. All rights reserved.

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preventing an individual from reporting any form of trauma in order to reflect stoicism and conform to tradition, culture and peer dynamics.[12,23,30-33] 1.2 Data Collection

The source of injury data has become of increasing concern within the last few years as researchers attempt to understand the intricacies that define and lead to unnecessary trauma in sport. Prior studies have utilized simple investigator observation, retrospective survey mailings, single or multiple hospital sites, trauma centres, local clinics, emergency services, injury registries and various tertiary units.[7,9,10,15,18,19,34-36] In some instances, a rodeo may be a significant distance from a major trauma centre, or an injury may have been ascertained from more than one source.[15,35] Compounding the challenge are the continual incidences of self-treatment and subsequent nonreporting, especially when the injury does not limit ambulation.[15,17,35] This has resulted in questionable underreporting, the inability to monitor trauma patterns, variations in classification of injury and injury rate, incomplete or ambiguous data information, and debatable accuracy of diagnosis.[13,15,19,24] To remedy these challenges, authors have proposed incorporating various forms of follow-up procedures such as telephone interviews and mailed questionnaires. As injury recall time increases, however, memory and subsequently validity and reliability become suspect.[21-23,37-39] An additional factor limiting causative interpretation has been inadequate sample size in many studies resulting from a minimal number of cases, individuals lost to follow-up, or limited time span covering a single rodeo or regional finals. The loss of data hampers optimal statistical power and subsequent inference,[9,17,38,40] especially when attempting to quantify and compare specific body regions following trauma, or predisposing factors (i.e. behaviour, gender) that may influence injury occurrence. 1.3 No Uniform Definition of Injury or Reporting System

The numerous attempts in quantifying incidence and type of injury among rodeo athletes Sports Med 2010; 40 (10)

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have resulted in a plethora of injury definitions, exposures and injury-related categories in the literature. Whereas early studies were largely descriptive in nature and utilized tabular/frequency analyses,[7,9,10,19] standard injury classification by time loss has typically been avoided because of the rodeo athlete’s ephemeral response to pain and expedient return to competition regardless of injury severity.[7,24,40] More recently, injuries based on exposure hours and incidence rates have been reported, although variations in calculating exposure to injury (i.e. exposure hours, athlete exposures) often lead to contrasting findings, making comparison of studies challenging.[13,15,38] In addition, the majority of studies have failed to differentiate youth from adult trauma data, or document the incidence of recurrent injuries.[19,38] Although any definition of injury and level of trauma lacks universal agreement and has inherent shortcomings,[21-23,41-43] efforts need to be directed toward establishing a uniform definition of injury, as well as developing a clear, concise but comprehensive injury reporting system. This should include extensive discussion to identify the important variables to be documented, not only medically but also from a performance standpoint, an area that has not been established or agreed upon in this sport. This would allow for ease of comparison between studies and, more importantly, allow for accurate and expedient recommendations with regard to training and conditioning, injury prevention, triage and rehabilitation, leading to optimal performance.[13,38,44] 1.4 Predisposing Factors Prior to Injury

Although specific indices associated with trauma have been thoroughly addressed, prior studies have indicated the limitations in attempting to isolate the extensive array of intrinsic and extrinsic factors that predispose an athlete to a particular injury.[22,26,37,41,44,45] Areas such as personality and the physical traits of rodeo athletes have not been extensively quantified in the literature, although there is widespread agreement that high trait anxiety, poor concentration, a high-risk mentality and extroverted behaviour contribute to injury in this population.[25,37,46-50] ª 2010 Adis Data Information BV. All rights reserved.

Attempting to accurately define physical traits has also been avoided due to perceived difficulty in categorizing somatotype, skill level, biomechanics, musculoskeletal fitness and orthopaedic soundness, but collecting this type of information has been strongly encouraged to determine potential predisposing factors to injury.[19,37] Additional elements such as the type, amount and duration of instruction, prior or pre-existing injury, environmental or arena conditions, and quality of bucking stock are known to contribute to injury,[7,17,51] but are rarely found in the rodeo literature. Since injury cannot be attributed to a single risk factor,[26,44,45,50] the paucity of documentation on intrinsic and extrinsic factors in this sport attenuates a multifactorial approach in effectively identifying and subsequently minimizing causation, a situation that needs to be addressed.[31,38] With efforts directed toward earlier assessment and detection of musculoskeletal changes at all levels of competition, results would lead to alternatives that may prevent acute physical trauma from escalating to chronic conditions and permanent abnormalities as mentioned in prior work with other athletes.[7,37,52] By continuing to address risk factors inherent in this sport and implementing preventive measures and standards of care, the rate of injury and mortality may be reduced. In short, these attempts at identifying causation in rodeo athletes have resulted in convoluted and incomplete findings, with minimal application. A previous instalment to this review focused on the sport science for rodeo, the history behind the sport and what is currently known about the physical and physiological status, coronary risk profile, strength and power levels, event-specific kinesiological and biomechanical aspects, nutritional habits and psychological indices associated with the rodeo athlete.[53] This article reviews the limited body of information on the incidence of injury and physical challenges in rodeo, the mechanisms and risk factors associated with injury, and recommendations for optimal performance and reduction of trauma in this non-traditional sport. To ensure comprehensiveness, the literature search/data retrieval methodology employed Sports Med 2010; 40 (10)

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included all sport science and medicine databases encompassing an unlimited time period and all subdisciplines, as well as all references cited by prior authors in their respective areas of research. 2. Injuries An image exists in the public’s mind of thrillseeking individuals with unyielding determination, resilience and an ephemeral response to pain.[18,46] In reality, the number of injuries reported is staggering.[6,7,17,34] Research on acute injuries from rodeo competition, however, has been accomplished to only a limited extent. Initial studies addressed the characteristics and causes of injury in athletes during seasonal and finals competition according to frequency, severity and type of injury.[7,9-11,16,54-56] Researchers also indicated not only a high predisposition to trauma in rodeo, but a variety of injuries that these competitors experience. Renewed popularity and participation in this sport, especially at the younger levels of competition, have resulted in increased reports of severe musculoskeletal, cardiovascular and CNS trauma. When combined with the potentially violent interaction between human and livestock, the diversity of athletes entering the sport both in skill level and experience, as well as the limited criteria required to participate, creates an environment for imminent trauma.[57] 2.1 Incidence of Injuries

According to information derived from snapshot and anecdotal evidence to multiseason studies, many believe that rodeo is an extremely dangerous sport.[13,19,35,58,59] Over the last 30 years, the limited but growing number of studies seem to support this premise. Early documentation of rodeo injuries, according to frequency, extent and type of injury, has indicated high vulnerability to injury and a wide variation in the types of injury, with sprains, strains and contusions most prevalent.[9,10,54] However, data were limited to injuries incurred during finals competition, equivalent to playoffs, where athletes may have deliberately exceeded normal physiological parameters. At the ª 2010 Adis Data Information BV. All rights reserved.

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professional level, when combined with more recent information, approximately 28–50% of total injuries were attributed to bull riding, and 20–23% of total trauma reported in the saddle bronc and bareback riding events.[9,10,13,35,59,60] Calf roping athletes accumulated 3–12% of injuries, with steer wrestling (8%), team roping (1–4%) and barrel racing (0–3%) comprising the remainder of trauma.[9,10,35,60] Limited information on bullfighting, not an official sanctioned event by all rodeos, has accounted for 8% of trauma.[9] More recent investigation at the professional level has calculated injury incidence rates of 2.4 per 100 competitive exposures (CEs) across all events.[35] When analysed by event, an injury rate of 1440 per 1000 exposure hours was reported among bull riders (based on a mean exposure time of 80.26 seconds from chute entry to arena exit), while others have noted injury rates of 3.2–3.6/100 CEs in this event.[15,17,24,61] The combined findings indicate a 1.6–13.0 times greater incidence of injury in rodeo than reported in boxing, rugby, ice hockey and American football.[13] Additional findings have indicated trauma rates of 4.6 per 100 CEs among bareback riders, 1.4 per 100 CEs among saddle bronc riders, 0.9 per 100 CEs among steer wrestlers and a combined rate of 1.4 per 100 CEs among non-bull riding events.[61] It is well documented that the majority of acute musculoskeletal injuries sustained during rodeo competition occur during roughstock riding events, accounting for 75–87% of all injured rodeo competitors during seasonal competition.[7,9,11,19,34-36,60] This is not surprising, based not only on the athlete’s direct contact with the animal from chute entry to dismount, but also on the observed number of self-imposed exposures to potential trauma seen in this sport. Professional roughstock athletes typically attempt 60–100+ rides each season[3] and, at the collegiate level, multi-event participation is common in order to maximize team scoring. An exposure to injury ratio of 6 : 1 among roughstock events exemplifies the magnitude of injury potential in this sport, with ~25% of roughstock competitors injured at any given time.[7] Sports Med 2010; 40 (10)

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When calculating opportunity for injury (number of injuries divided by number of participants), rodeo athletes face an 89% potential for injury per collegiate season compared with the injury potentials of 47% per year and 107% over 5 years reported by others in college football.[7,62] Many believe that the actual incidence of trauma is underestimated or just the ‘tip-of-the-iceberg’.[4,40,55,63] Comparing the frequency of rodeo injuries by performance indicates that 55 of 138 (40%) of all injuries occurred during the initial performance and 47 of 138 injuries (34%) occurred during the final performance. Second round and slack competition accounted for only 20% and 6% of the injuries, respectively.[7] Classification of total injuries commonly sustained during seasonal rodeo competition typically involves contusions and strains, comprising 42% and 16% of total injuries, respectively.[7] Most trauma is regarded as minor, comprising both acute and chronic injuries (e.g. bursitis, overuse, prodromal and nondescript arthralgia and oedema).[7,15,41] Severe trauma constitutes a limited number of injuries involving fractures, dislocations, subluxations, concussions, ligament ruptures, pneumothorax, and various neurapraxias ranging from 5% to 32% of cases at both the collegiate and professional level of competition.[7,15,34,56] Previous researchers have reported similar results, although recent evidence on injury rates of concussion are mixed.[9,10,13,34,35] When severe trauma (such as fractures, dislocations and lacerations requiring suturing) is diagnosed following emergency care, many athletes typically continue to compete throughout the season.[7] A primary concern is the stresses and trauma commonly observed from axial overload during riding and the numerous opportunities for injury during dismount.[6,25,64] When the roughstock data are specifically analysed by bareback, saddle bronc and bull riding events, bareback riders exhibit a similar incidence of total upper body injuries compared with bull riders (36% vs 39%, respectively). The results indicate, however, that limb trauma occurs primarily in bareback events as opposed to axial injury in bull riding. Distinct biomechanical riding techniques specific for each event, unique differences in riding ª 2010 Adis Data Information BV. All rights reserved.

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equipment, and the contrast in performance among livestock species (i.e. horses vs bulls) likely contribute specifically to certain injury patterns observed.[2,17] During bull riding, the athlete attempts to sit on his supinated, gloved hand, which is wrapped in a rosined plaited rope. This technique enhances the opportunity to align both the rider and the bull’s centre of gravity in close proximity, and decrease the potential for acceleratory and centripetal forces to dislodge the rider.[6] To maintain this position during an 8-second ride, however, the upper extremity must be under continual isometric contraction, limiting articular range of motion and exacerbating transmission of forces throughout the limb. In contrast, the bareback event utilizes a leather rigging that is cinched around the horse’s heart girth. The athlete initially wedges his rosined, gloved hand into a rigid, rawhide handle, resulting in a semipronated grip with minimal flexibility during the ride. Rather than sitting up, the rider lays back in a supine position and, with the aid of leg motion/spurs, pulls himself back into position each time the horse bucks. The fixed grip and riding style result in the enhancement of the acceleratory and eccentric forces that produce hyperextension, degenerative joint disease (DJD), ulnar thickening, rotator cuff/impingement syndromes and carpal fractures observed in this event.[2,3,25,35,65] Following the ride, the potential for upper extremity injury increases when the rider is thrown or dismounts on the off-side (opposite side of riding hand) of the animal. This prevents the wedged hand from naturally releasing itself from the rigging, as the rider remains attached and subjected to further trauma.[2,15] Following eventual release, an athlete may experience additional trauma by falling in an unnatural, twisted position on an outstretched arm or leg, or being thrown into metal gates or railing, resulting in further limb hyperextension, various fractures and articular damage.[7,65,66] In summary, the incidence of both fatal and non-fatal trauma in rodeo continues to be reported in North America. The variation in the number and variety of injuries may reflect the variation in data collection, ranging from seasonal finals and single season designs, to multiseason Sports Med 2010; 40 (10)

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studies incorporating only a percentage of total rodeos,[7,9,10,24,35] as well as studies specifically targeting bull riding.[13,15,17,67] With a greater number of participants combined with a more progressive style of riding, many have perceived that the incidence of rodeo injury has increased and has become more violent;[1,13] in contrast, others have postulated minimal severe injury.[34] The increased rate of injuries might simply be a function of enhanced reporting of injuries among a larger number of inexperienced athletes entering the sport as well as the increasing promotion and emphasis of professional bull riding events. No studies, however, have been conducted on injury-reporting patterns across age or type of event to support this belief. 2.2 Head and Neck Injuries

Head and neck trauma accounts for 10–29% of total trauma and up to 63% of upper body injuries.[7,10,15,19,68] Although craniocervical trauma in sport remains the primary factor among fatalities,[69] the severity of head trauma reported across rodeo studies has been both extensive and equivocal. While some report only a minimal level of head trauma,[9,34] others report the incidence of craniofacial and cervical trauma in the riding events to be double the rate observed in other sports.[13,19,41] Cranial trauma comprises 11–14% of all injuries ranging from Grade 1 or Grade 2 concussions with no neurological sequelae to traumatic brain trauma involving subdural haematomas, seizures, unconsciousness, permanent neurological deficits involving quadriplegia, and death.[7,13,15,19,24,64,67,69] Concussion injury incidence rates of 3.4/1000 CEs have been documented, but when combined with cervical trauma, trauma comprises 13–27% (8.7/ 1000 CEs) of total cases observed each season.[15,34] Reported injuries have included not only a high incidence of concussions, but a substantial incidence of orofacial trauma, similar to other high-collision sports. Isolated and non-isolated skull fractures with intracranial lesions, dentoalveolar trauma, oral and maxillofacial and fractures primarily involving orbital/zygomatic and mandibular regions, avulsed ears, retinal detachª 2010 Adis Data Information BV. All rights reserved.

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ment, eye loss and soft tissue damage have been reported.[15,19,66,70] Although head and facial cases comprise 15 injuries/1000 rides, many believe that facial trauma is underreported, with many athletes not seeking proper medical care.[17,71] Dental injuries typically involve extruded, intruded or lateral tooth luxations, exarticulations, crown fractures involving enamel, dentin and/or pulp, and apical, middle and cervical root fractures similar to other collision sports.[7,15,70,72] Fractures have been associated with both high-speed and subsequent high-impact cases, as well as lower velocity mishaps with livestock via head or chute contact, or being stepped on, kicked or landed on following dismount.[10,15,19] The high number of orofacial injuries found among steer wrestlers, caused by livestock horn contact, indicates a need for mouthguard protection, which is already prevalent in other sports.[7,19,20,37,70] As opposed to the standard stock and mouth-formed styles, customfabricated mouth guards utilizing ethylene vinyl acetate provide the optimal fit for longterm protection with minimal respiratory interference.[70] Injuries to the cervical spine typically involve the vertebral facets and posterior spinous processes as well as intervertebrate bodies and subarachnoid spaces, especially following head-first dismounts.[25,73,74] Neural injuries resulting from cervical trauma include slightly decreased strength in the muscles of the arms, hands and back, and flaccid muscles in the lower extremity. Chronic cervical nerve root neurapraxia, commonly referred to as ‘burner syndrome’ in many sports, is commonly observed following direct blunt trauma to the neck/shoulder region.[74-76] The cervical spine is the most vulnerable and frequently injured segment of the spine associated with highvelocity trauma,[73,74] with craniocerebral/spinal cord trauma from bull riding reported in the literature.[34,77] Craniocerebral and cervical injuries, however, are not limited to the roughstock events. Facial blows from horn contact with subsequent maxillofacial and dental insult, and awkward dismounts from horses have been reported in steer wrestling and calf roping, respectively.[7,10,24] Sports Med 2010; 40 (10)

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Various cervical and brachial plexus syndromes have been observed in these events.[25] In summary, recent observations have noted wide variation in the incidence and severity of head and neck trauma cases reported based on level of competition, reporting source and event. These incidences are considered excessive, resulting in greater treatment, rehabilitation care and cost when compared with other sports. 2.3 Thoracoabdominal and Spine Injuries

Although not specifically addressed in all studies concerning the rodeo athlete, the incidence rates of thoracic, back and abdominal injuries vary widely, accounting for 11–84% of all trauma during competition.[7,15,24,34,68,78] The majority of trauma include various rib, low back and sternal contusions as well as fractures, sprains, sternoclavicular subluxations, pectoralis major and latissimus dorsi ruptures, and costal cartilage separations.[9,10,66,78,79] Low back injuries, comprising 5–8% of trunk trauma, are typically limited to contusions from being stepped on or landed on by livestock or the gradual onset of overuse.[7,9,10] Groin and adductor strains constitute 2–8% of total trauma reported from riding.[7,9,10,15] Although life-threatening, limited reports of pneumothoracic trauma have been reported, as well as isolated cases of syndesmotic disruption with aortic transection and ventricular septal rupture.[15,80,81] It has been theorized, but not proven, that the adoption of protective thoracic vests has reduced the incidence of organ ruptures, cardiac contusions and internal derangement typically observed but unreported in the past.[25] Isolated cases of a transient seizure caused by lumbosacral trauma with no neurological sequelae, brachial plexus neurapraxia, and thoracic outlet damage primarily attributed to livestock or ground impact have been documented.[7,15,25] In most cases the participants were not held for observations, were released under their own recognizance, and continued to compete.[7] In summary, the spine is vulnerable to improper movements of sufficient force, which may enhance the opportunity for injury and lead to ª 2010 Adis Data Information BV. All rights reserved.

permanent disability. As with the cervical region, high-collision injuries typically involve thoracolumbar facets and posterior spinous processes, and the intervertebrate bodies and subarachnoid spaces between vertebrae.[25,73,74] Subsequently, the minimally protected anatomy of the trunk, combined with violent impact and compression forces during and following riding, predisposes the spine to serious injury, with lethal or irreversible sequelae as well as polytrauma involving various organs. 2.4 Upper Extremity Injuries

Early studies have reported the proportion of upper extremity trauma constituting 27–45% of the total number of rodeo injuries reported primarily in roughstock riders.[7,10,35] Besides routine contusions and sprains, up to 33% of trauma to this area is diagnosed as severe, involving fractures, dislocations, subluxations and ligament ruptures.[9,15,55] This is comparatively higher than the 13–37% upper extremity injuries reported in football, softball, downhill skiing and basketball,[12,20,41,62,82] but lower than the 68–91% of trauma documented in other nontraditional high-risk activities, i.e. rock climbing, skateboarding.[83,84] Shoulder injuries account for 8–15% of upper extremity cases.[7,10,24,68] Unlike other sports such as softball or snowboarding, where a large percentage of injuries may occur on the dominant side of the body,[12,20] laterality is typically not observed among roughstock athletes.[2,3] Subluxations, dislocations and other maladies have been reported in both riding and free arm.[2,3,9,15,24] Injuries to the shoulder typically include anterior and/or posterior arthralgia, inflammation, instability and increasing weakness due to overuse.[2] Various musculoskeletal strains, primarily involving the rotator cuff complex, are common.[9,10,65,85] Excessive reaction forces during riding have resulted in rotator cuff tendinopathy, subacromial and glenoid impingement syndromes, glenohumeral subluxation/dislocations, labral tears, acromioclavicular disorders and fractures.[15,25] Shoulder subluxations/dislocations and tendon ruptures have occurred during Sports Med 2010; 40 (10)

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steer wrestling from ground impact when a steer was missed or improperly grabbed.[8,15,25,79] Upper arm trauma is attributed to significant impact forces with livestock and chute gates. Point tenderness, contusions from blunt trauma and prodromal pain are common.[10] Biceps tendon strains and rupture from bull riding have also been reported.[24,61] In most rodeo studies, the elbow receives a significant proportion of acute trauma, comprising 9–14% of upper extremity cases (table I).[2,3,7,10,65,86] Acute injuries range from standard abrasions, contusions, sprains and hyperextension during the ride and dismount, to epicondyle, radial head, coronoid and olecranon tip fractures.[2,63,65,85] Ulnar nerve compression, with neuritis, arthralgia and tenderness with concomitant oedema, are injuries attributed to direct blunt trauma, riding mechanics and overuse specific to the elbow region.[3,87-89] There have also been numerous cases involving complete ligament rupture associated with being hung up as the rider attempted to dis-

Table I. Radiographic findings of the upper extremity in collegiate rodeo athletes (reproduced from Meyers et al.,[2] with permission) Location

No. of subjects (%)

Hand/wrist Fractures healed non-union DJD

19 (23.2) 5 (6.1) 14 (17.1)

Joint calcification

3 (3.7)

DISI

1 (1.2)

Scapholunate dissociation

1 (1.2)

Forearm Ulnar cortical thickening

15 (18.2)

Healed stress fracture

2 (2.4)

Plates and screws

1 (1.2)

Elbow Ulnar/humeral DJD

12 (14.7)

Joint space narrowing with loose bodies

3 (3.7)

Traction spurs

3 (3.7)

Joint calcification

2 (2.4)

Posterior olecranon tip fracture Total

1 (1.2) 82 (100.0)

DISI = dorsal intercalated segment instability; DJD = degenerative joint disease.

ª 2010 Adis Data Information BV. All rights reserved.

mount.[15] Sprains with concomitant ulnar nerve contusion have been reported in calf roping following hyperextension from the arm caught in a rope coil.[10] Extrinsic nerve compression may be elicited by acute or recurrent trauma, with ulnar nerve entrapment being the second most common compressive neuropathy in the upper extremity after carpal tunnel syndrome.[87,89] The ulnar nerve is vulnerable to injury because of its inherent compressive anatomy and superficial location. Direct blunt trauma may escalate to acute compression through tissue inflammation, ecchymosis, effusion and subsequent oedema.[90] Acute cases of ulnar neuropathy are usually reported following isolated trauma, such as an elbow dislocation or by direct blunt trauma resulting in a contusion.[87,88] The elbow, considered to be the second most common site of overuse injury in sport after the knee,[91] is also exposed to chronic abuse in this sport. As noted in earlier studies addressing trauma among professional rodeo competitors,[3,65,85] long-term degenerative changes involving bilateral ulnar/humeral DJD, calcification, olecranon fractures, medial collateral ligament (MCL) traction spurs and joint space narrowing with loose bodies has recently been observed at the collegiate level of competition.[2] Joint impingement from repeated elbow hyperextension during a ride, or direct trauma as a result of a fall, may account for the increased prevalence of chondral loose bodies and olecranon fractures observed in this sport. Chronic joint laxity, due to either valgus extension overload similar to that seen in baseball pitchers or frank dislocation, may also play a role.[88] The majority of minor discomfort associated with the forearm involves abrasions, contusions and tendinitis, with forearm injuries comprising 1–5% of upper extremity trauma.[7,10] Radial stress fractures and ulnar cortical thickening or hyperostosis, as a result of repetitive axial overload and concomitant musculotendinous forces acting upon the upper extremity, were the most common findings in the forearm area.[2] These results are consistent with injuries previously documented across other rodeo studies.[3,9,92] Sports Med 2010; 40 (10)

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Hand and wrist injuries are usually diagnosed in 3–12% of upper extremity cases, primarily attributed to riding and roping mishaps and from falling on outstretched arms during dismount.[7,10,18,55] A similar incidence of finger/hand injury is commonly observed in other sports, with contusions and metacarpophalangeal sprains being the predominant injuries reported.[7,12,88] Injuries to the triangulofibrocartilage complex and radioulnar joint, ulnocarpal ligament disruption, joint impingement from repetitive carpal hyperpronation and numerous fractures of various carpal/ metacarpals have been documented. Tendinitis, inflammation and diminished neurological sensation are especially prevalent. These findings, along with an extensive number of cases involving substantial DJD and non-union fractures, are consistent with the severity of injury reported across most professional rodeo studies.[6,9,10,15,65] Among collegiate roughstock athletes, hand/wrist findings comprised 52% of total abnormalities observed in both the riding and nonriding limb.[2] Of these, 29% were healed and non-union fractures involving scaphoid, styloid, interphalangeal, phalanx and various metacarpals. The 17% of cases of DJD involved the scaphoid/radius, scapholunate, triangulofibrocartilage and carpometacarpal areas. Approximately 6% of findings included joint calcification, dorsal intercalated segment instability (DISI) and scapholunate dissociation. Ropers and steer wrestlers also incur injuries to the finger/hand areas. Both events expose the metacarpophalangeal area to contusions, sprains, abrasions, lacerations, various fractures, avulsions and interphalangeal amputations from rope dallying or livestock contact as the cowboy attempts to flank or turn calves or steers.[2,7,18,35,36,55,93,94] Crushing forces from rope entrapment ranging from 13 290 to 16 500 N have been calculated, minimizing opportunity for successful thumb replantation.[18] Severe digital vascular trauma followed by equivocal success in replantation and revascularization has been reported.[18,55] Since roping gloves cannot be expected to minimize the tremendous crushing forces, current thinking in injury prevention suggests continually educating ropers in the proper use of ª 2010 Adis Data Information BV. All rights reserved.

dallying. The use of gloves, however, would aid in reducing the number of minor hand and finger injuries.[7,55] Preventive orthotic implementation with sportspecific modification for the upper extremity should be addressed by rodeo coaches or advisors. Minimal orthoses, adapted from sports such as motocross racing, have been introduced to this sport[95] but with improper application, injury problems commonly observed in the phalangeal and carpal regions of roughstock athletes may be transferred to major limb and torso articulations.[2,7] More recently, custom fitted, functional bracing and additional padding have been suggested as an alternative to prophylactic taping to reduce the severity of injury to the upper extremity.[2,3,85] In summary, the extensive and pronounced trauma to the upper extremity continues to be a concern. With a significant level of joint loading forces, a high potential for articular trauma and subsequent premature osteoarthritis is evident.[2,75] 2.5 Lower Extremity Injuries

Lower extremity trauma accounts for 26–34% of total injuries in a single rodeo season.[7,10] Up to 5–30% of all cases involve the knee, followed by groin/pelvis (10–17%), ankle (0–18%), and upper (5–11%) and lower leg (7–23%) trauma.[7,9,10,24,68] Although these areas comprised 19% of total body injuries in roughstock, knee and leg/calf trauma comprised 67% of lower body trauma and 44% of total injuries incurred by saddle bronc riders.[7] When addressing the high incidence of lower body trauma in roughstock athletes, the leg placement technique required during riding competition is a factor in injury exposure.[7] Groin/pelvis trauma involves muscle strains and incomplete tears requiring nonoperative treatment and rehabilitation, as well as pubic symphysis contusions and diastasis from direct blows while riding.[9,10,15,24] Groin contusions have been associated with subsequent phlebothrombosis and femoral neuropathy.[28,96] Predisposing factors contributing to adductor trauma in most sports include excessive loads to the adductor Sports Med 2010; 40 (10)

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longus musculotendinous region. In addition, the spur placement in rodeo may result in lower leg malalignment commonly observed in the roughstock events.[28,97] Injuries to the upper leg consist of various contusions, quadriceps and hamstring strains and femoral fractures.[7,10,15] Impact trauma to the quadriceps typically results in myofibril damage and capillary haemorrhage and may result in myositis ossificans if left untreated. Knee trauma consisting of patellofemoral arthralgia and inflammation, hyperextension and Grade 3 tears involving anterior and posterior cruciate and medial collateral ligaments, are consistent with excessive reaction forces while riding or during awkward landings and entanglement with the bull rope at dismount.[7,9,10,15] These types of injuries can result in early osteoarthrosis.[75,98] Lower leg trauma consists of various contusions, associated peroneal neurapraxia, acute compartment syndromes, and tibia/fibular fractures following impact with chute gates and railings, being stepped on by livestock, or chute and ground impact during steer wrestling.[4,7,9,10,15] Among female athletes, the incidence of trauma is minimal at most levels of competition.[7,9,10,35] However, barrel racing cases often include impact trauma to the lower extremities as the horse dives into, rather than around, a barrel.[7,25] Female athletes may also experience articular damage to the knee during high speed dismount in the goat tying events or following an unexpected dismount in breakaway roping.[7] Even though trauma to the foot and ankle region is perceived to be limited due to purported boot protection,[9] various contusions, syndesmotic disruption with associated fractures, metatarsophalangeal dislocations and metatarsal fractures have been reported.[7,10,15,35,99] These were mainly attributed to ground contact during dismount and to being stomped on by livestock. However, in this sport, ankle trauma is similar when compared with other traditional collision sports, where up to 18% injury rates have been reported.[7,41,62,100,101] In summary, due to increasing concerns in both lower leg fractures and continued knee trauma, further efforts in equipment design are warª 2010 Adis Data Information BV. All rights reserved.

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ranted. The inherent space restrictions within the chute area, as well as designing a boot to absorb and minimize forces upon landing, or due to blunt force, will continue to pose challenges. 2.6 Fatal Injuries

Although numerous deaths attributed to rodeo have been reported in the media, there is a paucity of information on rodeo fatalities in the scientific literature, with findings primarily involving roughstock athletes experiencing craniocervical trauma with various neurological sequelae.[13,15,67,69] A death associated with acute respiratory distress syndrome with subsequent respiratory failure was cited in one study.[34] Although only four fatalities have been reported in the literature,[15,34] this statistic should be interpreted with caution due to the number of injuries unaccounted for in nonsanctioned events and rodeos with limited or nonexistent injury reporting systems. 3. Aetiology The primary mechanisms of rodeo injury include excessive fatigue, age and level of experience, the inherent violent and unpredictable nature of the sport and the lack of adequate medical intervention resulting in the use of unproven prophylactic intervention (e.g. braces, taping and padding) typically self-administered before competition.[7-9,25,46,102] The equipment design and riding style specific to each event, and the lack of conditioning and training observed at all levels of competition, also contribute extensively to the excessive trauma.[2,7,103] With this sport comes additional factors contributing to trauma involving arena conditions and behavioural patterns leading to unsound decision making.[7,25,55,58] It must be stressed, however, that a multiplicative array of intrinsic and extrinsic factors may simultaneously contribute to trauma, rather than simply an isolated cause.[26,44,45,63,104] 3.1 Fatigue and Overuse

Over an entire year, rodeo athletes are repetitively driven to excessively high physical stress Sports Med 2010; 40 (10)

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due to the tremendous strength and weight differences between athletes and the livestock they are competing against.[7,9,65] This is further compounded by limited and often inappropriate conditioning programmes as a result of subjective evaluation by fellow athletes and coaches, and a scientific/medical community not often familiar with this type of sport.[7,105,106] Sports that particularly involve a high degree of repeated eccentric movement, such as rodeo, enhance the early onset of fatigue and subsequent musculoskeletal damage at the myofibril and sarcomere levels where crossbridge attachments are forcibly compromised.[9,12,88,107-113] This type of muscle damage occurs in athletes who suddenly experience short-term, high-intensity structural overloading, resulting in nondescript and articular arthralgia,[15,20,111] musculoskeletal dysfunction, joint laxity/instability,[7,114,115] various degenerative changes[75,76,98,116] and a significant loss in acute strength reported to be as high as 40% of 1 repetition maximum quadriceps and forearm flexion strength.[109,117] Significant phosphocreatine degradation and compromised glycogen replenishment and storage have also been observed following excessive resistance workloads.[16,110,112] In summary, fatigue may escalate the opportunity for musculoskeletal trauma in this high-contact, high-injury-potential sport.[2,44,51,118] The physical requirements of the sport, coupled with repetitive training and competition with limited rest, can usually lead to what is referred to as overtraining syndrome (OTS).[108,119] The increasing prevalence of OTS among rodeo athletes has also been attributed to inconclusive evidence that clearly defines the optimal training zone for an athlete,[105,120] improper care and rehabilitation of injuries[2,9] and poor communication between coaches, medical staff and the athlete.[119] Although predicting OTS is often difficult and inconclusive with subclinical trauma, over 94 physiological, psychological, immunological and biochemical markers are associated with this increasing problem in sports.[119] Additional factors reported to contribute to musculoskeletal compromise include the inherent hypovascularity of connective tissue and the ª 2010 Adis Data Information BV. All rights reserved.

limited ability of bone to remodel at an expedient rate to match the demands of the sport.[20,121,122] When coupled with limited periods of recovery, the excessive static loading ultimately leads to muscular fatigue, compromised coordination and predisposition to injury.[2,9,95,118,120] Since these athletes are typically nursing some form of injury, progression of training from a normal adaptive microtrauma to subclinical injury may be minimized by varying day-to-day training routines to decrease repetitive musculoskeletal stress on the same joints.[108,123,124] Athletes with a seasonal history of chronic pain, muscular fatigue, diminished performance, or showing sudden changes in riding or rope throwing biomechanics so as to alleviate prodromal discomfort, are demonstrating hallmark signs of overt orthopaedic trouble.[124] No research has been directed toward the effects of fatigue and overuse on subsequent injury in rodeo, or muscle activity patterns in rodeo athletes during competition. Other studies, however, have documented an increasing number of injuries with increasing training time.[20,37,83] In contrast, a rodeo athlete’s typical ephemeral response to impending fatigue and trauma is well known, despite the high incidence of injuries incurred.[14,33,46,48,49] It should be noted, however, that an injured individual’s perception of the level of trauma and prognosis are not often in juxtaposition with medical opinion.[47,119] The information regarding the influence of fatigue on rodeo injuries remains an area of increasing interest and deserves further study. 3.2 Age and Skill Level

As with most sports, where psychomotor development is a prerequisite in maintaining optimal performance, one of the significant contributors to injury is an athlete’s age and skill level, where hand-eye coordination, sport-skill technique and split-second decision-making processes have not been adequately developed.[12,20,37,44,45,84] Although studies have typically documented a substantial number of injuries at the professional level, there is growing concern regarding the effects of this form of trauma on young and Sports Med 2010; 40 (10)

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inexperienced athletes at all levels of competition.[2,10,17,24,59,67] The beginning of the competitive season is an especially vulnerable time for injuries, since an individual has probably not achieved an optimal level of physical conditioning, or has had limited opportunity to refine motor skills.[105,125] The combination of limited instruction, inadequate skill development and increased anxiety affecting concentration and sound judgement has had a profound effect on the number and severity of injuries.[37,120,125] In one study, however, limited differences in injury incidence were documented between experienced and inexperienced rodeo athletes, although a higher rate of upper extremity trauma was observed among inexperienced competitors.[40] The study, however, only represented 19% of all professional rodeos, the inexperienced competitors did not compete on the same level of livestock (i.e. bulls vs steers), the actual amount of experience was not quantified and the study only included the most financially lucrative rodeos, which may have deterred inexperienced individuals from competing against the top athletes. In addition, increases in age, regardless of skill level, have resulted in the rise in injuries in this sport. Limited forethought and adherence to tradition tends to cloud judgement, leading to minimal helmet use, with a concomitant increase in the number of cranial, facial and cervical injuries that exceed injuries observed in other sports.[13] In addition, with increasing age accompanied by rapid maturation, physical strength and independence, the number of injuries tend to escalate. The enhanced physical strength and independent behaviour consistent among adolescents typically result in greater risk-taking and subsequently more severe impact injuries.[37,44,120] There is also an erroneous consensus among many adolescents that skill proficiency carries over from more traditional sports to rodeo, which tends to decrease inhibitions, resulting in greater potential for injury.[12] Again, findings are often difficult to interpret due to a lack of consistent definition of skill level (i.e. beginner, intermediate, advanced) and the possibility of more enhanced reporting of injuries, especially when sustained by young athletes, thereby contributing to a perceived increase ª 2010 Adis Data Information BV. All rights reserved.

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in injury incidence when compared with older, experienced populations.[37] 3.3 Inadequate Medical Care

Unfortunately, the often marginal on-site preventive care and education leading to inadequate self-treatment, as well as traditional disregard for orthotic concerns, delayed early sports medicine advances in rodeo, with these problems still existing at various levels of the sport.[4] The extensive year-long season, with a concomitant number of performances each week in a septic environment, typically results in acute trauma exacerbating into chronic conditions.[4,7,9,25] Although trauma is a daily part of this sport, and some medical personnel may not feel comfortable working in this challenging environment, it is essential that appropriate triage and education in trauma care, drug intervention and proper rehabilitative management be spearheaded by the medical community for these athletes’ optimal return to play.[9,19] Medical services should encompass not only local emergency medical oversight for catastrophic trauma, but should move toward preventive care at all levels of competition. At present, a large proportion of rodeos in the US rely solely on on-site emergency medical services with allied or ancillary staffing and ambulatory transport. In some instances, a physician may be in attendance. With the exception of corporatesponsored medical care – i.e. Justin Sportsmedicine, Mesquite, TX, USA, Wrangler Chiropractic Sports Medicine, Greensboro, NC, USA – preand post-medical services are primarily elicited by an association with a limited number of rodeos, or obtained by a rodeo promoter to handle catastrophic injuries, rather than encouraging preventive care.[4] In an attempt to overcome the shortfall in medical assistance and potential for debilitating injury, ~75% of rodeo athletes use some form of makeshift orthotic/tape support to withstand repetitive trauma during competition.[7,35] Although generally accepted by the athletes as an adjunct in dampening appendicular forces, the efficacy of prophylactic taping has been questioned in light of the excessive number of appendicular Sports Med 2010; 40 (10)

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injuries reported in the rodeo literature, as well as in other sports.[7-9,85] In most circumstances, due to the independent nature of rodeo athletes, the limited space available under standard riding equipment (e.g. glove) and the post-injury rather than preventative medical support observed at the majority of rodeos, the orthosis/tape is often selfadministered.[4,7] In addition, orthotic procedures are typically modified from other sports with minimal education.[95] This results in inadequate support, which is ineffective in altering the transmission of forces to more proximal limb and torso articulations.[2,7] Premature osteoarthritis, chronic neurapraxias and various degenerative conditions are typically the long-term prognosis when orthotic use is compromised or omitted.[9,75,76] Injury care techniques from other sports are now being readily adapted for rodeo use. Wrist taping, comprising 39% of orthotic use, has become popular in roughstock events. The use of helmets, elbow braces and tape (13%), and forearm padding (5%), adopted from football, lacrosse, hockey and tennis, may signal increasing concerns among rodeo athletes.[7,17] Although sport injury awareness is increasing, 25% of competitors abstain from using orthotic support, indicating a major area for improvement. Ninetythree percent of the athletes using some form of orthosis or taping were already managing injuries previously incurred while competing in other rodeo competitions, often with minimal therapeutic education.[7,50] Information on the level of self-medication following injury in this sport has been equivocal. Drug addictions resulting from indiscriminant use of inappropriate animal narcotics to overcome pain has been mentioned.[25] When athletes were questioned concerning drug use following injury at the collegiate level, 97% reported abstaining from any medication because of assumed drug interference on reflexes and coordination while still competing, and because of a lack of awareness of available drugs and their use in injury rehabilitation.[7] In either case, it is apparent that this sport population has not been educated in the efficacy and appropriate use of NSAIDs or other drugs for optimal return to competition.[111] ª 2010 Adis Data Information BV. All rights reserved.

In summary, as injuries continue to mount, studies continue to indicate significant concerns over the minimal medical care. These include inconsistent standards and a lack of quality control as they relate to on-site facilities, qualified personnel and the athlete’s general malaise, indifference, nondisclosure of injury or misunderstanding of the need to ensure an optimal environment to meet impending/expected trauma.[8,9,41] Others have noted that even when medical facilities are available, often the rodeo athlete still lacks the necessary understanding of remediation of acute trauma, and optimal triage and rehabilitation to optimize performance in the long term.[4,7,25] Due to the independent nature of these athletes, on-site allied health professionals can have a significant educational impact by incorporating education during treatment and encouraging follow-up contact, resulting in enhanced treatment and rehabilitative compliance, preventing acute trauma from leading to a chronic injury state.[7-10,51] Ultimately, findings continue to reflect risk factors, with limited research and education directed toward resolving these challenges. 3.4 Environmental Conditions

Rodeo athletes across all ages are constantly exposed to adverse climatic conditions, safety issues within the arena, inconsistent quality of livestock, inherent equipment mishaps, as well as controversy over equipment adoption. Although advances in rodeo sport technology and equipment development have been observed, the limited efforts have not deterred the continued rate of trauma. Characteristic of the inherent nature of the sport, >70% of total injuries occur during completion of an athlete’s ride. Of these, confrontation with chutes, gates and ropes, and other equipment mishaps, account for up to 21% of reported trauma.[2,7,10,65,102] Surprisingly, limited efforts have been directed toward documenting surface properties and climatic conditions in relation to injury incidence. Arena soil characteristics, such as composition, moisture content and compaction, affect both ground hardness and livestock and human stability. As discussed in other sport environments, improper Sports Med 2010; 40 (10)

Injuries in Rodeo

selection and management of soil composites lead to increased impact forces and abrasiveness during ground collisions.[20,26,41,44,57,104,126,127] Occasionally disking the arena floor without regard to surface properties, especially during inclement weather, will continue to contribute to trauma in this sport. Adoption of protective guidelines requiring selective padding of chutes, gates or equipment essential for performance, as seen in other major athletic events, may avert numerous injuries encountered by competitors.[7,10,57] Selection of stock contractors based on the performance quality of their livestock may improve quality and uniformity of competition and decrease injury, especially at the collegiate and high school level. As noted in an earlier study,[7] a significant correlation existed between the injury rate and the quality of livestock used, with variation in livestock performance between contractors for collegiate competition evident throughout the season. Others have also expressed concern over the questionable quality of livestock used at the smaller rodeos.[57] 3.5 Equipment Concerns

During the last two decades, equipment design, modification and usage, as well as development and testing of rodeo-specific equipment, have come to the forefront in this sport.[15,17,19] Recommendations concerning equipment use to minimize predisposition to trauma, however, have not been fully accepted by competitors based on tradition, stoicism, lack of education and equipment not designed specifically for rodeo. Regarding equipment use and mishaps, over the last two decades, the adoption of protective vests has demonstrated promise in minimizing the severity of trauma but not prevention among roughstock athletes. Minimal research into the actual use and efficacy of preventive equipment has been conducted, but further efforts in this area are warranted.[1,8,15,36] Various cervical collars, braces and headgear adapted from other sports have been observed at all levels of the sport.[7,15,17] Often, however, athletes may forego the protective equipment or orthotics that are available, subsequently enhancing the opportunity for needless injury.[2,3,7,65] For instance, research ª 2010 Adis Data Information BV. All rights reserved.

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has noted that only 53% of athletes entered in team roping used some form of glove, either leather or cotton, on their throwing hand.[7] The recommendations for helmet use have been controversial and continue to draw both supporters and detractors. Whereas minimal evidence has been found indicating the need for helmet use in rodeo,[34] helmet use has become standard practice in many sports, with research suggesting a profound decrease in craniofacial trauma and traumatic brain injury (TBI) up to 60% among young athletes wearing protective headgear.[84,128-131] Minimal findings in rodeo have also supported this trend, indicating a lower incidence of trauma primarily involving benign closed head injury.[17,19] Among rodeo scientists and medical professionals, the need for increased use of headgear has been strongly encouraged.[8,15,17,19,132] Regardless of a reduction in head injuries, which many consider an underestimation across most sports,[13,19,133,134] there is concern with potential cervical trauma from the extra weight, greater biomechanical twisting forces, and hindered agility, sight and hearing that helmet use may induce.[120,129,135] No concrete evidence, however, has been documented. Of greater concern, as noted in other sports, is that helmet use would actually increase tactically aggressive behaviour among athletes, resulting in higher velocity impacts than present helmet designs could absorb.[37,136,137] Further research efforts, rather than anecdotal evidence, has been strongly encouraged to determine the efficacy of helmet use in reducing axial trauma among rodeo athletes at all competitive levels.[19] To encourage acceptance, further development of rodeo equipment should be directed toward improvement of function and reliability. Major efforts should be focused on the biomechanical needs of the rodeo athlete, while ultimately preserving the sport’s heritage and tradition. 3.6 Lack of Fitness and Training

Improvements in performance in this sport are primarily achieved by ‘playing yourself into shape’, as substantiated in an earlier study Sports Med 2010; 40 (10)

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evaluating 156 collegiate rodeo athletes from 15 universities.[7] Overall, there was a lack of adherence to traditionally organized conditioning/training programmes, similar to other sports, with 27% of athletes stating that they participated in general exercise, 24% in strength training and 5% in jogging. The remaining 43% did not participate in any conditioning regimen, with this trend evident across all events and among both injured and non-injured athletes.[7] More recently, only 62% of Professional Rodeo Cowboys Association (PRCA) athletes regularly participated in standard, non-rodeo-specific exercise at least twice weekly at an intensity insufficient to improve performance or decrease injury rate.[103] In lieu of training protocols that closely duplicate the biomechanical and psychological aspects of this unique sport, actual competition is primarily relied on for hand/eye response refinement. This method of training, from an injury standpoint, is obviously not recommended. When compared with other high-collision sports, the exposure : injury ratio in rodeo is staggering.[7] The potential for musculoskeletal damage to occur is high, when one considers the size differential of the average 70 kg rider competing against livestock weighing 500–1000 kg at most levels of competition.[2,7,58,105] Although anecdotal evidence indicates some improvement in this area, the advantages of training to enhance core stability as well as agility, balance and strength essential for most rodeo events has not been readily accepted as in other sports.[44,107,108] This casual mindset may subsequently contribute to the observed physical fatigue resulting from a decline in muscle control, function and concomitant loss of energy absorption, ultimately escalating the opportunity for acute musculoskeletal trauma and premature degenerative changes to the spine and the upper and lower extremity areas.[2,51,75,118,138,139] Based on the premise that injury potential appears to be inversely related to an athlete’s state of training and muscular strength,[51,104,107,113,118,138,140,141] the lack of structured conditioning among many of these athletes – as well as minimal scientific efforts to quantify training indices for optimal performance in this sport – is disconcerting. ª 2010 Adis Data Information BV. All rights reserved.

3.7 Behaviour

The rodeo athlete’s success and potential income is primarily determined by the ability to compete on a continual basis. Factors such as adverse climatic conditions, severity of injuries, extensive travel and delayed or compromised medical/rehabilitative care rarely seem to influence the rodeo athlete’s competitive drive, a perception magnified when the potential for financial rewards is considered.[9,58,65,99] Romantic tradition holds that the rodeo athlete endures such hardships with quiet dignity, has a perceived supranormal tolerance to pain and an unyielding demand for perfection against ever-changing competitive conditions.[1,2,4,7,46] The unique stoicism, pain perception and consequent dissociation exhibited among this population, however, can result in detrimental behaviour and practices. Often, following emergency care, most athletes continue to compete throughout the season.[7,18,58] This form of behaviour has been defined as a form of positive deviance supported by strong psychosocial constructs developed through tradition and culture.[25,30,142] Others have commented on the narcotic or addictive behaviour that transcends certain and continued risk following injury, thus preventing essential trauma management and optimal return to competition.[1,7,9,25] Additional contributing behavioural factors associated with injuries among young athletes typically involve inexperience combined with high anxiety, limited attentional focus and low coping skills.[27,44,120,125] Among adolescents, inherent personality factors typically reflect low trait anxiety, high pain tolerance and self-reliance, and extroverted tendencies.[31,47,63] When combined with peer pressure and social norms, rules of caution and safety are often disregarded as the adolescent competitor pushes the bounds beyond physical capabilities.[12,32,37] In either case, these behaviours result in a plethora of unnecessary injuries. Proposed theories of self-concept affecting injury potential, the dissociation of pain as a function of individual perception, cognitive approaches to performance demands and emotional/somatic Sports Med 2010; 40 (10)

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response on rehabilitative processes have been defined in earlier research.[46-49,143] Research into psychosocial behaviour of rodeo athletes, however, is lacking in the literature, preventing further understanding of underlying mechanisms, and ultimately minimizing the efficacy of injury prevention programmes in this population. Future investigation of psychological influences upon performance, injury potential, pain and rehabilitation in rodeo may also provide additional insight into these unique athletes. 4. Prevention 4.1 Education and Instruction

A greater emphasis on coaching and athlete education has been recommended to reduce the frequency of injury in this type of sport.[7,50] Besides the inherent nature of the sport, as mentioned in sections 3.2 and 3.7, several studies have linked a number of both upper and lower extremity traumas to poor techniques or mechanics, inexperience and simply poor judgment.[2,7,10,19,25,58] Since skill level remains the primary impetus in minimizing sport injury,[12,50,84,144,145] implementation of basic sports medicine and science education at all levels of sanctioned competition, by the respective associations and/or rodeo coaches, has been strongly suggested.[7,19] A more formal multidisciplinary attempt as a way to reduce rodeo trauma is highly recommended in lieu of the more simplistic trial and error methodology commonly adopted in this sport. The physical trauma encountered at this level of competition may be diminished by a greater understanding of proper biomechanics, preventive injury techniques, post-injury care, nutritional requirements and proper conditioning techniques, thereby enhancing the opportunity for athletes to participate successfully. Recent efforts to develop guidelines for management and prevention of head trauma in this sport by a North American group of sports medicine professionals, is encouraging.[19] Rodeo organizations, however, should take the lead in initiating greater discussion, planning and cooperation, culminating in a minimal standard ª 2010 Adis Data Information BV. All rights reserved.

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of care as recommended in other high-collision sports.[8,19,146] Additional adjunct efforts should include establishing a centralized injury surveillance system for injury tracking and medical follow-up, as well as establishing teams of full-time healthcare professionals across the country, supported by a computerized referral system linked to all rodeo associations and appropriate medical staff at all levels of competition. This would enhance the opportunity to accumulate the essential injury data to more accurately assess the incidence, mechanisms and severity of trauma, leading to more appropriate interpretation of what rodeo athletes experience, as well as scientific and medical recommendations to properly prepare these athletes for the rigors of this sport. This would enable athletes to receive optimal care from rodeo-literate medical staff and allow tracking of athletes, especially those with multiconcussion syndromes and other life-threatening maladies, as they matriculate from one level of competition to the next. 4.2 Fitness/Conditioning

With a lack of fitness participation observed in a large segment of athletes interviewed, there is definitely an opportunity for foundation work, especially involving core strengthening, flexibility, balance, endurance and stability.[7,103,107,123,147] Efforts in this area would enhance spine and pelvic stabilization, anticipatory postural adjustment, critical power output and kinetic chain transfer of force and power to upper and lower extremities during riding.[108,123] The focus, however, must be directed toward rodeo event-specific movements for optimal pre-programmed core muscle activation, neuromuscular adaptation, distal joint loading and motor control. Isolation of specific muscle groups or joints (i.e. open chain kinetic exercise) should be avoided.[107,123] At the very least, a greater involvement in general physical conditioning by rodeo athletes, either individually or through a team concept, may minimize pathological changes leading to sprains and strains, and enhance earlier recovery.[10,108,113,123] Many of these competitive Sports Med 2010; 40 (10)

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injuries are associated with insufficient muscular synergy, endurance, strength and limb flexibility.[10,51,118] It must be understood from the start that improvement will only come with a focus on attention to detail and sport-specific technique, as well as overcoming an athlete’s false sense of true athleticism, commonly observed in today’s sport environment. Therefore, a commitment from both the coach and athlete is essential for a successful outcome. Despite physical conditioning programmes being recommended at all levels of competition,[7,10,65,86,103,148] there is a limited amount of information available on rodeo, especially studies that focus on the efficacy of such programmes. This prevents an objective appraisal of any training programme presently being used in this sport. In addition, development of a comprehensive conditioning regimen, within the limited time-frame that the year-long competitive season allows, will be a challenge.[2,105,106] Although there is a lack of consensus on what constitutes a rodeo-specific evaluation at this time, physiological testing and evaluation to establish the present physical fitness status of rodeo athletes at all levels of competition should be coordinated through the cooperative efforts of the respective rodeo associations, utilizing a computerized referral system of participating centres and universities with the expertise in assessing human performance.[7] Evaluation of core strength and bilateral symmetry in strength and range of motion (ROM) has been recommended as a starting point for many sports.[51,107,123] Since rodeo competition at the high school, college and professional levels includes a variety of events lasting approximately 8–20 seconds, rodeo competitors require substantial muscular strength and power supported by anaerobic capacity.[86,105,106,148] Given the varied nature of rodeo events, it is difficult to fully assess the strength and power requirements for each athlete. However, since 40–80% of all injuries are attributed to stresses induced through the riding motion, specific attention to upper torso, shoulder and rotator cuff musculature should be a priority, as well as lower body, hip and leg strength. This would seem to be particularly cogent for roughª 2010 Adis Data Information BV. All rights reserved.

stock riders, who attempt to stay on a bull or bronc for 8 seconds. Although adductor and abductor thigh strength have not been measured in any study, these are important leg muscle groups involved in riding; therefore, future investigation in this area should better characterize leg strength among this population.[140] Although year-round conditioning that focuses on improving sport-specific strength, power, explosive movement and full ROM is recommended, athletes experiencing constant injuries should be encouraged to spend a period of time conditioning outside the arena. The volume, intensity, recovery and duration of activities can be varied as the athlete moves through a series of phases that progress from general preparation through basic strength, functional strength, power, maintenance and active rest.[123,149] In summary, sport-specific conditioning is considered to be the cornerstone of today’s successful athletic programme. As both rodeo livestock and athletes become more physical, it is imperative to design programmes that emphasize a combination of power, acceleration, flexibility, technical skill, functional capacity and injury prevention. Rodeo athletes tend to invest a significant amount of time training for skill acquisition in their particular events. However, comprehensive assessment of these athletes could greatly improve training and rodeo performance, while decreasing predisposition to injury. 5. Conclusions Rodeo competitors experience a significant level of trauma. Although only limited preventative measures exist, which will never totally eradicate competitive injury, even moderate attention to preventative recommendations may decrease the number and severity of problems. Of greater concern is the underestimation of trauma because of the limited body of knowledge currently available in this sport. For example, out of over 2800 professional, collegiate, high school and youth rodeos, injury surveillance data are only available on a very small percentage of rodeos. The majority of reported injuries have been limited to isolated case studies of small sample Sports Med 2010; 40 (10)

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sizes and hospital records and represents only a modest fraction of participating athletes. Therefore, broadening efforts and funding to increase accessibility of surveillance systems to a greater number of programmes would provide a clear and accurate picture of the prevalence of injury in this sport. As mentioned in sections 2 and 3.6, prior efforts to quantify this non-traditional sport are still in the initial stages. Although definitive identification of the optimal aerobic base, critical power and strength requirements, as well as the optimal skill technique of rodeo athletes to achieve maximal performance levels, have not been adequately assessed and remain unanswered, these factors all positively or negatively contribute to injury potential, and further investigation is encouraged. Since biomechanics often plays a decisive role in performance potential and injury causation,[144] it has been stressed that kinetic analysis should be an essential component of future epidemiological research in this sport.[50,51,145] In regards to the psychological challenges of this sport, further efforts in understanding the competitive mindset required to successfully overcome the unique adversities experienced by these athletes would lead to increased receptiveness to conditioning and training, and, most importantly, treatment and rehabilitation of injuries. From a treatment standpoint, earlier and more accurate recognition and diagnosis of injury is still needed and could result in a pronounced decrease in the number and severity of overuse injuries. In addition, conditioning and rehabilitation protocols that specifically affect the successful functional outcome of an injury in this sport have not been firmly established and warrant further investigation. It is imperative that future studies take a more comprehensive, standardized, multifactorial approach to data collection allowing researchers to isolate predisposing risk factors more specifically and subsequently recommend alternatives to minimize trauma. Finally, as with all sports, rodeo performance is based on an extensive array of variables and, therefore, interdisciplinary efforts encompassing expertise across medicine, science and coaching are encouraged. However, whether increases in ª 2010 Adis Data Information BV. All rights reserved.

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knowledge, education and technology will be able to significantly reduce predisposition to injury among this population remains to be seen. As with all high-risk sports, the answer may lie in the increased wisdom and responsibility of the athlete, coach and/or parent to ensure an adequate level of ability, self-control and, simply, common sense as they participate in this unique sport. Acknowledgements No sources of funding were used to assist in the preparation of this article. The authors have no conflicts of interest that are directly relevant to the content of this article.

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Correspondence: Senior Research Scientist Michael C. Meyers, Department of Health and Human Development, 139 Reid Hall, POB 172940, Montana State University, Bozeman, MT 59717-2940, USA. E-mail: [email protected]

Sports Med 2010; 40 (10)

Sports Med 2010; 40 (10): 841-858 0112-1642/10/0010-0841/$49.95/0

REVIEW ARTICLE

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The Extent to Which Behavioural and Social Sciences Theories and Models are Used in Sport Injury Prevention Research Angela J. McGlashan and Caroline F. Finch School of Human Movement and Sport Sciences, University of Ballarat, Mt Helen, Victoria, Australia

Contents Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Search and Selection Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Classification and Review of Selected Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Prevention Measures in Sport Injury Prevention Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Summary Characteristics of the Reviewed Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Theories and Models Used in Sport Injury Prevention Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Implications for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Abstract

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Behavioural and social science theories and models (BSSTM) can enhance efforts to increase health and safety behaviours, such as the uptake and maintenance of injury prevention measures. However, the extent to which they have been used in sports injury research to date is currently unknown. A systematic review of 24 electronic databases was undertaken to identify the extent to which BSSTM have been incorporated into published sports injury prevention research studies and to identify which theories were adopted and how they were used. After assessment against specific inclusion and exclusion criteria, the full text of 100 potentially relevant papers was reviewed in detail. These papers were classified as follows: (i) explicit – the use of BSSTM was a stated key aspect in the design or conduct of the study; or (ii) atheoretical – there was no clear evidence for the use of BSSTM. The studies that explicitly mentioned BSSTM were assessed for how BSSTM were specifically used. Amongst the 100 identified papers, only eleven (11% of the total) explicitly mentioned BSSTM. Of these, BSSTM were most commonly used to guide programme design/implementation (n = 8) and/or to measure a theory/ construct (n = 7). In conclusion, very few studies relating to sport safety behaviours have explicitly used any BSSTM. It is likely that future sports injury prevention efforts will only be enhanced, and achieve successful outcomes,

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if increased attention is given to fully understanding the behavioural determinants of safety actions. Appropriate use of BSSTM is critical to provide the theoretical basis to guide these efforts.

1. Introduction There has been, and continues to be, widespread concern about sport and recreational (hereafter referred to as sport) injury worldwide.[1-8] Prevention of sport injuries is a complex process because of the multi-factorial nature of their causes and risk factors.[9,10] Accordingly, a multidimensional approach is required to address the problem and this must be implemented within the context of the prevailing sports culture and player behaviours.[11-13] Although a range of injury prevention measures have been evaluated within sport,[14,15] a lack of rigorous, directed behavioural and social sciences research into sport injury prevention, either in isolation or in combination with other approaches, has been suggested as contributing to difficulties in achieving uptake and dissemination of effective preventive measures.[12,16,17] While there has been increasing attention directed at establishing the efficacy of many and varied sport injury measures or interventions to prevent injury, much less attention has been given to the development of, and research into, effective methods for broader uptake, dissemination and diffusion of interventions in this context.[12,13,18] A recent systematic review[17] has emphasized the lack of behavioural and social science theories and models (BSSTM) being applied to unintentional injury prevention in general. These authors noted the paradox that while integration of BSSTM in other health research areas has grown significantly over recent years, it does not yet appear to have been adopted widely by injury prevention researchers.[17] Several other publications in the general injury prevention area have also emphasized the need to integrate BSSTM with the development of injury interventions.[19-22] More recently, in the context of sport, Finch[12] highlighted research into this area as a key knowledge requisite in her Translating Research into Injury Prevention Practice (TRIPP) framework. ª 2010 Adis Data Information BV. All rights reserved.

The importance of BSSTM is that they can provide tools for moving beyond intuition about what might work, or efficacious evidence from controlled trials, to the design and evaluation of interventions requiring adoption and maintenance of safety behaviours in the real world. They do this by providing a theoretical and conceptual basis for understanding safety behaviours and their determinants,[19,23-25] thereby presenting a systematic way of better understanding the events or situations that can explain or predict injury events, as well as the relationships between them.[23] Models draw on a number of theories to help understand a particular problem in a certain setting or context,[23] as, for example, was recently applied to understand protective eyewear behaviours in squash players.[24] Using BSSTM as a foundation for the development of interventions and planning for their delivery is consistent with the rationale for broaderbased evidence-based interventions in public health and behavioural medicine.[19,23,25] Use of behavioural theory, in particular, provides a framework for studying problems, identifying target groups and behaviours for intervention, developing appropriate interventions, measuring change in relevant behaviours and for evaluating intervention success.[19] In turn, this can lead to greater insights for programme planners and implementers to translate stronger programmes with higher uptake. Considerations from BSSTM framed within an ecological framework contribute to this by explaining the dynamics of safety behaviours, including processes for changing them and both the positive and negative influencing factors associated with both social and physical environments.[24] It has been argued that intervention programme planning, implementation, and evaluation processes based on BSSTM are more likely to succeed than those developed without the benefit of a theoretical perspective.[19,26] As these approaches work for general public health and other safety initiatives, it would seem Sports Med 2010; 40 (10)

Behavioural Models for Sports Injury Research

likely that they would also make a significant contribution to the prevention of sports injuries.[24,27] Although a number of recent systematic reviews of sport injury prevention measures shown to be efficacious have been reported,[14,15,28-32] none have described the role of BSSTM in the reviewed interventions, even though almost all interventions trialled to date have required some form of behaviour change on the part of a player, athlete or coach. In contrast, there is a major knowledge gap in relation to the effectiveness, or real-world uptake, of sports injury prevention interventions. This article reviews and summarizes the extent to which use of BSSTM has been reported across a range of sports injury prevention studies, as a precursor to better understanding intervention effectiveness. In doing so, it identifies which BSSTM have been most commonly used to date and categorizes the theoretical contexts in which they have been applied. 2. Methods 2.1 Search and Selection Strategies

A comprehensive electronic database search strategy was developed to identify relevant published literature associated with BSSTM and sport injury prevention from the following 24 somewhat overlapping electronic databases: ‘Academic Search Premium’, ‘AUSPORT’, ‘AUSPORTMed’, ‘Health Science Consumer’, ‘Health Source: Nursing’, ‘SportsDiscus with full text’, ‘SpringerLink’, ‘Web of Science’, ‘Web of Knowledge’, ‘JSTOR’, ‘PsychArticles’, ‘PsycINFO’, ‘Psychology + Behaviour’, ‘Psychoanalytic Electronic Publishing (PEP)’, ‘CINAHL Plus with text’, ‘Meditext’, ‘Wiley Interscience’, ‘APA-FT’, ‘PubMed’, ‘BMJ Journals Online’, ‘Electronic Journals (EBSCO)’, ‘Science Direct’, ‘Informaworld’ and ‘MEDLINE’. The search covered all items in each database (including ‘in press’ items) from the earliest records available until July 2009. An initial broad search filter was completed using three keywords: ‘sport’, ‘injury’ and ‘prevention’. Initial searches combined this injury filter with keywords reflecting BSSTM including the names of common BSSTM (e.g. Health Belief ª 2010 Adis Data Information BV. All rights reserved.

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Model) identified from the broader injury prevention, health behaviour and health promotion literature.[19,23,25,33,34] The search was further refined and expanded to capture other potential studies through the use of specific keywords (in isolation or in combination) chosen as relating to the following: (i) BSSTM constructs – ‘attitude’, ‘perceptions’, ‘social norms’, ‘perceived behavioural control’, ‘perceived severity/susceptibility’, ‘barriers’, ‘knowledge’, ‘self-efficacy’, ‘behavioural capability’, ‘reinforcement’, ‘environment’, ‘empowerment’, ‘motivation’, ‘antecedents’, ‘behaviour’, ‘adoption’, ‘maintenance’, ‘implementation’, ‘intrapersonal/interpersonal’, ‘organizational/community’; (ii) common sports injury prevention measures – ‘protective equipment’, ‘mouthguards’, ‘headgear’, ‘eyewear’, ‘faceguards’, ‘warmup’, ‘education’, ‘training’, ‘exercises (including biomechanical and neuromuscular)’; (iii) specific sports activities – ‘football’, ‘hockey’, ‘soccer’, ‘rugby’, ‘squash’, ‘netball’, ‘basketball’, ‘tennis’, ‘volleyball’, ‘handball’, ‘baseball’, ‘softball’, ‘athletics’, ‘badminton’, excluding cycling/bicycling; and (iv) terms – ‘survey/questionnaires’, because these are commonly used tools in BSSTM studies. The Cochrane Database of Systematic Reviews (www.cochrane.org) was also checked to ensure that no similar review was in existence there. Figure 1 summarizes the systematic process underpinning the review search strategy and the numbers of relevant papers identified and retained at each stage. In the initial stage, all potential articles were identified upon a preliminary review of titles, abstracts and keywords screened according to the defined broad search criteria. All duplicate articles were removed. Any study not exactly matching the stated exclusion criteria was kept for further full text review. Hand searching of the reference lists, individual journals, and identified review papers was undertaken to identify any further relevant studies not retrieved via the initial database searches. An author and citation search was also conducted to identify further studies undertaken by authors of the retained studies. The final studies identified for more detailed review were assessed against a checklist of specified inclusion/exclusion criteria (see Appendix 1 of the Supplemental Digital Sports Med 2010; 40 (10)

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Potentially relevant articles identified and abstracts, titles and keywords screened for retrieval via electronic database searches (n = 125)

Articles selected based on search criteria and title/abstract information and full articles requested (n = 125) Hand searching of reference lists of full and potentially relevant articles (n = 69) Additional search − author and citation search (n = 16)

Full-length articles evaluated against inclusion/exclusion criteria (n = 210)

Excluded (n = 110)

Identified articles met full inclusion and exclusion criteria and included in the final review (n = 100) Fig. 1. Summary of the systematic literature search strategy and the numbers of studies selected or excluded at each stage.

Content 1, http://links.adisonline.com/sportsmedi cine/SMZ/A5). To be retained for final review, an article had to focus on a sport injury prevention measure and mention some aspect of safety behaviours (e.g. mouthguard use) as well as some behavioural determinant/s in relation to the measure (e.g. attitudes). Specific inclusion and exclusion criteria were developed and agreed to by the authors. Full text articles were obtained and their content assessed to determine whether they met the stated inclusion/exclusion criteria (as listed in table I). 2.2 Classification and Review of Selected Studies

The lead author (AMcG) summarized the key characteristics of the selected studies and classified the use of BSSTM in the studies where applicable. For studies reporting use of BSSTM, details were recorded for the particular BSSTM reported and how they were used. In the first stage, the use of BSSTM in the selected studies was categorized as belonging to only one of the following categories:  Explicit: whereby, BSSTM were a clearly stated key aspect in the design or conduct of the study. Studies assigned to this category were required to state that BSSTM were used ª 2010 Adis Data Information BV. All rights reserved.

and to specifically mention the name of the theories or models.  Atheorectical: where there was no clear evidence for the use of BSSTM in the design or conduct of the study (including unrelated to, lacking a theoretical basis or somewhat implied though not plainly expressed). For example, a number of studies only implied or presented information potentially relating to one or more BSSTM constructs such as risk perceptions, safety attitudes, self-efficacy or perceived behavioural control, with no direct relevance to BSSTM. In these studies, it was not evident whether the particular ‘construct’ used by the authors had been chosen by chance or because of its theoretical basis. Based on the information provided in the papers, each author independently classified all studies according to the BSSTM use. Any discrepancies in the classifications were resolved through consensus discussion. In the second stage, studies classified as having explicit BSSTM use were summarized and assessed against the Trifiletti et al.[17] categorization of BSSTM use. This categorization allowed studies to be classified in more than one category. Application of the Trifiletti et al.[17] categorization required use of BSSTM in these studies to be rated as follows: Sports Med 2010; 40 (10)

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Table I. Inclusion and exclusion criteria for selecting papers to be included in the systematic review Inclusion criteria

Exclusion criteria

Full-text (complete) peer-reviewed, English language, earliest records to July 2009 Original research studies Studies relating to all ages and both sexes Sports activities (team/individual) in formal, competitive and social/recreational settings Related to the prevention of acute or traumatic injuries Unintentional injury Target populations – e.g. sports participants (athletes, players), coaches, officials, parents (or significant others) Studies specifically related to specified safety behaviours or behavioural interventions to prevent acute sport injury – e.g. protective equipment, warm-up Mention of behavioural and social sciences themes, aspects or approach

Studies relating to chronic, recurrent or illness-related conditions Studies not published in the peer-review literature, reports, reviews, theses and conference proceedings: not reported as full peerreviewed paper Intervention/prevention measure studies not considering prevalence of use and determinants of safety behaviour Bicycle-related studies, including bicycle helmet use studiesa Reviews or commentaries on injury prevention interventions, even if peer reviewed Studies relating to violence-related behaviours or intentional injuries

a

Bicycle-related studies were excluded from this review because it is not clear to what extent the bicycling activity described would be related to sport and active recreation, rather than to transportation. Even though this means that many studies of bicycle helmets have been excluded from this review, it is appropriate because most of those helmet-wearing interventions were implemented and assessed in the context of road safety initiatives rather than sports safety.

(a) Theory was used to guide programme design and/or implementation and/or to select programme measures. (b) Measurement of a theory or construct or model was undertaken (e.g. data was provided that described predisposing or enabling factors of player safety practices). (c) A theoretical construct or an extension of a theory (i.e. whether changes or variation in outcomes as predicted by models) was tested (e.g. whether the theory of reasoned action was helpful in understanding variations in beliefs, attitudes, subjective norms and safety practices). (d) Other: the use of BSSTM did not conform to the aforementioned categorization or when the study authors did not adequately explain the role of theory or models. The categories (a) to (c) represent Trifiletti et al’s.[17] increasing levels of theory application, from (a) low to (c) high, whilst the ‘other’ category (d) did not correspond to a ‘level’ of theory application. 3. Results 3.1 Prevention Measures in Sport Injury Prevention Research

Table II shows the total number of potential studies identified, total exclusions and inclusions, ª 2010 Adis Data Information BV. All rights reserved.

and the number of studies according to prevention measure categories. 3.2 Summary Characteristics of the Reviewed Studies

Table III summarizes the characteristics of the 100 studies that met the inclusion criteria. Most studies (n = 74) related to personal protective equipment (PPE) as the major injury prevention measure. The sporting activities varied from team ball sports, to team bat and ball sports, racquet sports, target and precision sports, individual water sports, individual athletic activities, equestrian activities and wheeled non-motorized sports. Most studies focused on the athletes/players themselves (n = 61 studies) but other common groups were coaches (n = 11), officials (n = 4) and dentists (n = 4). Sixteen studies related to multiple types of participants. Table III also indicates the categorization of each study according to its use or non-use of BSSTM. Overall, of the 100 studies that met the inclusion criteria, only eleven (11%) studies mentioned explicit use of BSSTM. 3.3 Theories and Models Used in Sport Injury Prevention Research

Table IV summarizes the specific BSSTM used in the eleven studies stating explicit use. Of the Sports Med 2010; 40 (10)

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studies that explicitly mentioned BSSTM, seven were related to the use of PPE.[16,24,38,39,41,58,107] Only the Theory of Reasoned Action/Theory of Planned Behaviour[39,107,109,122] and Diffusion of Innovation[58,129] were used in more than one study. When explicit studies were rated according to the Trifiletti et al.[17] categorization of BSSTM use, it was apparent that the majority (n = 8) had used BSSTM to guide programme design and/or implementation, or to measure a specific theory or a theoretical construct (n = 7); only four studies formally tested a theory and three studies did not meet any of the aforementioned criteria and was specified as ‘other’. 4. Discussion It is critical that sports safety interventions have a strong evidence-base for their efficacy and effectiveness before they are delivered to players, coaches and sporting bodies. It is equally important that they are both effective from a public health perspective and can be readily adopted and maintained in the ‘real world’. Although it is now accepted that behavioural approaches are useful for understanding, explaining and changing behaviour related to injury problems[19,21] and are an important consideration in intervention effectiveness,[13] this review highlights the lack of BSSTM applications to published sport injury prevention research. This is a concern because most solutions to preventing the sport injury problem rely on some form of behaviour

change or modification on the part of players, athletes, coaches, officials, administrators or peak sports bodies.[12,16,18,24] Whilst this review found quite a large number of studies relating to sport injury prevention measures with some behavioural basis, only 11% applied any formal theoretical considerations to their study, suggesting that most authors in this area are either not aware of the importance of BSSTM, or do not appreciate the value of theoretical underpinnings and their application to practice, or may simply lack the knowledge, expertise or requisite skills/training to utilize them. When BSSTM were used in the published sports injury studies, this tended to be in relation to individual-level (intrapersonal/interpersonal) theories. These included the Health Belief Model,[38,131] Theory of Reasoned Action/Theory of Planned Behaviour,[39,107,109,122,132] Attitude-Social Influence Self Efficacy (ASE) model (an elaboration of the Theory of Planned Behaviour),[41,133] and Social Cognitive Theory.[16,134] This is quite appropriate and not surprising given the focus on ensuring the safety of individuals involved either in team sports or as individual participants of activities such as skating. However, recent commentary has stressed that it is more than just individual (i.e. player) factors that affect uptake and adoption of safety measures, and hence sustained behaviour change.[13] Such factors relate to the capacity of the full sports delivery system to deliver and implement preventive measures for the benefits of sports participants.[13]

Table II. Overall summary of identified sport injury prevention measure studies at different stages in the review process Prevention measure

No. of potential studies

Total studies excluded

Total studies included

Atheoretical studies

BSSTM explicit studies (%a)

Equipment

7

5

2

1

Multi-focused

6

0

6

4

1 (50.0) 2 (33.3)

General IP

5

0

5

4

1 (20.0)

Education

8

2

6

5

Protective equipment

109

36

74

68

6 (8.2)

Specialized exercise

74

67

7

7

0 (0.0)

210

110

100

89

11 (11.0)

Total a

1 (16.7)

% denotes BSSTM out of total studies included per prevention measure.

BSSTM = behavioural and social science theories and models; IP = injury prevention.

ª 2010 Adis Data Information BV. All rights reserved.

Sports Med 2010; 40 (10)

Country of study

Categorizations of BSSTM use and sport; level of playb atheoretical

Australia USA Ireland USA

PPE, general PPE, general PPE, general

USA France USA

PPE, general PPE, general

The Netherlands India

PPE, eyewear

Australia

PPE, eyewear

Australia

PPE, eyewear

Australia

PPE, eyewear PPE, eyewear

Australia Australia

PPE, eyewear PPE, eyewear

Australia Australia

PPE, eyewear PPE, facial protection PPE, faceguard

Australia USA USA

Squash; adult (pennant) Ice hockey (indoor); adult/recreational Baseball; junior/youth league

PPE, headgear PPE, headgear PPE, headgear PPE, headgear PPE, headgear PPE, headgear PPE, headgear PPE, headgear PPE, headgear

Australia Australia USA Australia USA USA USA USA USA

Rugby union; junior/interschool Australian football; adult/amateur/community Rugby union; adult/university Surfing; non-specific level Organized equestrian; non-specific level Wrestling; collegiate/division 1

Australian football; adult/community Rugby (female); various Hurling; adults/inter-county Various sports (12 sports); junior/high school (athletes) In-line skating; adult/recreational In-line skating; adult/non-specific In-line skating, skateboarding and snowboarding; junior/adolescent extreme sports In-line skating; junior/children recreational to high performance Various sports; junior to adult/high school, college and university Squash; adult/competitive and social/recreational Squash; adult/competitive and social/recreational to state Squash; adult/competitive and social/recreational Squash; non-specific level Squash; adult/competitive and social/recreational Squash; non-specific level Squash; non-specific

Skiing and snowboarding; adult/non-specific Skiing; adults/non-specific Skiing and snowboarding; adults/non-specific level

Reference

Players Players Players Players, coaches

35 36 37 16

Participants Participants Participants

38 39 40

Participants

41

Coaches

42

Players

43

Players

44

Players

45

Players Players

46 47

Venue operator Players, venue managers Players Players Players, coaches and parents Players Players Players Participants Participants Wrestlers Participants Ski-shop owners Ski patrollers

48 24 49 50 51 52 53 54 55 56 57 58 59 60

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PPE, general PPE, general PPE, general PPE, general

Study focus

explicit

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ª 2010 Adis Data Information BV. All rights reserved.

Table III. Characteristics of the 100 studies included in this review and classification of their use of behavioural and social science theories and models (BSSTM) Safety behavioura

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Table III. Contd Safety behavioura

Country of study Canada

PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards

PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards

Australia Australia Australia Australia, Scotland, Ireland, Wales UK UK UK UK USA USA USA USA USA Australia China

PPE, mouthguards

USA

PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards

USA UK Turkey Japan

PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards

Singapore Nigeria USA Nigeria Switzerland Brazil

PPE, mouthguards

USA

PPE, mouthguards

USA

atheoretical Rugby union; junior to adults/high school to national level Rugby; adult/elite international Rugby; adult/elite international Rugby; adolescents/high school (private) Rugby; adult/elite international

Rugby; adult/elite international Rugby; non-specific level Rugby league; adult/elite super league Rugby; adult/various levels Rugby; adult/elite international American football; adult/university (freshman) Football; junior/high school varsity Football; junior/high school varsity Basketball; junior/high school varsity Basketball; junior to adult/social to elite Basketball; adult/professional and semiprofessional Ice hockey; adult/university NCAA men’s division 1 Ice hockey; junior/high school Field hockey; adult/elite premium division Tae Kwon Do; junior/elite Various sports (4 sports); adolescents/high School Various sports; junior/high school Various sports; junior to adult Various sports; junior/high school Various sports; junior/high school Various sports; adult/national level Various sports; adult/semi-professional to professional American football; adult/university NCAA division I-A Ice hockey; adult/university NCAA division I, II, and III, and independent varsity ice hockey programme

Study focus

Reference

Players, coaches

61

Players Players Players Players

62 63 64 65

Players Players Players Players Players Players Players Players Players Players Players

66 67 68 69 70 71 72 73 74 75 76

Players

77

Players Players Players Players

78 79 80 81

Players Players Coaches Coaches Players, officials Players

82 83 84 85 86 87

Coaches

88

Athletic trainers

89

explicit

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Sports Med 2010; 40 (10)

PPE, headgear

Categorizations of BSSTM use and sport; level of playb

Safety behavioura

Country of study

PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards PPE, mouthguards

USA USA USA USA USA USA USA Singapore Nigeria Turkey

PPE, mouthguards

Turkey

PPE, mouthguards

PPE, mouthguards

Switzerland and Germany Switzerland, Germany and France USA

PPE, mouthguards

UK

PPE, mouthguards

Australia

PPE, mouthguards

USA and Canada USA

PPE, mouthguards

USA UK

General injury prevention General injury prevention General injury prevention General injury prevention

Australia USA Australia Australia

Study focus

Reference

Football; adult/university NCAA division I-A Football; adult/university NCAA division I-A Soccer; junior/competitive Various sports; junior/public school Soccer; junior/non-specific Various sports; non-specific Various sports; non-specific level Various sports; non-specific level Various contact sports; non-specific level Various sports; junior/high school coaches and university athletes Various sports; adult/university coaches and players Handball; adult/amateur, semi-professional

Officials Officials Parents Parents Parents Dentists Dentists Dentists Dentists Coaches, players

88 90 91 92 93 94 95 96 97 98

Coaches, players

99

Coaches, players

100

Squash; junior, adult/juniors, amateur, semiprofessional and professional

Coaches, players

101

Football; junior/high school varsity

Coaches, trainers Players, parents

102

atheoretical

explicit

Rugby union; adult/elite players and community level parents of junior players Australian football; junior to adult/amateur

Players, spectators (family and friends) Players, trainers, dentists President

Ice hockey; junior to senior/all levels Baseball; junior/little league

104

105 106

Skiing; adult/non-specific

Skiers Players

107 108

Australian football; junior/elite

Players Players Participants Coaches

109 110 111 112

English football (soccer); adults/professional non-specific level Ice Hockey; junior/non-specific level Little athletics; junior/non-specific level Rugby union; junior/community

103

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Equipment, safety baseballs Equipment-ski bindings General injury prevention

Categorizations of BSSTM use and sport; level of playb

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Table III. Contd

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Table III. Contd Safety behavioura Specialized exercise, tackling ‘spearing’ and rule enforcement Specialized exercise, tackling ‘spearing’ Specialized exercise, warm-up Specialized exercise, preexercise stretching Specialized exercise, intervention Specialized exercise, nonintervention Specialized exercise, nonintervention Multi, non-intervention (SEE) Multi, intervention Multi, intervention

Categorizations of BSSTM use and sport; level of playb

Country of study

atheoretical

Study focus

Reference

USA

American football; junior/high school

Officials

113

USA

Football; junior/high school level

Players, coaches

114

Australia

Golf; adult/non-specific level

Players

115

USA

Various sports; junior/high school level

Coaches

116

USA

Soccer; NCAA division 1 (female)

Coaches

117

Australia

Australian football; adult/elite

Coaches

118

UK

Cricket; adult/first-class county

Coaches

119

Australia

Skiing/snowboarding; adults/various levels (beginners/intermediate/advanced)

Skiers

120

Rugby union; population wide

Multi-focused Multi-focused

27 121

Basketball; high school varsity, junior varsity, division III Massachusetts South Coast conference coaches

Players, coaches

122

New Zealand USA

explicit

Skiing/snowboarding; junior, adult/ non-specific level

USA

Multi, non-intervention (SEE) Multi, non-intervention (SEE) Education intervention

New Zealand

Soccer; junior

Players

123

Australia

Skiing/snowboarding; adults/various levels (beginners/intermediate/advanced) Running; adults/non-specific

Skiers

124

Runners

125

Skiing; various participants/levels (beginners to advanced) Soccer; adults/various club officials Netball and soccer; various levels/ non-specific level

Skiers

126

Officials Coaches

127 128

Coaches

129

Players, coaches parents

130

Education intervention Education intervention

The Netherlands The Netherlands Australia New Zealand

Education intervention

USA

Education intervention

Australia

Education intervention

Sports Med 2010; 40 (10)

Various sports; adolescent/high school athletic coaches Basketball and rugby; junior/non-specific

a

General PPE refers to multiple types of PPE considered in the one study, e.g. helmets, wrist guards, knee and elbow pads.

b

Non-specific denotes authors did not specify level of sport.

NCAA = National Collegiate Athletic Association; PPE = personal protective equipment; SEE = specialized exercise and education.

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Table IV. Summary of behavioural and social science theory and models (BSSTM) explicitly stated as being used in sports injury research studies BSSTM

Safety behaviour under investigation

Trifiletti et al. categorization[17] of BSSTM use

Health Belief model

Protective equipment

Tested theory

38

Theory of Reasoned Action/Theory of Planned Behaviour

General injury Prevention

Guided programme design and/or implementation; measured theory or construct

109

References

Theory of Reasoned Action/Theory of Planned Behaviour

Multi-intervention (SSE)

Measured theory or construct

122

Behavioural Intention model (otherwise known as Theory of Reasoned Action)

Equipment, ski bindings

Guided programme design and/or implementation; measured theory or construct; tested theory

107

Theory of Reasoned Action/Theory of Planned Behaviour (including threat perceptions)

Protective equipment

Guided programme design and/or implementation; measured theory or construct; tested theory

39

Social Cognitive Theory

Protective equipment

Guided programme design and/or implementation; measured theory or construct

16

Attitude-Social Influence Self-Efficacy model

Protective equipment

Guided programme design and/or implementation; tested theory

41

Refined Ecological model

Protective eyewear

Guided programme design and/or implementation; other

24

Diffusion of Innovation Theory

Protective headgear

Other

58

Diffusion of Innovation Theory

Coach education, general injury prevention

Guided programme design and/or implementation; measured theory or construct

129

PRECEDE-PROCEED modela

Multi-intervention

Guided programme design and/or implementation; measured theory or construct

27

Ottawa Chartera

Multi-intervention

Other

27

a PRECEDE-PROCEED model and Ottawa Charter applied in the same study;[27] (identified 12 BSSTM; n = 11 studies). SEE = specialized exercise and education.

Despite the increasing availability of evidencebased sports injury prevention measures, sports safety efforts to date have been hampered because limited research attention has focused on understanding the intervention implementation context and processes, including barriers and facilitators to sustainable programmes.[12,13,18] This knowledge gap requires not only the use of individual-level theories but also the application of organization- and community-level theories. Our review has confirmed that organizational- and community-level theories have rarely been used, with the exception of a refined Ecological model,[24] the Diffusion of Innovation Theory,[58,129] the PRECEDE-PROCEED planning model[27] ª 2010 Adis Data Information BV. All rights reserved.

and the Ottawa Charter.[27] Further application of BSSTM at multiple-levels of behavioural influence (i.e. aligning individual, organizational and community) in this area of research should strengthen the design of intervention strategies and ensure sustainability of implemented programmes. Their direct application could be used to develop different intervention strategies and methods when working with either individuals or communities[19,25] in different sports settings. For example, at the individual level, intervention strategies could include a variety of behavioural, educational, counselling, skills development and training methods.[25,135] At the organizational and community level, the use of Sports Med 2010; 40 (10)

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social marketing, mass media and media advocacy are important, as well as coalition building, social planning and community development.[25,135] Another significant gap highlighted by this review is that many of the common theories from the behavioural literature were not identified in the reviewed sports injury studies; these include the Protection-Motivation Theory,[136] Stages of Change/Transtheoretical model,[135,137] Precaution Adoption Process model,[138] Applied Behavioural Analysis,[19] Social Networks and Social Support,[139] Self Efficacy,[140] Community Organisation and Mobilisation Theories (including Empowerment, Capacity, Participation and Relevance),[141] Communication Theories,[142] Organisational Development Theory (including Organisational Culture, Climate and Capacity),[143] the RE-AIM (Reach, Effectiveness-Adoption, Implementation and Maintenance) model[144] and Social Marketing.[145] Given the success of application of these BSSTM to other safety behaviours and health issues,[146,147] there could be considerable merit in also applying them to the sports injury context.[13] For instance, the Applied Behavioural Analysis[19,25] theory has been used in many injury settings (e.g. road safety,[148,149] child safety,[150] and occupational settings[151]) to change behaviour, but has yet to be applied to sports injury. Unlike the review conducted by Trifiletti et al.,[17] which found the PRECEDE-PROCEED planning model was most commonly used in unintentional injury prevention, our study has highlighted that this model has only been used in one sports injury prevention study to date.[27] The reasons for this are unclear, but could reflect the relative infancy of the application of BSTTM underpinnings to sports injury prevention. All BSSTM can be applied at various stages of the research process. We applied the Trifiletti et al.[17] categorization to ascertain how theory had been used in the sports injury studies that adopted it. The most common application was used to guide programme design and/or implementation, and/or select programme measures of a study. This implies a low level of theory application according to Trifiletti et al.,[17] and demonstrates a significant absence of the systematic application of BSSTM to sports injury research. Most studies ª 2010 Adis Data Information BV. All rights reserved.

that did apply theory to programme design were also categorized as measuring a theory, construct or model, thereby strengthening their theory application moderately. There was little evidence of testing theories, to determine what might be most applicable to the sports injury context. Without this information, researchers who want to apply BSSTM appear to just select random constructs that they think may be relevant, without formal justification or rationale for their choice. Often it seems that sport injury studies address constructs relevant to behaviour change in general, but there is little evidence of studies actually committing to the application of specific theory and systematically designing methods, such as questionnaires, accordingly. This is reflected in the large number of atheoretical studies. This could indeed lead to results that are neither replicable nor generalizable to other player groups or different interventions. Moreover, atheoretical studies are unlikely to build on existing behavioural knowledge and run the risk of omitting important psychosocial determinants and processes central to behaviour change. Although theory-based studies are more likely to provide a strong empirical foundation for evidence-based prevention approaches, this does not mean that nothing can be learnt from theoretical approaches. There is still a role for them in informing future theoretical studies, guiding implementation efforts and highlighting future research questions. 4.1 Limitations

Although an extensive search strategy was adopted, it is possible that the ability to locate relevant papers for the review was limited by the use of specific keywords or series of keywords. Using search terms relating to common theories only resulted in two studies being found; these two studies were also identified using alternative search terms. This restriction to only common theories may have limited identification of other useful or newly emerging theories. However, we do not expect this to be a major omission because our search strategy did identify one study that used the ASE model[41] and another that applied a refined Ecological model,[24] which are uncomSports Med 2010; 40 (10)

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mon in the general literature. It is acknowledged that a recent review of behavioural research in the broader injury prevention context[17] identified a larger range of theory applications (e.g. Health Belief model, Theory of Reasoned Action/Theory of Planned Behaviour, Social Cognitive Theory, Diffusion of Innovations, PRECEDE-PROCEED and Social Marketing Theory) than we were able to find in the sports injury prevention literature. Moreover, although excluded from this review, there is recognized use of BSSTM in studies of bicycle helmets and bicycle safety (see references[152-154] for examples). Our process of searching, which included hand searching reference lists and additional author searches, did identify further studies and point to the problem of how articles are indexed in databases. We excluded non-peer-reviewed (grey) literature, such as conference proceedings and dissertations, and this may have limited the identification of theory applications in sport injury prevention contexts, though we consider this unlikely. It is possible that some authors of peerreviewed studies are not reporting full details of their use of BSSTM due to factors such as length restrictions applied by journals. If this were the case, then the number (and proportion) of papers we assigned to the atheoretical categories of BSSTM use may be overestimated. If the field is to progress and researchers are to benefit from the accumulated wisdom of others, it would be pertinent for authors to include these details in their papers and for journal editors to require it formally. Without this, it is likely that researchers will continue to make the same ‘mistakes’ resulting in critical components of interventions, their target behavioural variables and maximal implementation strategies not being identified. The initial search for articles relied on abstract content only; it was, however, apparent that some studies seemed to have a behavioural approach (i.e. implementing an exercise programme) and did not clearly link to the stated exclusion criteria in the first instance. A full-text review of these ‘unclear exclusion’ studies was undertaken. None of the unclear exclusion studies mentioned theory applications; however, some did mention outcome measures (e.g. attitude, knowledge and beª 2010 Adis Data Information BV. All rights reserved.

853

haviour) in the method/discussion section and, subsequently, were included in the review (see Braham et al.[35] for example). 4.2 Implications for Future Research

The lack of evidence supporting the widespread use of BSSTM in the design, implementation and evaluation of sports injury interventions results in difficulty providing clear direction or strategies to enhance uptake of sport injury prevention interventions. Unfortunately, the current status of the field also does not appear to assist in enhancing theory development in sport injury prevention research. Previous reviews of sports injury studies have also noted many problems with the quality of their research designs[15] and until these are addressed uniformly, this may have implications for sports injury prevention. Having said this, whether the application of BSSTM to sport injury prevention contexts will improve the uptake of sport injury interventions is largely unanswered, but the evidence from other areas of public health priority suggests it should play a key role. To date, very few studies have used BSSTM; when applied, their use has been varied, with no studies being undertaken in the same sporting setting to enable comparisons of theories or consistency of findings to be established. Extending current work to the evaluation of the robustness of behavioural findings when theory is applied to particular sport injury prevention issues, and determination of what theories and models work best for specific sport injury prevention topics is needed. It is recommended that further research be conducted to compare or even integrate theories, so that the safety recommendations arising from future research studies take into account the complexity of sports behaviours and settings and the multitude of factors contributing to injury risk. It is unlikely that a single theory will be shown to explain the dynamics of safety behaviours in sporting contexts fully. Rather, it is likely that existing theories will need to be extended or refined to incorporate multi-level approaches. The extended ecological model of Eime et al.[24] is one step in this direction. Sports Med 2010; 40 (10)

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Finally, given the widespread use of BSSTM in other application areas (such as exercise promotion, occupational safety and road safety) valuable lessons could be synthesized and translated to the sport injury prevention area to reduce investment in unnecessary and costly duplication of efforts. Importantly, the sports injury prevention research field needs to embrace interdisciplinary collaborations and partnerships. This will enhance the applicability and relevance of research programmes to real-world safety applications and contexts (and vice versa). Significant injury reductions will only be achieved at a population level if research efforts contribute to collectively changing individual behaviours, environmental conditions and social structures to develop supportive safe sports contexts.[12,13] 5. Conclusions This review has highlighted the general lack of use of BSSTM in studies relating to unintentional sport injury prevention research. Future research in this area, incorporating such approaches is needed in studies that are rigorously designed and analysed. It will also be important to interweave BSSTM approaches into the mainstream of sport injury prevention research, through increasing multidisciplinary/interdisciplinary research teams. There already exist a number of BSSTM applications that researchers could use in enhancing the uptake of sport injury prevention measures, and new behaviour change theories and models are constantly emerging.[135] The field needs researchers who are willing to put these theories to the test. Advances in BSSTM development, as well as increased attention to behaviour change research, will provide new opportunities for reducing injuries and enhancing the uptake of preventive measures. By combining the usual sports injury prevention methods with BSSTM, the field will obtain a better understanding of how and why sports participants (and the settings they play in) make safety-related decisions and what enhancements can be made to injury prevention strategies to ensure their sustained uptake. As Trifiletti et al.[17] posits ‘‘It will take creative researchers to find the nexus’’ (page 305). ª 2010 Adis Data Information BV. All rights reserved.

Acknowledgements The authors have no financial or conflicting interests. Angela McGlashan was supported by a Postgraduate Research Scholarship funded from a National Health and Medical Research Council (NHMRC) funded research project (Project ID: 400937). Caroline Finch is supported by an NHMRC Principal Research Fellowship. Comments on a draft version of the paper were received from Dr Dara Twomey, Dr Peta White and Ms Rebecca McQueen.

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cussion prevention toolkit: Centres for Disease Control and Prevention’s heads up, concussion in high school sports. Health Promo Prac 2010; 11 (1): 34-43 Jalleh G, Donovan RJ, Clarkson J, et al. Increasing mouthguard usage among junior rugby and basketball players. Aust N Z J Public Health 2001; 25 (3): 250-2 Janz NK, Becker MH. The health belief model: a decade later. Health Educ Q 1984; 11 (1): 1-47 Azjen I. The theory of planned behavior. Organ Behav Hum Decis Process 1991; 50: 179-211 De Vries H, Backbier E, Kok G, et al. The impact of social influence in the context of attitude, self-efficacy, intention and previous behaviour as predictors of smoking onset. J Appl Soc Psychol 1995; 25: 237-57 Bandura A. Social cognitive theory: an agentic perspective. Ann Rev Psychol 2001; 52: 1-26 DiClemente CC, Crosby RA, Kegler MC. Review of emerging theories in health promotion practice and research: strategies for improving public health. Health Educ Res 2004; 19 (3): 349-50 Rogers EM. A protection motivation theory of fear appeals and attitude change. J Psychol 1991; 93: 91-114 Prochaska JO, Redding CA, Evers KE. The transtheoretical model and stages of change. In: Glanz K, Rimer BK, Lewis FM, editors. Health behavior and health education: theory, research and practice. 3rd ed. San Francisco (CA): Jossey-Bass, 2002: 99-120 Weinstein ND, Sandman PM. A model of the precaution adoption process: evidence from home radon testing. Health Psychol 1992; 11: 170-80 Israel BA. Social networks and health status: linking theory, research and practice. Patient Couns Health Educ 1982; 4: 65-79 Bandura A. Self efficacy: toward a unifying theory of behaviour change. Psychol Rev 1977; 84 (2): 191-215 Minkler M, Wallerstein N. Improving health through community organisation and community building. In: Glanz K, Lewis FM, Rimer BK, editors. Health behaviour and health education: theory, research and practice. 2nd ed. San Francisco (CA): Jossey-Bass, 1997: 241-69 Aldoory L, Bonzo S. Using communication theory in injury prevention campaigns. Inj Prev 2005; 11: 260-3 Beyer JM, Trice HM. Implementing change: alcoholism policies in work organisations. New York: Free Press, 1978

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144. Glasgow RE, Vogt TM, Boles SM. Evaluating the public health impact of health promotion interventions: the RE-AIM framework. Am J Public Health 1999; 89: 1322-7 145. Maibach EW, Rothschild ML, Novelli WD. Social marketing. In: Glanz K, Rimer BK, Lewis FM, editors. Health behavior and health education: theory, research and practice. 3rd ed. San Francisco (CA): Jossey-Bass, 2002: 437-61 146. Williams AF, Wells JK, Farmer CM. Effectiveness of Ford’s belt reminder system in increasing seat belt use. Inj Prev 2002; 8: 293-6 147. Davidson LL, Durkin MS, Kuhn L, et al. The impact of the safe kids/health neighbours injury prevention program in Harlem, 1988-1991. Am J Public Health 1994; 84 (4): 580-6 148. Sleet DA, Hollenbach K, Hovell M. Applying behavioral principles to motor vehicle occupant protection. Edu Treat Child 1986; 9: 320-33 149. Streff FM, Geller ES. Strategies for motivating safety belt use: the application of applied behavioral analysis. Health Educ Res 1986; 1: 47-59 150. Thomson JA, Ampofo Boateng K, Lee DN, et al. The effectiveness of parent in promoting the development of road crossing skills in young children. Br J Educ Psych 1998; 68: 475-91 151. Boyce TE, Geller ES. Applied behavioral analysis and occupational safety: the challenge of response maintenance. J Org Beh Manag 2001; 21 (1): 31-60 152. Ivers R. Systematic reviews of bicycle helmet research [letter]. Inj Prev 2007; 13: 190 153. O’Callaghan FV, Nausbaum S. Predicting bicycle helmet wearing intentions and behaviour among adolescents. J Safety Res 2006; 37: 425-31 154. Lajunen T, Rasanen M. Can social psychological models be used to promote bicycle helmet use among teenagers? A comparison of the health belief model, theory of planned behaviour and the locus of control. J Safety Res 2004; 35: 115-23

Correspondence: Professor Caroline F. Finch, School of Human Movement and Sport Sciences, University of Ballarat, PO Box 663, Mt Helen, Ballarat, VIC 3353, Australia. E-mail: [email protected]

Sports Med 2010; 40 (10)

Sports Med 2010; 40 (10): 859-895 0112-1642/10/0010-0859/$49.95/0

REVIEW ARTICLE

ª 2010 Adis Data Information BV. All rights reserved.

Neuro-Musculoskeletal and Performance Adaptations to Lower-Extremity Plyometric Training Goran Markovic and Pavle Mikulic School of Kinesiology, University of Zagreb, Zagreb, Croatia

Contents Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Search Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Plyometric Training (PLY) on Rigid Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Musculoskeletal Adaptation to PLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Bone Adaptation to PLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Muscle-Tendon Complex and Joint Adaptations to PLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Neuromuscular Adaptations to PLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Muscle Fibre Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Whole Muscle and Single Fibre Contractile Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Whole Muscle and Single Fibre Hypertrophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Muscle Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Neural Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6 Muscle Strength and Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.7 Stretch-Shortening Cycle Muscle Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Athletic Performance Adaptation to PLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Jumping Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Sprinting Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Agility Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Endurance Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. PLY on Non-Rigid Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Neuromuscular and Performance Adaptations to Aquatic- and Sand-Based PLY. . . . . . . . . . . . 4. PLY in Prevention of Lower-Extremity Injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Practical Application of PLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusions and Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Abstract

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Plyometric training (PLY) is a very popular form of physical conditioning of healthy individuals that has been extensively studied over the last 3 decades. In this article, we critically review the available literature related to lower-body PLY and its effects on human neural and musculoskeletal systems, athletic performance and injury prevention. We also considered studies that combined lower-body PLY with other popular training modalities, as well as studies that applied PLY on non-rigid surfaces. The available evidence suggests that PLY, either alone or in combination with other typical training modalities, elicits numerous positive changes in the neural and musculoskeletal

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systems, muscle function and athletic performance of healthy individuals. Specifically, the studies have shown that long-term PLY (i.e. 3–5 sessions a week for 5–12 months) represents an effective training method for enhancing bone mass in prepubertal/early pubertal children, young women and premenopausal women. Furthermore, short-term PLY (i.e. 2–3 sessions a week for 6–15 weeks) can change the stiffness of various elastic components of the muscle-tendon complex of plantar flexors in both athletes and non-athletes. Short-term PLY also improves the lower-extremity strength, power and stretch-shortening cycle (SSC) muscle function in healthy individuals. These adaptive changes in neuromuscular function are likely the result of (i) an increased neural drive to the agonist muscles; (ii) changes in the muscle activation strategies (i.e. improved intermuscular coordination); (iii) changes in the mechanical characteristics of the muscle-tendon complex of plantar flexors; (iv) changes in muscle size and/or architecture; and (v) changes in single-fibre mechanics. Our results also show that PLY, either alone or in combination with other training modalities, has the potential to (i) enhance a wide range of athletic performance (i.e. jumping, sprinting, agility and endurance performance) in children and young adults of both sexes; and (ii) to reduce the risk of lower-extremity injuries in female athletes. Finally, available evidence suggests that short-term PLY on non-rigid surfaces (i.e. aquatic- or sand-based PLY) could elicit similar increases in jumping and sprinting performance as traditional PLY, but with substantially less muscle soreness. Although many issues related to PLY remain to be resolved, the results of this review allow us to recommend the use of PLY as a safe and effective training modality for improving lower-extremity muscle function and functional performance of healthy individuals. For performance enhancement and injury prevention in competitive sports, we recommend an implementation of PLY into a well designed, sport-specific physical conditioning programme.

Plyometric training (PLY) is a very popular form of physical conditioning of healthy individuals and certain patient populations (e.g. osteoporotic patients). It involves performing bodyweight jumping-type exercises and throwing medicine balls using the so-called stretch-shortening cycle (SSC) muscle action. The SSC enhances the ability of the neural and musculotendinous systems to produce maximal force in the shortest amount of time, prompting the use of plyometric exercise as a bridge between strength and speed.[1] In this regard, PLY has been extensively used for augmenting dynamic athletic performance, particularly vertical jump ability.[2-4] Indeed, the vast majority of the earliest PLY studies examined the effects of SSC jumping programmes on vertical jump height.[5-12] Several other reviews on this topic have also been published.[2-4,13] ª 2010 Adis Data Information BV. All rights reserved.

However, the focus and application of PLY has evolved over the last 15 years. Specifically, PLY has been frequently used for improving human neuromuscular function in general,[14-16] as well as for improving performance in both explosive[9,17,18] and endurance athletic events.[19,20] Furthermore, a number of studies have shown that PLY (i) could improve biomechanical technique and neuromuscular control during high-impact activities like cutting and landing;[21-28] and (ii) has the potential for reducing the risk of lower-extremity injuries in team sports.[25,29-31] Finally, experimental evidence suggests that PLY appears to induce not only favourable neuromuscular, but also bone[32,33] and musculo-tendinous adaptation.[34,35] Our aim in this article is to critically review the available literature related to PLY and its effects on the human neural and musculoskeletal systems, Sports Med 2010; 40 (10)

Physiological Adaptation to Plyometric Training

athletic performance and injury prevention. Given that the vast majority of PLY studies focused on lower body, we reviewed only lower-body PLY that involved SSC jumping-type exercise. We also considered studies that combined lower-body PLY with other popular training modalities such as weight training (WT), endurance training, sprint training or electromyostimulation. 1. Search Strategy Computerized literature searches of articles published between January 1966 and April 2009 were performed with the use of MEDLINE, Scopus and SportDiscus databases. The following keywords were used in different combinations: ‘plyometric’, ‘pliometric’, ‘stretch-shortening cycle’, ‘drop jump’, ‘jump training’, ‘performance’, ‘muscle strength’, ‘muscle power’, ‘injury prevention’, ‘muscle-tendon’ and ‘bone mass’. All titles were scanned and the abstracts of any potentially relevant articles were retrieved for review. In addition, the reference lists from both original and review articles retrieved were also reviewed. The present literature review includes studies published in peer-reviewed journals that have presented original research data on healthy human subjects. Regarding training studies, we only considered PLY studies (and studies that combined PLY with other training modalities) which lasted ‡4 weeks. The size of the effect of PLY on each performance variable (i.e. muscle force or torque, muscle power, rate of force/torque development, vertical jump height, horizontal jump distance, sprint running performance, agility performance and endurance performance) is given either by the difference between the mean change in performance of subjects in the plyometric group and the control group (controlled trials), or by the difference between the mean change in performance of subjects in the plyometric group (singlegroup trials). To be able to compare the effects of PLY on different muscular and performance characteristics, we expressed the size of the effect either relative to the mean value of the control group (controlled trials), or relative to the mean pre-test value of the PLY group (single-group trails) – that is, in percentage values. ª 2010 Adis Data Information BV. All rights reserved.

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2. Plyometric Training (PLY) on Rigid Surfaces 2.1 Musculoskeletal Adaptation to PLY 2.1.1 Bone Adaptation to PLY

It is well established that physical exercise has a positive effect on bone mass. This is particularly evident for dynamic loading[36] of high magnitude, i.e. high strain rate.[37] Since plyometric jump training is associated with high ground reaction forces (up to 7 times bodyweight),[38] this type of exercise could be particularly suitable for increasing bone mass. Our literature search identified 18 studies that examined bone adaptation to PLY in humans (table I); 13 involved children or adolescents, two involved young adults and three involved pre- and/or post-menopausal women. Most studies incorporated PLY into either school- or home-based exercise programmes; only two studies combined PLY with WT. Training interventions in these studies mainly included 50–100 jumps per session, three to five sessions per week and lasted between 5 and 24 months, considerably longer than PLY interventions that are focused on performance enhancement (see sections 2.2 and 2.3). Twelve of 13 studies performed on children or adolescents reported significant positive effects of PLY on bone mass, with relative gains ranging from 1% to 8%. However, bone adaptation to mechanical loading in children is not homogenous but depends on the skeletal site and the maturity status of the participants. Specifically, positive effects of PLY on bone mass appear to be highest in early pubertal children, are somewhat lower in prepubertal children and are the lowest in pubertal children.[33,41,43,44] Furthermore, increases in bone mineral content and density tended to be greater at the femoral neck than at the lumbar spine, trochanter or proximal femur. Importantly, school-based jump training programmes not only increase bone mass in children, but also improve bone structure and strength.[44,49] Finally, recent longitudinal studies showed that PLY in early childhood has a persistent longterm effect over and above the effects of normal growth and development.[52,55] Sports Med 2010; 40 (10)

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Table I. Chronological summary of studies examining the effects of plyometric jump training on bone tissue adaptation Study

No. of subjects; design

Training protocol

Measures

Relative effectsa(%)

Bassey and Ramsdale[39]

27 pre-menopausal women: 14 underwent a high-impact training programme; RCT

CMJ training programme performed 5 d/wk for 6 mo

Femoral neck BMD Wards’s triangle BMD Trochanteric BMD Lumbar spine BMD

› fl › fl

Bassey et al.[40]

55 pre-menopausal women: 30 underwent a training intervention 123 post-menopausal women: 69 underwent a training intervention; RCTs

CMJ training programme (50 jumps) performed 6 d/wk for 6 mo (pre-menopausal) or 12 mo (post-menopausal)

Pre-menopausal: Femoral neck BMD Trochanteric BMD Lumbar spine BMD Post-menopausal: Femoral neck BMD Trochanteric BMD Lumbar spine BMD Post-menopausal (hormone replacement): Femoral neck BMD Trochanteric BMD Lumbar spine BMD

Witzke and Snow[33]

53 adolescent girls: 25 underwent a training intervention; non-RCT

Combined PLY and resistance training programme performed 3 ·/wk for 9 mo

Total body BMC Lumbar spine BMC Femoral neck BMC Trochanteric BMC Femoral mid-schaft BMC

Heinonen et al.[41]

58 pre-menarcheal girls: 25 underwent a training intervention 68 post-menarcheal girls: 64 underwent a training intervention; non-RCT

Combined aerobic step and jump training programme (100–200 jumps) performed 2 ·/wk for 9 mo

Pre-menarcheal girls: Lumbar spine BMC Femoral neck BMC Post-menarcheal girls: Lumbar spine BMC Femoral neck BMC

Fuchs et al.[42]

89 pre-pubescent children: (51 boys, 38 girls) 55 underwent a training intervention; RCT

Jump training programme (50–100 jumps) performed 3 ·/wk for 7 mo

Femoral neck BMC Femoral neck BMD Lumbar spine BMC Lumbar spine BMD

MacKelvie et al.[43]

70 pre-pubertal girls: 44 underwent a training intervention 107 early pubertal girls: 43 underwent a training intervention); RCT

Jump training programme (50–100 jumps) performed 3 ·/wk for 7 mo

Pre-pubertal girls: Total body BMC Total body BMD Lumbar spine BMC Lumbar spine BMD Femoral neck BMC Femoral neck BMD Trochanteric BMC Trochanteric BMD Proximal femur BMC Proximal femur BMD Early pubertal girls: Total body BMC Total body BMD Lumbar spine BMC Lumbar spine BMD Femoral neck BMC Femoral neck BMD Trochanteric BMC Trochanteric BMD Proximal femur BMC Proximal femur BMD

2.1 0.3 2.9b 0.3

› 1.6 › 2.6b fl 0.8 fl 1.1 fl 0.4 fl 0.1 › 0.2 fl 0.5 fl 0.3 fl › › fl ›

0.4 0.9 1.4 0.4 0.9

› 3.3b › 4.0b › 1.1 › 0.2 › 4.9b › 1.2 › 3.4b › 2.0b 20 20 › 0.4 › 0.2 20 › 0.2 fl 0.6 fl 0.2 fl 0.9 fl 0.6 › › › › › › › fl › ›

4.3 0.3 1.8b 1.9 3.8b 2.7b 0.3 0.2 1.3 0.8

Continued next page

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Table I. Contd Study

No. of subjects; design

Training protocol

Measures

Relative effectsa(%)

Petit et al.[44]

70 pre-pubertal girls: 44 underwent a training intervention 107 early pubertal girls: 43 underwent a training intervention; RCT

Jump training programme (50–100 jumps) performed 3 ·/wk for 7 mo

Pre-pubertal girls: Femoral neck BMD Femoral neck BA Intertrochanter BMD Intertrochanter BA Femoral shaft BMD Femoral shaft BA Early pubertal girls: Femoral neck BMD Femoral neck BA Intertrochanter BMD Intertrochanter BA Femoral shaft BMD Femoral shaft BA

fl fl fl fl fl fl

0.6 1.0 0.5 0.2 0.8 1.0

› › › › › ›

2.7b 0.6b 1.8b 1.2 0.4 0.3

MacKelvie et al.[45]

121 pre-pubertal boys: 61 underwent a training intervention; RCT

Jump training programme (50–100 jumps) performed 3 ·/wk for 7 mo

Total body BMC Lumbar spine BMC Lumbar spine BMD Femoral neck BMC Femoral neck BMD Trochanteric BMC Trochanteric BMD Proximal femur BMC Proximal femur BMD

› 1.5b › 1.3 › 0.7 20 › 0.2 20 › 1.3 › 1.2 › 1.1b

Johannsen et al.[46]

54 children (age: 3–18 y; 31 girls): 28 underwent a training intervention; RCT

Jump training (25 jumps) performed 5 ·/wk for 12 wk

Total body BMC Legs BMC Spine BMC Spine BMD Femoral neck BMC Femoral neck BMD Distal tibia BMC Distal tibia BMD

› 1.1b › 1.7b 20 › 0.6 › 1.5 › 1.2 fl 1.3 fl 1.5

Iuliano-Burns et al.[47]

36 pre-pubertal and early pubertal girls: 18 underwent a training intervention; RCT

Jump training performed 3 ·/wk for 8.5 mo

Total body BMC Lumbar spine BMC Femur BMC Tibia/fibula BMC

› fl fl ›

1.4 2.5 1.5 2.0b

MacKelvie et al.[48]

75 girls (age: 9.9 y): 32 underwent a training intervention; RCT

Jump training programme (50–132 jumps) performed 3 ·/wk for 20 mo

Total body BMC Lumbar spine BMC Femoral neck BMC Trochanteric BMC Proximal femur BMC

› › › fl ›

2.3 6.0b 3.9b 3.1 0.6

MacKelvie et al.[49]

64 pre-pubertal or early pubertal boys: 31 underwent a training intervention; RCT

Jump training programme (50–132 jumps) performed 3 ·/wk for 20 mo

Total body BMC Lumbar spine BMC Femoral neck BMC Trochanteric BMC Proximal femur BMC

› › › fl ›

1.7 2.0 3.9b 3.1 4.3

Vainionpa¨a¨ et al.[50]

80 pre-menopausal women: 39 underwent a training intervention; RCT

Jump training combined with walking, running and stamping performed 3 ·/wk for 12 mo

Lumbar spine BMD Femoral neck BMD Trochanter BMD Intertrochanter BMD Ward’s triangle BMD

› › › › ›

0.3 1.4b 0.9 1.0b 1.7

Continued next page

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Sports Med 2010; 40 (10)

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Table I. Contd Study

No. of subjects; design

Training protocol

Measures

Relative effectsa(%)

McKay et al.[51]

124 children (age: 10.1 y): 51 (23 boys and 28 girls) underwent a training intervention; non-RCT

CMJ training (10 jumps) performed 3 ·/wk for 8 mo

Total body BMC Total body BA Lumbar spine BMC Lumbar spine BA Proximal femur BMD Proximal femur BA Intertrochanter BMC Intertrochanter BA Trochanter BMC Trochanter BA Femoral neck BMC Femoral neck Area

fl fl fl fl › › › › › › fl fl

1.3b 1.5b 0.8 0.3 2.6b 1.3 2.9b 2.2 1.9 0.6 0.2 0.3

Kato et al.[32]

36 female college students (age: 20.7 y): 18 underwent a training programme; RCT

CMJ training (10 jumps) performed 3 ·/wk for 6 mo

Lumbar spine BMD Proximal femur BMD Femoral neck BMD Ward’s triangle BMD Trochanter BMD

› › › › ›

1.7b 1.8 3.6b 2.6 1.5

Gunter et al.[52]

199 children (94 boys, 105 girls): 101 underwent a training intervention; RCT

Jump training (~100 jumps) performed 3 ·/wk for 7 mo

Total body BMC Lumbar spine BMC Femoral neck BMC Trochanter BMC

› › › ›

7.3b 7.9b 7.7b 8.4b

Weeks et al.[53]

81 adolescents (37 boys, 44 girls): 43 underwent a training intervention; RCT

Jump training (~300 jumps) performed 2 ·/wk for 8 mo

Boys: Total body BMC Femoral neck BMC Femoral neck BA Trochanter BMC Lumbar spine BMC Lumbar spine BA Girls: Total body BMC Femoral neck BMC Femoral neck BA Trochanter BMC Lumbar spine BMC Lumbar spine BA

› › › › › ›

4.2b 2.1 1.1 6.7b 3.6b 1.7

› › › › fl ›

1.9 7.8b 0.3 6.9 1.9 1.7

Guadalupe-Grau et al.[54]

66 physical education students (43 males, 23 females): 28 underwent a training intervention; RCT

PLY (40–70 jumps) combined with WT performed 3 ·/wk for 9 wk

Men: Total body BMC Total body BMD Lumbar spine BMC Lumbar spine BMD Lower limbs BMC Lower limbs BMD Femoral neck BMC Femoral neck BMD Ward’s triangle BMD Trochanter BMD Intertrochanter BMD Women: Total body BMC Total body BMD Lumbar spine BMC Lumbar spine BMD Lower limbs BMC

› 0.3 › 0.8 › 1.9 › 0.9 20 20 › 1.5b fl 2.8 › 1.0 fl 2.2 20 › 1.0 fl 0.9 › 0.7 20 › 0.6

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865

Table I. Contd Study

No. of subjects; design

Training protocol

a

[(Post-training – pre-training) – (post-control – pre-control)]/pre-control.

b

Significantly (p < 0.05) greater increase in the exercise vs control group.

Measures

Relative effectsa(%)

Lower limbs BMD Femoral neck BMC Femoral neck BMD Ward’s triangle BMD Trochanter BMD Intertrochanter BMD

20 › 4.2b fl 1.0 fl 2.3 20 20

BA = bone area; BMC = bone mineral content; BMD = bone mineral density; CMJ = countermovement jump; PLY = plyometric training; RCT = randomized controlled trial; WT = weight training; ·/wk = sessions times per week; › indicates increase in performance; fl indicates decrease in performance; 2 indicates no change in performance.

Bone adaptation to PLY in adults has been much less studied (table I). The available data suggest that PLY effects on bone mass in women are age specific. More precisely, significant positive gains in bone mass (1–4%) following PLY have been observed in young and pre-menopausal women, but not in post-menopausal women.[32,39,40,50,54,56] Taken together, these results suggest that PLY, performed three to five times a week over 5–24 months, represents an effective training method for enhancing bone mass in prepubertal and early pubertal children, young women and premenopausal women. More studies are needed to test the effectiveness of PLY on bone mass in other populations (e.g. athletes and the elderly). 2.1.2 Muscle-Tendon Complex and Joint Adaptations to PLY

In SSC movements, the elastic behaviour of muscles, ligaments and tendons plays a decisive role.[57-59] In that regard, the importance of stiffness characteristics of the muscle-tendon complex in SSC exercise performance has been particularly stressed in scientific literature. Indeed, many authors have suggested that a stiff muscle-tendon complex is optimal for performance of SSC activities since it allows a rapid and more efficient transmission of muscle force to skeleton and, consequently, higher rates of force development.[60-63] However, a number of cross-sectional studies have proven otherwise by showing that the stiffness of the muscle-tendon complex correlates negatively to the augmentation of performance in concentric motion during SSC exercises.[64-68] Furtherª 2010 Adis Data Information BV. All rights reserved.

more, Stafilidis and Arampatzis[69] recently showed that faster sprinters have significantly lower stiffness of vastus lateralis tendon and aponeurosis compared with slower sprinters. The authors also reported that maximum elongation of vastus lateralis tendon and aponeurosis (i.e. lower stiffness) was significantly correlated (r = -0.57) with 100 m sprint performance time. Finally, Wilson et al.[70] have observed that flexibility training increased performance in upper-body SSC exercise with a reduction in the muscle-tendon complex stiffness. The authors suggested that a more compliant muscle-tendon unit can store and release more elastic energy, which in turn could improve SSC performance. A more compliant muscletendon unit could also improve SSC performance by allowing the muscle fibres to operate at a more optimal length over the first part of their shortening range. Collectively, these findings suggest that a more compliant muscle-tendon complex could be advantageous for SSC performance and that training could change the elastic behaviour of joint sub-components. In that regard, our literature review revealed several human studies that examined the effects of short-term (6–15 weeks) PLY on stiffness of various anatomical structures and/or their combinations as follows: joint stiffness,[34] musculo-articular stiffness[71,72] or the stiffness of particular elastic components within the Hill’s three-component model – parallel elastic component (i.e. passive muscles),[72,73] serial elastic component[19,35,74] or just passive part of the serial elastic component (i.e. tendons).[34,72,75,76] For example, Kubo et al.[34] Sports Med 2010; 40 (10)

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reported an increase of 63.4% in ankle joint stiffness assessed during drop jumps (DJs) with no significant changes in Achilles tendon stiffness following 12 weeks of PLY. Notably, the authors also observed that PLY significantly increased (i) the maximal Achilles tendon elongation and the amount of stored elastic energy; and (ii) the SSCtype jumping performance. No change in Achilles tendon stiffness and a significant increase in the SSC-type jumping performance following 8 weeks of PLY was also observed by Foure et al.[72] In addition, Wu et al.[76] recently reported a significant increase in jump performance and Achilles tendon elastic energy storage and release following 8 weeks of PLY; however, the authors also reported a significant increase in Achilles tendon stiffness following PLY intervention.[76] Similarly, Burgess et al.[75] also reported that 6 weeks of PLY significantly increased the Achilles tendon stiffness by 29% in young adults, together with a significant increase in concentric-only explosive muscular performance. Furthermore, several research groups focused on the entire serial elastic component of plantar flexor muscles and observed either a significant increase[19,74] or a decrease[35] in its stiffness following PLY. Interestingly, the two studies that reported conflicting findings regarding PLY effects on the serial elastic component stiffness also reported significant increases in the same SSC jump performance.[19,35] Two studies from the same research group focused on the musculo-articular stiffness of the ankle joint and showed either significant increase,[71] or no change[72] in musculoarticular stiffness of the ankle joint following PLY. Notably, in these two experiments the authors used different techniques for determination of the global musculo-articular stiffness. Finally, Malisoux et al.[73] observed that PLY induced increases in passive stiffness of fast-twitch muscle fibres, and Foure et al.[72] reported a significant increase in the passive stiffness of the gastrocnemii (i.e. predominantly fast-twitch muscle) after 8 weeks of PLY. Overall, these studies showed that PLY has the potential to change the various elastic components of the muscle-tendon complex. However, the cited studies provided conflicting findings that ª 2010 Adis Data Information BV. All rights reserved.

are difficult to interpret, particularly if we take into account the complexity of the relationships between the elastic properties at different anatomical levels[66,77] and methodological limitations of certain approaches in studying stiffness of biological tissues.[78] The recently reported results by Foure et al.[72] shed some light on this complex issue by showing that 8 weeks of PLY induced a significant relative increase of 33% in the passive stiffness of the gastrocnemii without changes in the Achilles tendon stiffness or global passive musculo-articular stiffness of the ankle joint. As a possible explanation of the results, the authors put forward a hypothesis that the muscle-tendon complex of gastrocnemii (bi-articular muscle) and soleus (mono-articular muscle) may have a different response to PLY. Further studies are needed to test this hypothesis, as well as to focus on the specific effects of PLY on particular elastic components of the muscle-tendon complex, and the overall joint behaviour during SSC movements. 2.2 Neuromuscular Adaptations to PLY 2.2.1 Muscle Fibre Type

Several animal studies have shown that PLY could induce fibre type transition in trained muscles. Specifically, in the soleus muscle of a rat, PLY induces a significant relative increase in type II fibres.[79-82] In humans, only three studies examined the muscle fibre transition as a result of PLY.[83-85] Similar to the results of animal studies, Malisoux et al.[83] also found a significant increase in the proportion of type IIa fibres of the vastus lateralis muscle. In contrast, Kyrolainen et al.[84] and Potteiger et al.[85] did not observe any significant changes in fibre-type composition of the lateral gastrocnemius and vastus lateralis muscles, respectively. When PLY was combined with WT, Perez-Gomez et al.[86] observed a significant increase in percentage of type IIa fibres in vastus lateralis, whereas Hakkinen and coworkers[16,87] found no changes in fibre composition. Combination of PLY with endurance training also had no effect on fibre composition of vastus lateralis muscle.[85] Collectively, the results of a limited number of human studies are inconclusive regarding the effects of PLY on Sports Med 2010; 40 (10)

Physiological Adaptation to Plyometric Training

human muscle fibre-type composition. When taking into account the results of animal studies, it is possible that PLY-induced fibre-type transition in leg extensor muscles could be muscle specific. Future studies should test this hypothesis. 2.2.2 Whole Muscle and Single Fibre Contractile Performance

Numerous previous studies examined the effects of various training paradigms such as resistance training, endurance training and sprint training on whole muscle or single fibre contractile performance.[88-91] Surprisingly, however, we found only three studies published in peer-reviewed journals that examined the effects of PLY on human muscle contractile performance.[34,35,73] Grosset et al.[35] recently showed that 10 weeks of PLY increased twitch peak torque and rate of torque development in the gastrocnemius muscle. The authors also observed a slight decrease in contraction time. In another study, Kubo et al.[34] observed that 12 weeks of PLY significantly decreased plantar flexors contraction time, with no changes in twitch peak torque and rate of torque development. These data generally suggest that PLY can increase the contractility of plantar flexor muscles. Malisoux et al.,[73] on the other hand, focused on the contractile properties of single fibres of vastus lateralis muscle and reported that 8 weeks of PLY induced significant increases in peak force and maximal shortening velocity in type I, IIa and hybrid IIa/IIx fibres, while peak power increased significantly in all fibre types. Note that these changes in a single fibre function were accompanied by significant improvements in the whole muscle strength and power. The latter results are particularly important since they suggest that PLY-induced improvements in muscle function and athletic performance could be partly explained by changes in the contractile apparatus of the muscle fibres, at least in knee extensor muscles. Further studies are needed to examine whether PLY induces similar adaptive changes in single fibres of plantar flexors. 2.2.3 Whole Muscle and Single Fibre Hypertrophy

The effects of strength and endurance training on human muscle and/or fibre size are well docuª 2010 Adis Data Information BV. All rights reserved.

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mented in the literature. Regarding PLY effects on human muscle size, we found one study that focused on the whole muscle[34] and three studies that focused on single muscle fibres.[73,84,85] Kubo et al.[34] used the MRI technique and showed that 12 weeks of PLY induced a significant increase in plantar flexor muscle volume (~5%), and this effect was similar to the effect induced by WT of similar duration. Furthermore, Malisoux et al.[73] reported significant increases in a cross-sectional area of type I (+23%), type IIa (+22%) and type IIa/IIx fibres (+30%) in vastus lateralis muscle following 8 weeks of PLY. Potteiger et al.[85] also reported significant increases in a type I and type II fibre cross-sectional area of the vastus lateralis muscle, but these effects were of smaller magnitude (+6–8%). In contrast, Kyrolainen and co-workers[84] observed no changes in a fibre cross-sectional area of gastrocnemius muscle following 15 weeks of PLY. When PLY was combined with WT, Hakkinen et al.[87] observed no changes in a fibre crosssectional area of the vastus lateralis muscle in women. However, a similar training protocol did induce a significant increase (~20%) in the mean area of fast-twitch fibres in men.[16] Furthermore, Perez-Gomez et al.[86] reported that combined PLY and WT increased lower-limb lean mass (+4.3%), as determined by dual energy x-ray absorptiometry. Finally, an 8-week combined PLY and endurance training also resulted in a significant fibre hypertrophy (~6–7%) in vastus lateralis muscle.[85] Overall, these data suggest that short-term PLY, alone or in combination with WT, has the potential to induce a moderate hypertrophy of both type I and type II muscle fibres; however, these effects (i) are generally lower compared with those induced by WT; and (ii) appear to be more pronounced in knee extensors than in plantar flexors. 2.2.4 Muscle Geometry

It is well known that a muscle’s geometry strongly influences its force and power output and that it can be changed with WT.[92] To our knowledge, only one study examined muscle architectural adaptations to PLY, and it was combined with sprint training.[93] The authors showed that 5 weeks of combined PLY and sprint training Sports Med 2010; 40 (10)

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intervention decreased fascicle angle and increased fascicle length in knee extensor muscles. Differential muscle architectural adaptations were observed when WT was added to PLY and sprint training; however, both training groups improved athletic performance to a similar extent.[93] Obviously, more studies are needed before any firm conclusions can be drawn regarding PLY effects on muscle geometry. 2.2.5 Neural Adaptation

The neural control, including central and peripheral components, plays a key role in force potentiation during the SCC-type exercises. Of particular importance are muscle activation prior to the ground impact (pre-activation) and reflex facilitation during the late eccentric and early concentric phase.[94] Thus, it is reasonable to assume that PLY-induced changes in human muscle function and performance have a neural origin. Our literature search revealed six PLY studies[28,34,76,84,95,96] and three combined PLY and WT studies[87,97,98] that focused on neural adaptation. Notably, most research groups used only surface electromyography (EMG) during maximal voluntary contractions (MVC) or during vertical jumps to detect changes in muscle activity following an intervention. Regarding PLY, several studies focused on changes in leg muscle activation during vertical jumping and provided conflicting findings. Chimera et al.[28] reported that adductor muscle pre-activation and adductor and abductor coactivation both increased after PLY during DJ performance. No changes in the EMG activity of quadriceps and hamstrings muscles were observed. Kyrolainen and co-workers[95,96] showed that leg muscle activity patterns during DJ did not change following an intervention; however, in one of these studies the authors did observe a significant increase in the pre-activity of leg extensors during DJ performance.[95] Kubo et al.[34] observed no changes in plantar flexor muscles activity during pre-landing and eccentric phases of vertical jumps following PLY. However, they reported a significant increase in plantar flexor muscles activity during the concentric phase of all studied vertical jumps. Moreover, using the ª 2010 Adis Data Information BV. All rights reserved.

twitch interpolation technique, these authors also assessed the activation level of plantar flexors prior to and after PLY, and reported a significant increase in both MVC (+17.3%) and activation level (+5.6%) of plantar flexor muscles. Wu et al.[76] used another technique – root mean square EMG – that was normalized to the respective Mwave, and showed that soleus (but not gastrocnemius) normalized EMG increased significantly after PLY, without any change in maximal M-wave amplitude. Furthermore, Kyrolainen et al.[84] reported that PLY significantly increased both MVC and muscular activity of plantar flexors, but not of knee extensors. Finally, there is limited evidence from both human[99] and animal[82] experiments that PLY may change the stretch reflex excitability. These findings suggest that neuromuscular adaptation to PLY is not only limited to the motor pathways to the muscle, but also concerns its sensory part. Regarding studies that combined PLY with WT, all of them reported significant training-induced increases in leg extensor muscle activity during either maximal isometric contractions[16,87] or during vertical jump performance.[97,98] Taken together, the reviewed studies generally suggest that PLY alone can increase MVC and voluntary activation of plantar flexors. This enhanced voluntary activity of plantar flexors could be accounted for by an increase in motor unit recruitment or discharge rate,[76,100] both mediated by changes in descending cortical outflow. Other possible aspects of neural adaptation to PLY include (i) changes in leg muscle activation strategies (or inter-muscular coordination) during vertical jumping, particularly during the preparatory (i.e. pre-landing) jump phase; and (ii) changes in the stretch reflex excitability. When PLY was combined with WT, a greater potential for increasing the EMG activity of leg extensors was observed compared with when PLY was the only training modality. However, one should use considerable caution in interpreting the EMG amplitude following training, as changes in EMG amplitude can be attributed to alterations in central neural drive, muscle factors such as muscle hypertrophy or a variety of technical factors not reflective of physiological Sports Med 2010; 40 (10)

Physiological Adaptation to Plyometric Training

changes.[101] Although some of these problems can be overcome by using EMG normalization procedures, single motor unit recording techniques and measurements of evoked reflex responses (Hoffman reflex, F-wave – an electrophysiological variant of the Hoffman reflex),[56] these methods have rarely been used in human PLY studies. Therefore, our current knowledge about PLY-induced changes in neural function is limited. 2.2.6 Muscle Strength and Power

Numerous previous studies have examined the effects of short-term PLY on the strength and power of lower-extremity muscles (table II) and have reported variable results. Specifically, relative changes in maximal strength of lower-extremity muscles induced by PLY ranged from +3.2% to +45.1%; however, most (i.e. 12 of 25) studies reported positive effects and these were mainly ‡10%. For ‘explosive’ muscle strength or rate of force/torque development, these relative effects were more variable (range -22.3% to +33.0%; table II). Still, most (i.e. 8 of 10) studies did observe a relative increase in ‘explosive’ muscle strength following a PLY intervention. Finally, PLY produced a relative increase in muscle power in 13 of 16 studies, and these positive effects ranged between +2.4% and +31.3%. Importantly, positive strength and power gains as a result of PLY were observed in both athletes and non-athletes, and in both males and females. A recent metaanalytical review supports this conclusion by showing that PLY significantly improves strength performance and that PLY gains are independent of the fitness level or sex of the subject.[127] Although numerous studies examined the effects of PLY on muscle strength and power, only four studies actually focused on the possible neuromuscular mechanisms behind these effects. Kyrolainen et al.[84] showed that 15 weeks of PLY improves the strength of plantar flexors but not the rate of force development, and these changes were accompanied by a significant increase in muscle activity without any changes in musclefibre distributions and areas. The authors found no change in maximal strength and muscle activation for knee extensor muscles but reported a ª 2010 Adis Data Information BV. All rights reserved.

869

significant increase in the rate of force development. In contrast, Kubo et al.[34] showed that PLY-induced changes in plantar flexors strength were accompanied by both significant hypertrophy and an increase in the activation level of those muscles. Furthermore, Potteiger et al.[85] showed that PLY increased leg extensors muscle power (+3–5%), and these changes were accompanied by a significant increase in the cross-sectional area of vastus lateralis type I (+4.4%) and type II (+7.8%) muscle fibres. Finally, Malisoux et al.[73] showed that PLY significantly increased leg extensors strength and power by +12–13%, and these changes in performance were accompanied by significant increases in single-fibre diameter, peak force, shortening velocity and power. Collectively, these data, together with the data presented in previous sections (see sections 2.2.1– 2.2.5), suggest that increases in muscle strength and power after PLY could have both a neural and muscular origin. Note, however, that some of these changes could be different from the changes induced by other resistance training modalities, namely (i) changes in muscle architecture (i.e. a decrease in fascicle angle and an increase in fascicle length of knee extensors[93]); (ii) changes in the stiffness of various elastic components of the muscle-tendon complex of plantar flexors;[35,66,71,72] and (iii) changes in single fibre mechanics of knee extensors (i.e. enhanced force, velocity and, consequently, power of slow and fast muscle fibres[90]). When PLY is combined with WT, its potential for augmenting human muscle strength and power is further increased (table III). Indeed, all studies that compared PLY with combined PLY and WT reported significantly greater relative changes in muscle strength and power after combined PLY and WT.[10,15,102] This conclusion is further supported by the results of a recent meta-analytical review that showed significantly higher strength gains after combined PLY and WT compared with after PLY alone.[127] The relative increase in maximal strength and power after combined PLY and WT is present in all published studies and it ranges from +5–43%, and from +2–37%, respectively (table III). Limited data exist regarding the effects of combined PLY and WT on the rate of force/torque development (table III). Sports Med 2010; 40 (10)

870

ª 2010 Adis Data Information BV. All rights reserved.

Table II. Chronological summary of studies examining the effects of plyometric training (PLY) on skeletal muscle function and athletic performance Study

No. of subjects; sex; fitness level; control group

PLY

Relative effects (%)

intervention exercise (wk/sessions)

maximal strength (performance variable)

explosive strength

muscle power

jumping performance (jump type)

Blattner and Noble[5]

26; M; N-A; yes

DJT (8/24)

Dvir[8]

16; M; N-A; yes

DJT (8/24)

› 6.4

CMJT (8/24)

› 5.7

Hakkinen and Komi[97]

10; M; N-A; no

COMB (24/72)

› › › › ›

Brown et al.[9]

26; M; A; yes

DJT (12/34)

› 5.0 (CMJ) › 6.0 (CMJA)

Hortobagyi et al.[12]

25; M; N-A; yes

COMB (10/20)

› › › ›

Bauer et al.[102]

8 NS; N-A; no

COMB (10/30)

› 15.1 (F/T) › 5.7 (F/T) › 7.1 (F/T)

14; F; N-A; yes

COMB (16/48)

› 27.5 (F/T)

Hortobagyi et al.

19; M; N-A; yes

COMB (10/30)

fl 3.2 (F/T)

Wilson et al.[103]

27; M; N-A; yes

DJT (5/10)

› 3.3 (F/T)

DJT (10/20)

› 0.2 (F/T)

Hakkinen et al.[87] [11]

Holcomb et al.[104]

19; M; N-A; yes

sprinting performance (distance, m [yd]a)

agility performance

endurance performance (measure)

› 8.5 (CMJA) › 13.0 (CMJA) › 6.9 (CMJA)

21.2 (SJ) 17.6 (CMJ) 25.0 (DJ) 26.8 (DJ) 32.4 (DJ)

6.1 (CMJA) 12.1 (CMJA) 2.9 (HJ) 1.4 (HJ)

› 5.5 (CMJA)

› 8.2 fl 0.6 (30)

› 1.1

› 6.7 (SJ) › 7.8 (CMJ)

› 1.1 (30)

CMJ (8/24)

fl 0.9 › 7.2

› 3.3 (SJ) › 6.7 (CMJ)

DJT (8/24)

› 4.6 › 10.2

› 7.3 (SJ) › 9.4 (CMJ)

DJT (8/24)

› 3.1 › 7.7

› 6.4 (SJ) › 6.9 (CMJ)

› 2.4

Continued next page

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Sports Med 2010; 40 (10)

› 3.9 (HJ) › 2.7 (HJ)

Study

No. of subjects; sex; fitness level; control group

PLY

Relative effects (%)

intervention exercise (wk/sessions)

maximal strength (performance variable)

explosive strength

fl 6.9

muscle power

jumping performance (jump type)

sprinting performance (distance, m [yd]a)

Wilson et al.[105]

27; M N-A; yes

DJT (8/16)

fl 2.4 (1RM)

Hewett et al.[25]

11; F; A; no

COMB (6/18)

› 12.2 (F/T) › 24.3 (F/T)

Cornu et al.[71]

19; M; N-A; yes

COMB (7/14)

› 14.3 (F/T)

Wagner and Kocak[106]

40; M; A; yes

COMB (6/12)

› 23.2

› 2.2 (CMJA)

› 1.7 (50)

40; M; N-A; yes

COMB (6/12)

› 19.8

› 2.7 (CMJA)

› 1.3 (50)

10; M: 11; F; N-A; yes

DJT (12/24)

9; M: 8; F; N-A; yes

CMJT (12/24)

Gehri et al.

[107]

agility performance

endurance performance (measure)

› 12.2 (CMJ) › 43.6 › 22.3

› 10.8 (SJ) › 10.8 (CMJ)

Physiological Adaptation to Plyometric Training

ª 2010 Adis Data Information BV. All rights reserved.

Table II. Contd

› 10.1 (DJ) › 10.8 (SJ) › 9.0 (CMJ) › 8.6 (DJ) Young et al.[108]

14; M; N-A; yes

DJT (6/18)

› 6.1 (F/T)

fl 1.7 (SJ) › 4.3 (CMJA) › 9.0 (DJ)

20; M; N-A; yes

DJT (6/18)

› 0.8 (F/T)

fl 3.7 (SJ) › 1.6 (CMJA) › 7.4 (DJ)

Potteiger et al.

[85]

COMB (8/24)

Paavolainen et al.[109]

18; M; A; yes

COMB (9/0)

› 20.4 (F/T)

Fatouros et al.[15]

21; M; N-A; yes

COMB (12/36)

› 8.2 (1RM) › 11.4 (1RM)

Rimmer and Sleivert[110]

17; M; A; yes

COMB (8/15)

› 2.9 › 5.8

› 6.0 (HJ)

› 25.9

. › 13.8 (VO2max)

› 4.6 (CMJA)

› 5.7 (20)

. fl 5.8 (VO2max) › 0.8 (LT)

› 10.3 (CMJA)

› › › ›

2.2 (40) 1.8 (30) 1.6 (20) 2.6 (10) Continued next page

871

Sports Med 2010; 40 (10)

8; M; N-A; no

872

ª 2010 Adis Data Information BV. All rights reserved.

Table II. Contd Study

No. of subjects; sex; fitness level; control group

PLY

Relative effects (%)

intervention exercise (wk/sessions)

maximal strength (performance variable)

explosive strength

muscle power

jumping performance (jump type)

› 16.6

› 14.3 (SJ) › 20.0 (CMJ)

Diallo et al.[111]

20; M; A; yes

COMB (10/30)

Matavulj et al.[17]

22; M; A; yes

DJT (6/18)

› › › fl

11.5 (F/T) 10.7 (F/T) 2.0 (F/T) 1.8 (F/T)

Miller et al.[112]

27 NS; N-A; yes

COMB (8/16)

› › › fl › ›

2.2 (F/T) 0.9 (F/T) 7.9 (F/T) 1.7 (F/T) 2.8 (F/T) 2.9 (F/T)

Spurrs et al.[19]

17; M; A; yes

COMB (6/15)

› 13.3 (F/T) › 15.4 (F/T)

Turner et al.[113]

8; M: 10; F; A; yes

COMB (6/18)

Luebbers et al.[114]

19; M; N-A; no

COMB (4/12)

fl 1.4 › 3.7

fl 3.5 (CMJA)

COMB (7/21)

› 0.3 › 6.3

fl 0.3 (CMJA)

COMB (6/18)

fl 5.2

› 2.9 (CMJ)

20; F; N-A; yes

Chimera et al.[28]

16; F; A; yes

COMB (6/12)

Irmischer et al.[26]

28; F; N-A; yes

COMB (9/18)

Robinson et al.[116]

16; F; N-A; no

COMB (8/24)

Kato et al.[32]

36; F; N-A; yes

CMJT (24/60)

6.6 3.0 22.7 18.2

› 31.3 › 21.0

fl 0.2 (CMJA)

› 18.2 (CMJ) › 7.0 (HJ)

7.8 (RE) 6.4 (RE) 5.1 (RE) 1.2 (ERPT) . 3.1 (VO2max) . fl 0.4 (VO2max)

› › › › fl

2 0.0 (SJ) › 4.8 (CMJ)

› 3.7 (DJ)

› › › ›

25.2 (F/T) 45.1 (F/T) 44.5 (F/T) 24.3 (F/T)

endurance performance (measure)

› 15.6 (CMJ) › 13.8 (CMJ)

› 0.4

› 15.4

agility performance

fl 0.3 (37 [40 yd])

› 5.7 (CMJA) › 32.5 (CMJA)

› 6.2 (40)

› 5.6 (CMJ) Continued next page

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Sports Med 2010; 40 (10)

Canavan and Vescovi[115]

fl fl fl fl

sprinting performance (distance, m [yd]a)

Study

No. of subjects; sex; fitness level; control group

PLY

Relative effects (%)

intervention exercise (wk/sessions)

maximal strength (performance variable)

explosive strength

› 16.0 (F/T) › 4.2 (F/T)

› 17.6

muscle power

jumping performance (jump type)

23; M; N-A; yes

COMB (15/30)

Lehance et al.[117]

20; M; N-A; yes

DJT (6/12)

Tricoli et al.[118]

15; M; N-A; yes

COMB (8/24)

Herrero et al.[119]

19; M; N-A; yes

COMB (4/8)

Kotzamanidis[120]

30; M; N-A; yes

COMB (10/20)

Miller et al.[121]

19; M: 9; F; N-A; yes

COMB (6/12)

Malisoux et al.[73]

8; M; N-A; no

COMB (8/24)

› 11.2 (1RM)

› 7.5 (SJ) › 14.6 (CMJ)

Myer et al.[23]

8; F; A; no

COMB (7/21)

› 24.5 (F/T) › 18.0 (F/T)

› 5.4 (CMJA)

Saunders et al.[20]

15; M; A; yes

COMB (9/25)

Markovic et al.[14]

63; M; N-A; yes

COMB (10/30)

Stemm and Jacobson[122]

17; M; N-A; yes

COMB (6/12)

Kubo et al.[34]

10; M; N-A; no

COMB (12/48)

Burgess et al.[75]

»7; M; N-A; no

DJ (6/»15)

endurance performance (measure)

› 31.8 (DJ) fl 5.1

fl 0.3 (F/T)

agility performance

› 17.8 (CMJ) › 15.8 (CMJA) › 25.4 (DJ)

› 1.6 (10)

› 3.6 (SJ) › 4.5 (CMJ)

› 1.4 (30) › 2.1 (10)

fl 3.8 (SJ) fl 0.3 (CMJ)

fl 0.3 (20)

› 39.3 (SJ)

› 3.0 (30) › 3.7 (20) › 2.6 (10)

› 2.0

› 5.5 › 3.0

› 14.2

› 2.5 (F/T)

› 3.6

› 8.0 (5JT)

› 7.1 (SJ) › 6.4 (CMJ) › 2.4 (HJ)

› 2.4 (RE) › 4.8 (RE) . fl 2.3 (VO2max) › 0.9 (20)

› 2.0

› 7.2 (CMJA) › 13.3 (F/T)

› 28.5 (SLSJ) › 35.3 (SLCMJ) › 42.0 (SLDJ) › 19.0 › 12.0

› 58.0 (USLCJ)

Continued next page

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Sports Med 2010; 40 (10)

Kyrolainen et al.[84]

sprinting performance (distance, m [yd]a)

Physiological Adaptation to Plyometric Training

ª 2010 Adis Data Information BV. All rights reserved.

Table II. Contd

874

ª 2010 Adis Data Information BV. All rights reserved.

Table II. Contd Study

No. of subjects; sex; fitness level; control group

PLY

Relative effects (%)

intervention exercise (wk/sessions)

maximal strength (performance variable)

Dodd and Alvar[123]

28; M; A; no

COMB (4/7)

de Villarreal et al.[124]

20; M; N-A; yes

DJT (7/7)

22; M; N-A; yes

explosive strength

muscle power

fl 2.9 (F/T) › 1.6 (1RM)

jumping performance (jump type)

sprinting performance (distance, m [yd]a)

agility performance

› 1.9 (CMJA)

› 0.1 (18 [20 yd]) fl 1.3 (37 [40 yd]) › 0.3 (55 [60 yd])

2 0.0

› 1.1 (CMJ) › 0.3 (DJ)

› 0.8 (20)

fl 1.4 (DJ) › 2.6 (DJ)

› 3.2 (20)

DJT (7/28)

› 14.9 (F/T) › 13.1 (1RM)

› 12.8 (DJ)

DJT (7/14)

› 11.5 (F/T)

› 16.0 (DJ)

endurance performance (measure)

› 19.3 (CMJ)

› 2.4 (1RM)

› 20.2 (DJ)

› 0.8 (20)

› 14.4 (CMJ) › 8.5 (DJ) › 5.1 (DJ) › 11.1 (DJ) Salonikidis and Zafeiridis[18]

32; M; A; yes

COMB (9/27)

Vescovi et al.[125]

18; F; N-A; yes

COMB (6/18)

Grosset et al.[35]

6; M: 3; F; N-A; no

COMB (10/20)

Potach et al.[126]

4; M: 12; F; N-A; yes

COMB (4/8)

Foure et al.[72]

17; M; N-A; yes

COMB (8/16)

Wu et al.[76]

21; M; N-A; yes

COMB (8/16)

› 3.6 › 3.4 › 9.0 (F/T)

› 16.3

› 10.2 › 9.6 › 1.5

› 3.8 (CMJ) › 10.0 (CMJA) › 6.3 (HJ)

› 2.0 (MAV)

› 8.1 (CMJA) › 4.3 (F/T)

› 17.6 (SJ) › 19.8 (HOP) › 12.9 (CMJA)

Studies that state imperial measurements are shown in metric measurement with the conversion to imperial in square brackets.

1RM = one repetition maximum; 5JT = five-jump test; A = athletes; CMJ = countermovement jump; CMJA = countermovement jump with the arms swing; CMJT = countermovement jump training; COMB = combination of various jump exercises; DJ = drop jump; DJT = drop jump training; ERPT = endurance running performance time; F = females; F/T = force/torque; HJ = horizontal jump; HOP = hopping; LT = lactate threshold; M = males; MAV = maximal aerobic velocity; N-A = non-athletes; NS = not specified; RE = running . economy (VO2 during submaximal running); SJ =.squat jump; SLCMJ = single-leg countermovement jump; SLDJ = single-leg drop jump; SLSJ = single-leg squat jump; USLCJ = unilateral straight-legged concentric jump; VO2max = maximal oxygen uptake; › indicates increase in performance; fl indicates decrease in performance; 2 indicates no change in performance.

Markovic & Mikulic

Sports Med 2010; 40 (10)

a

› 2.1 (12)

Physiological Adaptation to Plyometric Training

The results of a limited number of studies suggest that muscle hypertrophy[16,86] and increased neural drive to the agonist muscles[16,87,97,98] are the likely mechanisms behind significant increases in muscle strength and power following combined PLY and WT. Finally, note that electromyostimulation represents another training modality that can be successfully combined with PLY in augmenting lower extremity strength and power.[119,136]

875

could be the result of different types of plyometric exercises used.[38] Namely, although most PLY studies used DJ as the training exercise, the authors rarely described whether they applied countermovement-type (i.e. slow) or bounce-type (i.e. fast) DJ training.[3] Taken together, these results indicate that PLY could enhance both slow and fast SSC muscle function, but these effects appear to be specific with respect to the type of SSC exercise used in training.

2.2.7 Stretch-Shortening Cycle Muscle Function

SSC muscle function has been classified as either slow (ground contact time >0.25 seconds) or fast (ground contact time <0.25 seconds).[144] In both slow and fast SSC, a pre-stretch enhances the maximum force and work output that muscles can produce during the concentric phase. The mechanisms responsible for this enhancement could be (i) the time available for force development; (ii) storage and reutilization of elastic energy; (iii) potentiation of the contractile machinery; (iv) interaction between the series elastic component and the contractile machinery; and (v) the contribution of reflexes.[145-147] It is beyond the scope of this article to discuss these mechanisms in detail (for a review, see van Ingen Schenau et al.[147]). We will only mention that the relative contribution of each mechanism to muscle performance enhancement appears to be different in slow SSC versus fast SSC. The efficacy of the slow SSC in lower extremities is usually assessed through pre-stretch augmentation during vertical jumping and expressed in either centimetres:[148] (countermovement jump [CMJ], squat jump [SJ]), or in percentages:[64] ([CMJ – SJ]/SJ · 100). The efficacy of the fast SSC, also known as reactive strength,[108] is usually assessed by dividing the DJ height with ground contact time,[108] or by dividing the DJ flight time with ground contact time.[14] A limited number of studies showed that PLY significantly improves fast SSC muscle function.[14,15,108] Regarding the effects of PLY on slow SSC function, the results are conflicting.[14,34,103,107] However, a recent meta-analysis[3] strongly suggested that PLY produces greater effects in CMJ compared with SJ, and the present study confirmed these findings (see section 2.3.1). The observed discrepancies ª 2010 Adis Data Information BV. All rights reserved.

2.3 Athletic Performance Adaptation to PLY 2.3.1 Jumping Performance Vertical Jumping Performance

PLY has been extensively used for augmenting jumping performance in healthy individuals. Numerous studies (see table II) have shown that short-term PLY improves vertical jump height in both children and young adults, regardless of their previous athletic experience, sex and training status. The results of two recent meta-analyses further support this view by showing significant and practically relevant PLY-induced increases in vertical jump height in athletes and non-athletes of both sexes.[3,13] However, some studies[108,112-114,119,124] reported no change or even slight decreases in vertical jumping performance following PLY. While no effect on jumping performance in some of the studies[119,124] might be related to an insufficient training stimulus (i.e. £8 training sessions), the observed decreases in jumping performance following PLY[108,112,114] could be related to factors such as muscle damage and residual fatigue. Indeed, in one of these studies, a significant (+3%) increase in vertical jump height was observed after a short recovery period.[114] In the reviewed studies, vertical jumping performance was assessed using all four types of standard vertical jumps such as SJ, CMJ, CMJ with the arm swing (CMJA) and DJ. In addition, some studies[20,34,75] used one or more single-leg jumps (table II). Overall, the results of this review suggest that PLY considerably improves vertical jump height. The calculated relative improvements range, on average, from +6.9% (range, -3.5% to +32.5%) for CMJA, over +8.1% (range, -3.7% Sports Med 2010; 40 (10)

Study

Polhemus and Burkhardt[128]

PLY combined with exercise training type; control group

No. of subjects; sex; fitness level

PLY intervention; wks; no. of sessions; type of exercise

WT; no

34 M; A

6; 18; COMB

876

ª 2010 Adis Data Information BV. All rights reserved.

Table III. Chronological summary of studies examining the effects of plyometric training (PLY) combined with another form(s) of physical conditioning on skeletal muscle function and athletic performance Relative effects (%) maximal strength

explosive strength

muscle power

jumping performance

sprinting performance (m [yd]a)

› 4.4 (37 [40 yd])

agility performance

endurance performance

› 14.6 (1RM) › 23.3 (1RM)

WT; no

35 M; A

6; 18; COMB

› 17.4 (1RM) › 34.5 (1RM)

Polhemus et al.[129]

Ford et al.[7]

WT; yes

27 M; A

6; 18; COMB

› 6.9 (CMJA) › 5.1 (HJ)

WT; yes

31 F; A

6;18; COMB

› 14.6 (CMJA) › 1.8 (HJ)

› 5.4 (37 [40 yd])

› 7.6 (CMJA) › 9.1 (CMJA)

› 3.0 (37 [40 yd]) › 3.0 (37 [40 yd])

Wrestling, softball; no

12 M; N-A

10; 25; COMB

WT; no

15 M; N-A

10; 25; COMB

WT; no

16 M; N-A

16; 32; DJT

WT, volleyball training; no

16 M; A

16; 32; DJT

Hakkinen et al.[16]

WT; yes

18 M; N-A

24; 72; COMB

Hakkinen and Komi[97]

WT; no

10 M; N-A

24; 72; COMB

› 21.2 (SJ) › 17.6 (CMJ) › 25.0 (DJ) › 26.8 (DJ) › 32.4 (DJ)

Adams et al.[10]

WT, endurance running; no

31 M; A

10; 30; COMB

› 1.5 (CMJA)

Clutch et al.[6]

› 3.2 fl 0.7

› 6.9 (CMJA) › 4.7 (CMJA) › 9.3 (F/T)

› 21.6

Continued next page

Markovic & Mikulic

Sports Med 2010; 40 (10)

› 1.7 (46 [50 yd])

Study

Blakey and Southard[130]

Bauer et al.[102]

PLY combined with exercise training type; control group

No. of subjects; sex; fitness level

PLY intervention; wks; no. of sessions; type of exercise

WT; no

11 M; N-A

8; 16; DJ

WT; no

10 M; N-A

8; 16; DJ

WT; no

10 M; N-A

8; 16; CMJ

WT; no

6 NS; N-A

10; 30; COMB

Relative effects (%) maximal strength › 7.2 (1RM) › 7.4 (1RM) › 8.1 (1RM)

explosive strength

muscle power

jumping performance

sprinting performance (m [yd]a)

agility performance

endurance performance

› 13.7 › 21.8 › 11.8

› 14.3 (F/T)

› 10.0 (CMJA)

› 10.0 (F/T) › 6.7 (F/T) 7 NS; N-A

10; 30; COMB

› 17.5 (F/T) › 5.0 (F/T) › 18.8 (F/T)

› 7.6 (CMJA)

Paavolainen et al.[131]

WT, sprint training, endurance training; yes

15 M; A

6; NS; COMB

› 2.1 (F/T)

› 10.9 (SJ) › 8.0 (CMJ)

› 1.9 . (VO2max) 2 0.0 (AT)

Kramer et al.[132]

WT; no

12 F; A

9; 27; COMB

› 16.0 (1RM)

› 5.6 (CMJA)

› 3.5 (REPT)

Delecluse et al.[133]

Sprint training; yes

32 M; N-A

9; 18; COMB

Lyttle[134]

WT; yes

22 M; A

8; 16; DJ

Potteiger et al.[85]

Aerobic training; no

11 M; N-A

8; 24; COMB

Witzke and Snow[33]

WT; yes

53 F; N-A

»40; »120; COMB

› › › ›

5.2 6.7 5.3 17.4

› 2.0 (100) › 12.7 (1RM)

› 6.2 (F/T)

› 6.8

› 18.0 (SJ) › 11.4 (CMJ)

› 2.6 › 5.1

› 5.0 (CMJA)

› 0.2 (40) 2 0.0 (20) › 16.3 . (VO2max)

› 2.0

Continued next page

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Sports Med 2010; 40 (10)

WT; no

Physiological Adaptation to Plyometric Training

ª 2010 Adis Data Information BV. All rights reserved.

Table III. Contd

878

ª 2010 Adis Data Information BV. All rights reserved.

Table III. Contd Study

No. of subjects; sex; fitness level

PLY intervention; wks; no. of sessions; type of exercise

Relative effects (%)

Fatouros et al.[15]

WT; yes

20 M; N-A

12; 36; COMB

Hunter and Marshall[135]

WT; yes

11 M; N-A

10; 19; COMB

› › › ›

WT, flexibility training; yes

14 M; N-A

10; 19; COMB

› 14.1 (CMJ) › 8.2 (DJ) › 7.4 (DJ) › 8.7 (DJ)

Maffiuletti et al.[136]

Electrostimulation; no

20 M; A

4; 12; COMB

› 27.3 (F/T) › 24.6 (F/T)

› 19.5 (SJ) › 20.8 (SJ) › 12.8 (DJ) › 12.0 (CMJ) › 8.2 (CMJA)

Tuomi et al.[98]

WT; yes

14 M; A

6; 24; COMB

› 16.3 (F/T)

› 11.3 (SJ) › 13.2 (CMJ)

Wilkerson et al.[137]

Flexibility training, strengthening; no

11 F; A

6; NS; NS

› 8.1 (F/T) › 7.0 (F/T) › 11.7 (F/T) › 13.7 (F/T)

Moore et al.[138]

WT; no

2 M, 5 F; A

12; 33; COMB

› 169.8 (4RM)

› 6.7 (CMJA)

› 9.6 (25)

Herrero et al.[119]

Electrostimulation; yes

20 M; N-A

4; 16; COMB

› 13.0 (F/T)

› 7.3 (SJ) › 7.6 (CMJ)

› 1.7 (20)

maximal strength › 42.2 (1RM) › 26.8 (1RM)

explosive strength

muscle power

jumping performance

› 37.1

› 15.0 (CMJA)

sprinting performance (m [yd]a)

agility performance

endurance performance

8.6 (CMJ) 8.8 (DJ) 10.0 (DJ) 5.5 (DJ)

Continued next page

Markovic & Mikulic

Sports Med 2010; 40 (10)

PLY combined with exercise training type; control group

Study

PLY combined with exercise training type; control group

No. of subjects; sex; fitness level

PLY intervention; wks; no. of sessions; type of exercise

Dodd and Alvar[123]

WT; no

32 M; A

4; 7; COMB

Ratamess et al.[139]

WT, sprint training; no

6 F; A

10; 20; COMB

WT, sprint training; no

8 F; A

10; 20; COMB

Relative effects (%) maximal strength

explosive strength

muscle power

› 25.0 (1RM)

jumping performance

sprinting performance (m [yd]a)

agility performance

› 1.0 (CMJA)

› 0.6 (18 [20 yd]) › 0.3 (37 [40 yd]) › 0.3 (55 [60 yd])

› 2.3

2 0.0 (9.1)

› 3.8

› 3.3 (12)

› 7.6 › 7.4 › 2.7

endurance performance

› 10.8 (CMJA) › 9.7 (HJ)

WT; no

13 M; N-A

6; 12; COMB

Salonikidis and Zafeiridis[18]

Tennis-drill exercises; yes

32 M; A

9; 27; COMB

Perez-Gomez et al.[86]

WT; yes

37 M; N-A

6; 18; COMB

Chappell and Limpisvasti[24]

Core strengthening, balance training; no

30 F; N-A

6; 36; COMB

Marques et al.[141]

WT; no

10 F; A

12; 24; COMB

› 6.3 (CMJA) › 6.5 (HJ) › 8.1 (CMJA) › 6.0 (HJ)

› 42.9 (1RM) › 22.9 (1RM) › 41.7 (1RM) › 13.7 (1RM)

› 0.4 fl 10.5

› 4.9 › 5.0

› 6.7 (SJ) › 8.8 (CMJ)

› 2.7 (5) › 1.1 (10) › 0.8 (15) › 0.6 (20) fl 0.3 (25) 2 0.0 (30)

fl 1.7 . (VO2max)

› 8.2 (CMJA) › 13.0 (4RM)

› 3.9 (CMJ) › 9.6 (CMJ) › 10.3 (CMJ) › 12.7 (CMJ) Continued next page

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Sports Med 2010; 40 (10)

Faigenbaum et al.[140]

› 24.6 (1RM)

Physiological Adaptation to Plyometric Training

ª 2010 Adis Data Information BV. All rights reserved.

Table III. Contd

Markovic & Mikulic

Studies that state imperial measurements are shown in metric measurement with the conversion to imperial in square brackets.

1RM = one repetition maximum; 4RM = four repetition maximum; A = athletes; AT = anaerobic threshold; CMJ = countermovement jump; CMJA = countermovement jump with the arms swing; COMB = combination of various jump exercises; DJ = drop jump; DJT = drop jump training; F = females; F/T = force/torque; HJ = horizontal jump; M = males; N-A = non. athletes; NS = not specified; REPT = rowing ergometer performance time; SJ = squat jump; VO2max = maximal oxygen uptake; WT = weight training; › indicates increase in performance; fl indicates decrease in performance; 2 indicates no change in performance.

4; 4; COMB 6 M, 10 F; A WT; no

4; 8; COMB 5 M, 10 F; A WT; no Mihalik et al.[143]

ª 2010 Adis Data Information BV. All rights reserved.

a

› 10.0 (CMJA) › 8.2

› 5.1

› 5.6 (CMJA)

› 1.1 (10) › 0.2 (40) › 12.5 (SJ) › 2.8 (CMJ) › 2.3 › 3.3 › 4.3 15 M; A WT; yes Ronnestad et al.[142]

7; 14; COMB

› 20.2 (1RM)

explosive strength maximal strength

No. of subjects; sex; fitness level PLY combined with exercise training type; control group Study

Table III. Contd

PLY intervention; wks; no. of sessions; type of exercise

Relative effects (%)

muscle power

jumping performance

sprinting performance (m [yd]a)

agility performance

endurance performance

880

to +39.3%) for SJ and +9.9% (range, -0.3% to +19.3%) for CMJ, to +13.4% (range, -1.4% to +32.4%) for DJ. The estimated relative improvements could also be considered practically relevant since the improvement in vertical jump height of ~7–13% (i.e. ~2–7 cm, depending on the type of a vertical jump) may be of high importance for trained athletes in sports such as volleyball, basketball or high jump.[3] Our results also suggest that the relative effects of PLY are likely to be higher in fast SSC vertical jump (DJ) than is the case for slow SSC vertical jumps (CMJ and CMJA) and concentric-only vertical jump (SJ). These findings are largely in accordance with the previous suggestion by Wilson et al.[103] that PLY is more effective in improving vertical jumping performance in fast SSC jumps as it enhances the ability of participants to use neural, chemo-mechanical and elastic benefits of the SSC. Also, as discussed in previous section (see section 2.2.7), PLY has been found to significantly improve fast SSC muscle function while the results on the effects of PLY on slow SSC function have been conflicting, which could partly explain the greatest improvements in vertical jumping performance observed for DJ. The present study also confirmed the findings of a recent meta-analysis,[3] which strongly suggested that PLY produces greater effects in CMJ compared with SJ. Note that we remain cautious toward the relative improvements in vertical jump performance that exceed +30%, as reported in four studies (one for SJ,[120] one for CMJA[116] and two for DJ;[84,97] table II). Since these studies were heterogeneous with respect to subject characteristics and programme design, the observed unrealistically large gains in jump height following PLY intervention are difficult to explain. In the majority of athletic conditioning programmes PLY is combined with other training modalities, most commonly with some form of WT. The combination of PLY and WT (see table III) seems to have a greater potential in enhancing vertical jumping performance compared with PLY as the only training modality. For example, following combined PLY and WT, CMJA improved on average by +7.8%, whereas the average improvement when PLY was the only training Sports Med 2010; 40 (10)

Physiological Adaptation to Plyometric Training

modality was +6.9%. Given that both PLY and WT improve vertical jump performance (although with different adaptive changes in the neural and musculoskeletal systems[34,98]), it is likely that their combination elicits greater overall training adaptation in the athlete’s body. This view is supported by recent results of Kubo et al.[34] The authors showed that PLY improved concentric and SSC jump performance mainly through changes in mechanical properties of the muscle-tendon complex, while WT-induced changes in concentriconly jump performance were mainly the result of an increased muscle hypertrophy and neural activation of plantar flexors. The effects of PLY combined with electromyostimulation[133] have also been examined[119,136] and are worthy of discussion. Herrero et al.[119] observed that, while PLY effects indicated a slight relative decrease in both SJ (-3.8%) and CMJ (-0.3%) performance, the combined PLY and electromyostimulation resulted in a relative improvement in both SJ (+7.3%) and CMJ (+7.6%) performance, suggesting that PLY and electromyostimulation together may be used to enhance vertical jumping ability. The relative improvements in vertical jumping performance in another study that evaluated the effects of combined PLY and electromyostimulation[136] were of even greater magnitude (range +8.2–20.8%; table III). Unfortunately, the authors[136] did not address the effectiveness of the protocol compared with PLY or electromyostimulation alone.

Horizontal Jumping Performance

The effects of PLY on horizontal jumping performance have been investigated in six studies,[11,12,14,19,35,109] and horizontal jump performance was assessed using long jump,[11] standing long jump[12,14] and five bounding jumps.[11,12,19,35,109] On average, the relative improvement in horizontal jump performance was +4.1% (range +1.4–7.0%), and was observable in both athletes[19,109] and non-athletes.[14] This finding suggests smaller effects following PLY compared with the effects on vertical jumping performance. Of course, due to the relatively low number of studies, the results need to be interpreted with caution. ª 2010 Adis Data Information BV. All rights reserved.

881

It is generally assumed that transfer of PLY effects to athletic performance likely depends on the specificity of plyometric exercises performed. Therefore, athletes who require power for moving in the horizontal plane (e.g. sprinters, long jumpers) mainly engage in bounding plyometric exercises, as opposed to high jumpers, basketball or volleyball players who require power to be exerted in a vertical direction and who perform mainly vertical jump exercises. This corresponds to the well known principle of training specificity.[149] The findings of Hortobagy et al.,[12] however, could not support the above-mentioned assumption, as the two experimental groups that performed two distinctly different PLY routines did not yield specific gains in performance. The authors state that these unexpected findings may be explained by the high degree of generality among the jumping tests performed, as the vertical and horizontal jumping tests were highly correlated. Obviously, the issue of specificity of plyometric exercises in improving the horizontal jumping performance needs to be clarified in future studies. Two studies[129,140] investigated the PLY-induced effects on horizontal jump performance when PLY was combined with WT. The calculated relative improvement in standing long jump in 12- to 15-year-old boys following 6 weeks of combined PLY and WT[140] was +6.0%, possibly indicating that a combination of PLY and WT may be beneficial for enhancing horizontal jumping performance. Furthermore, in support of this assumption are almost identical findings (i.e. +5.1%) in a study by Polhemus et al.[129] Additional well designed studies evaluating the effects of PLY on horizontal jumping performance are needed before the magnitude of effect can be established more accurately. Collectively, the reviewed studies in this section strongly suggest that PLY alone, or in combination with WT, improves jumping performance in both athletes and non-athletes. 2.3.2 Sprinting Performance

Sprint running, in varying degrees, is essential for successful performance in many sports. It represents a multidimensional movement skill that Sports Med 2010; 40 (10)

Markovic & Mikulic

882

requires an explosive concentric and SSC force production of a number of lower-limb muscles. It is, therefore, expected that sprint performance could benefit from PLY. Our review of studies suggests improvements in sprint performance following PLY over distances from 10 to 55 m (60 yards),[14,18,105,106,109,110,116-118,120,123,124] although slight decreases in sprint performance following PLY have also been observed[11,28,119,123] (table II). The benefits of PLY for sprint performance are expected to be the greatest at the velocity of muscle action that most closely approximates the velocity of muscle action employed in training.[110] Therefore, it has been suggested[110,133] that the greatest effects of PLY on sprinting performance occur in the acceleration phase, since the velocity of muscle action in bounding plyometric exercises most closely approximates the velocities of muscle action in the acceleration phase of the sprint. The results of this review (table II) partly support the above-mentioned theory as the greatest relative effects of PLY were observed for a 10 m sprint performance (average improvement +2.2%; range +1.6–2.6%), reducing to the improvement of +2.1% for 12 m sprint performance, further reducing to the average improvement of +1.5% (range -0.3% to +5.7%) for a 20 and 18 m (20 yards) sprint performance, and finally reducing to the average improvement of +1.3% (range -0.6% to +3.0%) for 30 m sprint performance. However, the average improvement for 40 and 37 m (40 yards) sprint performance was +1.7% (range -1.3% to +6.2%) and for 50 m sprint performance was +1.5% (range +1.3–1.7%). An important question in everyday training practice as well as among scientists is the following: if PLY is an effective method of speed improvement, can it improve speed more so than the conventional speed training? In that regard, Rimmer and Sleivert[110] compared the effects of sprint-specific PLY against traditional sprint training on 10 and 40 m sprint performance times. Following 8 weeks of PLY, the PLY group significantly improved 10 m (+2.6%) and 40 m (+2.2%) sprint performance times, but these improvements were not significantly different from those observed in the sprint group. A study by Markovic et al.[14] could not support these findª 2010 Adis Data Information BV. All rights reserved.

ings as the authors found sprint training to be superior to PLY in improving the 20 m sprint performance time. It should be noted that PLY exercises used in this study were not sprint specific; possibly making the power transfer from PLY to sprint performance more difficult. Given the findings of the two described studies,[14,110] as yet, no evidence of superiority of PLY for speed improvement compared with traditional sprint training has been presented. Further work is also required to determine the exact mechanisms behind speed improvement as a result of PLY. PLY has most commonly been combined with WT to evaluate the effects on sprinting performance[7,10,86,123,129,134,138,140,142] (table III), and also with sport-specific training[7,18] and electromyostimulation.[119] When combined with WT over 6–12 weeks,[7,86,142] sprinting performance improved in the range of +0.2–3.0%, with one study[140] reporting no change in performance. Note that the relative improvements in one study[138] seem unrealistically large (+9.6%) compared with other related studies. Note also that the use of the Meridian Elyte athletic shoe during PLY could induce some additional positive effects on sprint performance of athletes, particularly on their sprint endurance ability.[139,150] Overall, the results presented in this section suggest that PLY alone, as well as its combination with WT, have the potential for improving sprinting performance in both athletes and non-athletes. 2.3.3 Agility Performance

Agility has been defined as a rapid whole-body movement with change of velocity or direction in response to a stimulus.[151] This definition recognizes both the cognitive (decision-making process) and physical (change of direction speed) components of agility. In this review, we will use the term ‘agility’ to denote only its physical component. Most agility tasks require a rapid switch from eccentric to concentric muscle action in the leg extensor muscles (i.e. the SSC muscle function). Thus, it has been suggested[152] that PLY can decrease ground reaction test times through the increase in muscular force output and movement efficiency, therefore positively affecting agility performance. The literature search revealed six Sports Med 2010; 40 (10)

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studies that examined PLY effects upon agility performance.[14,18,73,118,121,123] Moreover, a combination of PLY and WT,[7,123,140] and PLY and sport-specific training[7,18] upon agility performance has also been examined. Agility assessment has been performed using the following various agility tests: T-agility test,[121,123] Illinois agility run,[121] pro agility shuttle run,[140] 20 yard shuttle run,[7,14] square agility test,[118] 30 m shuttle run[73] and 4 m side steps.[18] The reviewed studies[14,18,73,118,121,123] indicate consistent findings in that PLY yielded improvements in agility performance, and the range of relative improvement was +1.5–10.2%. Only one study[123] reported no change in agility performance following PLY. When PLY was combined with WT[7,123,140] and sport-specific training,[7,18] similar relative improvements (i.e. +2.7–7.6%) were observed. Again, one study[7] reported a minimal relative decrease in agility performance (-0.7%). Agility tasks are relatively complex, certainly more so than jumping or sprinting. Tricoli et al.[118] found that a 6-week PLY, consisting of plyometric exercises executed in the vertical direction, improved agility performance by +2.0% but the magnitude of improvement was no different from the group that underwent Olympic weightlifting training. The authors speculate that the complexity of agility tasks makes power transfer from plyometric exercises to the tasks requiring agility difficult. In that regard, Young et al.[153] suggested that agility tasks could be more influenced by motor control factors than by muscle strength or power capacity. Miller et al.[121] assessed the effects of a 6-week PLY intervention on agility performance. An additional force-plate test was used to measure ground contact time while hopping. The participants improved their performance times in two agility tests (+5.5% and +3.0%) and the authors concluded that PLY improved performance in agility tests because of either better motor recruitment or neural adaptations. Ground contact times measured by a force plate were also reduced. Overall, although further research examining PLY effects on agility performance is needed, the current findings seem promising for the athletes requiring agility to perform their sport. ª 2010 Adis Data Information BV. All rights reserved.

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2.3.4 Endurance Performance

Endurance athletes (e.g. distance runners, cyclists, cross-country skiers, triathletes) have traditionally focused their training on improving cardiovascular and muscular endurance, as these factors are assumed to be the primary determinants of competitive success in endurance events. In distance runners for example, the primary factors known to affect performance in. clude maximum oxygen uptake (VO2max), lactate threshold and running economy.[154] However, in a homogenous group . of elite distance runners, similar levels of VO2max, lactate threshold and even running economy might be observed, suggesting that other factors (i.e. factors related to the anaerobic work capacity) might contribute to competitive performance at the elite level. In that regard, Noakes[155] suggested that muscle power factors may have a role in limiting endurance performance and . may be better performance predictors that VO2max when comparing elite aerobic athletes. Literature searching indicated four studies[19,20,109,113] that investigated the effects of PLY on endurance performance variables in moderately to highly trained distance runners, while two studies were also conducted using cross-country skiers[131] and rowers[132] as participants. In studies[19,20,109,113] examining the PLY effects in distance runners, the findings seem to be consistent in that the parameter that benefits the most in terms of endurance performance improvement is running economy (i.e. the oxygen cost of sub-maximal running), which, consequently, might lead to an improvement in distance running performance time. The findings of improved running economy following PLY are certainly beneficial to distance runners since even small improvements in running economy become very important over long distances. However, a true indicator of improved endurance performance is race performance time, and in that regard two studies[19,109] included pre- and post-race time data. In a study by Paavolainen et al.,[109] participants improved their running economy which, along with an increased muscle power, resulted in +3.1% relative improvement in a 5.km running performance time. Meanwhile, the VO2max decreased by -5.8%. Sports Med 2010; 40 (10)

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A study conducted by Spurrs et al.[19] yielded the following similar findings: improved running economy following PLY in a group of distance runners; improved musculotendinous stiffness and jumping performance variables; improved race . performance time (+1.2%); and a decrease in VO2max (-3.1%). The exact mechanism by which the improvement in running economy following PLY occurs remains unclear; however, it has been theorized that this improvement is a result of improvements in neuromuscular characteristics including motor unit recruitment and reduced ground contact time. Further supporting this assumption is the fact . that cardiovascular endurance variables (i.e. VO2max and lactate threshold) showed no change or even slightly decreased[19,20,113] following PLY in distance runners, while indicators of muscle strength and power,[19,20,109] as well as indicators of anaerobic work capacity,[109] improved. It appears likely that the improvements in anaerobic power and neuromuscular characteristics following PLY in distance runners transfer to the . improvement in running economy, since VO2max and lactate threshold values appear not to be affected or are even slightly reduced. The introduction of PLY in moderately to highly trained .endurance athletes[19,20,109,113] did not improve VO2max and/or lactate threshold, suggesting that PLY appears to produce an insufficient aerobic stimulus in moderately to. highly trained endurance athletes to improve VO2max and/or lactate threshold beyond values achieved by aerobic training alone. Contrary to the findings in endurance trained individuals, an 8-week PLY was found to produce improvements in . VO2max in physically active men by +13.8%.[85] Obviously, untrained and ‘physically active’ individuals allow greater room for improvement compared with the endurance-trained population. On the other hand, including PLY in the training programme of endurance athletes seems . to be justified for reasons other than VO2max and/ or lactate threshold improvement. Future studies aiming to assess the PLY effects on endurance performance should strive to include pre- and post-PLY data on endurance race performance time, as this parameter serves as a definitive ª 2010 Adis Data Information BV. All rights reserved.

Markovic & Mikulic

yardstick by which endurance performance of an . athlete can be evaluated. While VO2max, lactate threshold, running economy and neuromuscular characteristics are valuable measurements, their meaning is largely limited without an insight into the magnitude of improved race performance. Collectively, the reviewed studies clearly show that PLY, either alone or in combination with other training modalities, has a strong potential to enhance a wide range of athletic performance in children and young adults, regardless of their sex, previous athletic experience and training status. The mechanisms behind these improvements are still not fully understood; however, they appear to be muscle specific and may include:  an increased neural drive to the agonist muscles;  changes in the muscle activation strategies (i.e. improved intermuscular coordination);  changes in the mechanical characteristics of the muscle-tendon complex of plantar flexors;  changes in muscle size and/or architecture;  changes in single-fibre mechanics. 3. PLY on Non-Rigid Surfaces PLY is commonly performed on firm surfaces such as grass, athletic tracks and wood. An increased risk of muscle soreness and damage caused by the forces generated during ground impact and intense plyometric contraction, as suggested in a number of studies,[16,156-161] might be reduced when PLY is performed on a nonrigid surface. In this section, we review studies that investigated the application of aquatic- and sand-based PLY in healthy individuals. The retrieved studies only focused on PLY effects on muscle strength/power or athletic performance.[112,116,122,162,163] Therefore, our discussion is limited to adaptive changes in these neuromuscular and performance qualities. 3.1 Neuromuscular and Performance Adaptations to Aquatic- and Sand-Based PLY

Our literature review found four studies that applied aquatic-based PLY. Martel et al.[163] Sports Med 2010; 40 (10)

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reported a relative improvement in CMJA performance by +7.5% following 6-weeks of PLY conducted in 1.2 m of deep water. In addition, the authors observed a relative increase in knee extensor strength at high velocities (+9.6–26.5%), but also a relative decrease in knee flexor strength and knee extensor strength at low velocities (from -9% to -3.4%). Stemm and Jacobson[122] compared the effects of land-based and aquatic-based (knee-level water) PLY on vertical jump performance with identical PLY exercises performed by both groups. The aquatic-based group improved CMJA performance by +5.0% and the magnitude of improvement was similar to that achieved by the land-based PLY group. Furthermore, Robinson et al.[116] reported large relative increases in vertical jump performance (+33.5%), sprint performance (+6.7%) and concentric and eccentric knee extensor/flexor muscle strength (+25–52%) in an aquatic-based group, and the magnitude of improvements was not significantly different from the land-based group. As expected, the reported muscle soreness was significantly higher in the land-based group. Finally, Miller et al.[112] reported a small relative increase in vertical jump performance (+1.6%) and muscle power (+4.3%), with no relative changes in knee extensor/flexor muscle strength following 8 weeks of aquatic PLY. The usefulness of sand-based PLY has also been investigated. Impellizzeri et al.[162] recently compared the effects of 4 weeks of PLY performed on sand versus grass on vertical jump and sprint performance in soccer players. PLY on both surfaces yielded similar relative improvements in sprint performance (+2.5–4.3%) with PLY on sand inducing less muscle soreness than PLY on grass during the whole 4-week training period. Relative increases in SJ (+10.2%) and CMJ (+6.5%) were also observed following sand-based PLY; however, the results suggest that grass surface was superior in enhancing CMJ performance while sand surface tended to induce greater improvements in SJ. Collectively, current knowledge justifies the use of aquatic- and sand-based PLY for rapid movement performance enhancement. Of particular practical importance for coaches and athletes is the fact that aquatic- and sand-based PLY induces ª 2010 Adis Data Information BV. All rights reserved.

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significantly lower muscle soreness compared with land-based PLY. However, regarding the effects of PLY performed on non-rigid surfaces on muscle strength and power, the current results are inconclusive. Further studies should perhaps focus on determining (i) the optimal water level to elicit a training effect with measurement of impact forces; and (ii) the mechanisms behind performance changes following aquatic- and sandbased PLY. 4. PLY in Prevention of Lower-Extremity Injuries Aside from its benefits in enhancing both the muscle function and athletic performance, PLY combined with other neuromuscular training modalities (e.g. strength training, balance training, stretching and agility training) also represents an effective training paradigm for reducing the risk of lower-extremity injuries in team sports.[25,29-31] This is particularly evident for non-contact anterior cruciate ligment (ACL)[51] injuries in female athletes participating in sports that involve a substantial amount of jumping, landing, and pivot turns, such as soccer, basketball, netball and team handball.[30,164-167] In that regard, our literature search revealed the following two groups of studies related to the use of PLY for the prevention of lower-extremity injuries: (i) studies focusing on the reduction of lower-extremity injury rates in sports; and (ii) studies focusing on modifying lower-extremity injury risk factors, particularly those related to non-contact ACL injury. Given that several recent reviews have been published on this topic,[164-166] we will only briefly summarize the results of these two groups of studies. In total, we found 20 published neuromuscular interventions that included PLY, targeted toward lower-extremity injury prevention in athletes and/ or targeted toward modifying risk factors for lower-extremity (mainly ACL) injuries in athletes. Studies that included only technical aspects of jumping (e.g. landing technique) in their interventions were not included in this review. The details of the studies are given in tables IV and V. In eight of ten studies from the first group, the Sports Med 2010; 40 (10)

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applied multi-component training programme reduced the lower-extremity injury rates in female athletes (table IV). This was particularly evident for non-contact ACL injury rates. The two studies that did not observe a reduction in lowerextremity injury rates had some specificities and/ or limitations compared with the remaining eight studies. More precisely, Pfeiffer et al.[170] conducted the preventive intervention post-training, contrary to the remaining nine studies, while Steffen et al.[172] had very low compliance with the applied intervention programme. The results of the second group of studies (table V) show that the observed reduction of lower-extremity injuries in female athletes following interventions that incorporate PLY is likely to be the result of a modification of biomechanical and neuromuscular injury risk factors, particularly those related to non-contact ACL injury. Specifically, the reviewed interventions generally (i) reduced vertical ground reaction forces;[25-27] (ii) decreased valgus measures;[21,22,24,176] and (iii) increased effective knee and hip muscle preparatory and reactive activation during landing in female athletes.[27,28] Moreover, Zebis et al.[177] recently reported that neuromuscular training markedly increased pre-landing and landing EMG activity of medial hamstring muscles during a side-cutting manoeuvre, thereby decreasing the risk of dynamic valgus. These results generally highlight the importance of enhancing hip and knee muscle pre-activation while performing high-risk manoeuvres such as landings and pivot turns, and PLY appears to be a particularly effective training modality for inducing these changes in the neuromuscular control.[27,28] Finally, the reviewed interventions altered quadriceps dominance in female athletes by increasing hamstring strength and hamstring/quadriceps strength ratio.[23,25,137] Importantly, although mainly focused on injury prevention and/or alteration of injury risk factors, the reviewed interventions also have the potential for enhancing athletic performance.[23,25-27] Taken together, these results support the conclusions of recent narrative and meta-analytical reviews[164-166] that PLY represents one of the most important elements of effective injury-prevention ª 2010 Adis Data Information BV. All rights reserved.

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programmes and, therefore, should be an integral part of the year-round physical conditioning programmes of female athletes in team sports. However, given that all the reviewed studies have been conducted in young female athletes, it remains unknown whether these conclusions and recommendations are also valid for male athletes; hence, there is a clear need for similar studies in male athletes. Moreover, due to the large variation in the total duration of interventions among the reviewed studies (from 6 weeks to 8 months), future research should also determine the optimal duration of injury-prevention interventions. 5. Practical Application of PLY In this section we briefly discuss issues related to the practical applicability of PLY. Let us first focus on subject characteristics. In this regard, it should be observed that the reviewed studies were performed on both athletes (mainly national and regional level) and non-athletes with varying levels of physical fitness and skill. Nevertheless, the results of recent meta-analyses clearly show that the strength and jump performance benefits from PLY were similar in both athletes and non-athletes,[3,127] regardless of their age,[3,127] level of physical activity and previous athletic experience.[3,13,127] Some discrepancies, however, were observed in the results of these meta-analyses regarding the sex effects on improvements in vertical jump height, and these can probably be attributed to different statistical procedures applied and different methodology used to define the groups with mixed samples.[3,13] Regarding the programme design, the optimal exercise selection and the optimal combination of acute programme variables in PLY are still unknown. Most PLY studies that focused on performance enhancement used several PLY exercises for a period of 6–15 weeks for 2–3 sessions a week. A recent meta-analytical review showed that the optimal PLY strategy for maximizing gains in strength is to (i) combine PLY and WT; (ii) use a training intervention duration of <10 weeks (with >15 sessions); and (iii) use highintensity exercises with >40 jumps per session.[127] Another meta-analysis showed that the optimal Sports Med 2010; 40 (10)

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Table IV. Summary of neuromuscular training programmes involving plyometric training (PLY) aimed to reduce lower-extremity injury rates in athletes Study

No. of subjects; design

Training protocol

Results

Hewett et al.[168]

366 F soccer, basketball and volleyball players underwent training and were compared with the control of 434 M and 463 F; prospective cohort

6-wk training programme (PLY, stretching, strengthening) 3 d/wk

Significant reduction of ACL injury risk in the trained F athletes (p £ 0.05). The rate of ACL injuries was decreased 72% in the trained group compared with the untrained group

Heidt et al.[169]

300 high school F soccer players: 42 underwent a training programme; prospective cohort

7-wk pre-season conditioning programme (PLY, cardiovascular, strengthening, stretching, agilities)

Significantly fewer injuries in the trained group compared with the control group (p < 0.01). No differences in the occurrence of ACL injuries between the groups

Myklebust et al.[30]

900 F team handball players studied over a 3-y period; prospective cohort

15-min training programme (PLY, flexibility, balance and agility exercises) performed 3 d/wk for 5–7 wk, and then 1 ·/wk during the season

In elite team division, there was a significant reduction (p = 0.01) in the risk of ACL injury during the second intervention season among those who completed the programme compared with those who did not

Petersen et al.[31]

134 F team handball players underwent training and were compared with the control of 142 F players; prospective controlled

Training programme (PLY, balance) performed 3 d/wk for 5–7 wk and then 1 ·/wk during the season

A non-significant (p > 0.05) reduction in the number of ACL injuries in the training group compared with the control group, although ACL injury risk was 80% lower in the training group

Mandelbaum et al.[29]

5703 young F soccer players: 1885 underwent a training programme; prospective controlled cohort

20-min training programme (education, stretching, strengthening, PLY, specific agilities) performed 2–3 ·/wk

88% and 74% ACL injury reduction in the first and second season, respectively

Pfeiffer et al.[170]

1439 F soccer, basketball, and volleyball players: 577 underwent a training programme; prospective controlled design

20-min training programme (deceleration, agilities, PLY, body awareness) performed 2 ·/wk for 9 wk

Rate of non-contact ACL injuries per 1000 exposures was 0.167 in the treatment group and 0.078 in the control group. Odds ratio 2.05 (p > 0.05)

Gilchrist et al.[171]

1435 F soccer players: 583 underwent a training programme; prospective RCT

15-min training programme (stretching, strengthening, PLY, agilities, education) performed 3 ·/wk for 12 wk

Overall 41% reduction in ACL injuries and 70% reduction of non-contact ACL injuries

Steffen et al.[172]

2092 F soccer players: 1091 underwent a training programme; prospective cluster RCT

15-min training programme (balance, PLY, eccentric hamstrings exercises, landing technique) performed over 8 mo; 15 consecutive sessions then 1 ·/wk

No differences in overall injury rates or any specific injury between the intervention and control group

Soligard et al.[173]

1982 F soccer players: 1055 in the treatment group; cluster RCT

~20-min training programme (running, strengthening, PLY, balance) performed during each training session for 8 mo

A significant lower risk of injuries overall of overuse injuries and severe injuries in the intervention group

Pasanen et al.[174]

457 F floorball players: 256 in the treatment group; cluster RCT

20–30 min training programme (running, balance, PLY, stretching, strengthening) performed over 6 mo; 2–3 ·/wk for 10 wk and 1 ·/wk for 16 wk

The training group reduced the risk of non-contact leg injuries by 66%

ACL = anterior cruciate ligament; F = females; M = males; RCT = randomized controlled trial; ·/wk = sessions times per week.

PLY strategy for maximizing gains in vertical jump height is to (i) combine various PLY exercises; (ii) use a training intervention duration of >10 weeks (with >20 sessions); and (iii) use high-intensity exercises with >50 jumps per session.[13] While these data could be used as general ª 2010 Adis Data Information BV. All rights reserved.

guidelines, we have to acknowledge that PLY is rarely used in sports as a single training modality but, rather, is incorporated into a multi-component physical conditioning programme. This rationale is further supported by data presented in a previous section (see section 4) on injury prevention Sports Med 2010; 40 (10)

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Table V. Summary of neuromuscular training programmes involving plyometric training (PLY) aimed to modify risk factors for lower-extremity injuries in athletes Study

Targeted risk factor

No. of subjects; design

Training protocol

Results

Hewett et al.[25]

Excessive vertical ground reaction force during landing; quadriceps muscle dominance

11 F high school volleyball players in the treatment group; pre-/post-test control group

120-min training programme (PLY, strengthening, stretching) performed 3 ·/wk for 6 wk

A significant decrease (22%; p < 0.01) in peak landing force from a volleyball block jump and in knee adduction and abduction moments (50%; p < 0.01); increased hamstrings/ quadriceps ratio

Chimera et al.[28]

Knee and hip muscle activation strategies

20 F soccer and field hockey players (9 in the treatment group)/pre-/post-test control group

20–30 min PLY performed 2 ·/wk for 6 wk

A significant increase (p < 0.05) in hip adductor muscle preactivation and adductor to abductor co-activation

Wilkerson et al.[137]

Quadriceps muscle dominance

19 F basketball players (11 in the treatment group)/ pre-/post-test control group

PLY for 6 wk

Increased hamstrings strength and hamstrings/quadriceps ratio at a speed of 60/sec

Irmischer et al.[26]

Excessive vertical ground reaction force during landing

28 physically active F (14 in the treatment group); RCT

20 min PLY performed 2 ·/wk for 9 wk

Significant reductions in peak landing forces and rates of force development

Lephart et al.[27]

Excessive vertical ground reaction force during landing; poor jump-landing posture and poor muscle activation strategies

27 F soccer and basketball players (14 in the PLY and strength group); uncontrolled randomized pre-/post-test

30-min training protocol (PLY, strengthening, balance, stretch) performed 3 ·/wk for 8 wk

Increased initial and peak knee and hip flexion, and time to peak knee flexion during the task. Increased peak pre-active EMG of the gluteus medius and integrated EMG for the gluteus medius during the pre-active and reactive time periods

Myer et al.[21]

Poor jump-landing posture

53 F basketball, soccer, and volleyball players (41 in the treatment group); controlled single-group pre-/post-test

90-min training programme (PLY, strengthening, balance, speed) performed 3 ·/wk for 6 wk

Increased knee flexionextension range of motion during the landing phase of a vertical jump; decreased knee valgus (28%) and varus (38%) torques

Myer et al.[22]

Poor jump-landing posture

18 high school F athletes (9 in PLY group); uncontrolled randomized pre-/post-test

Eighteen 90-min PLY sessions during a 7-wk period

Reduced initial contact and maximum hip adduction angle, reduced maximum ankle eversion angle, increased initial contact and maximum knee flexion during the drop vertical jump. Decreased initial contact and maximum knee abduction angle during the medial drop landing

Myer et al.[23]

Decreased vertical ground reaction forces during landing; increased hamstrings strength

19 high school F athletes (8 in PLY group); uncontrolled randomized pre-/post-test

90-min PLY performed 3 ·/wk for 7 wk

Increased hamstrings strength and hamstrings/quadriceps ratio; improved centre of pressure measures during hop landings in the medial/lateral axis; no change in vertical ground reaction force

Pollard et al.[175]

Poor jump-landing posture

18 F soccer players; longitudinal single-group pre-/post-test

20-min in-season injury prevention programme (stretching, strengthening, PLY, agilities) before each soccer practice

Significantly less hip internal rotation and greater hip abduction at landing; no changes in knee valgus or knee angles Continued next page

ª 2010 Adis Data Information BV. All rights reserved.

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Table V. Contd Study

Targeted risk factor

No. of subjects; design

Training protocol

Results

Myer et al.[176]

Excessive knee abduction moment at landing (knee valgus)

27 F soccer and basketball players (12 in the ‘high-risk’ and 6 in the ‘low-risk’ treatment groups); prospective controlled trial

90-min training programme (PLY, strengthening, balance, speed) performed 3 ·/wk for 7 wk

A significant decrease in knee abduction moments by 13% in the ‘high-risk’ group; no change in the ‘low-risk’ or control groups

Chappell and Limpisvasti[24]

Extended knee and hip during landing; knee valgus

30 F soccer and basketball players; single-group pre-/post-test

15-min intervention (core strength, balance, PLY, agilities) performed 6 ·/wk for 6 wk

Stop jump (stance phase): dynamic knee valgus moment decreased; drop jump (stance phase): increased knee flexion

EMG = electromyographic activity; F = females; RCT = randomized controlled trial; ·/wk = sessions times per week.

in sport. Finally, we have to point out that enhancing bone mass in children and pre-menopausal women requires a considerably higher PLY volume (i.e. between 5 and 24 months, 3–5 sessions/week and 50–100 jumps/session), while the exercise intensity should be low to moderate. 6. Conclusions and Recommendations The available evidence suggests that PLY, either alone or in combination with other typical training modalities such as WT, elicits numerous positive changes in neural and musculoskeletal systems, muscle function and athletic performance of healthy individuals. Specifically, the reviewed studies have shown that long-term (6–24 months) PLY represents an effective training method for enhancing bone mass in pre-pubertal/early pubertal children, young women and pre-menopausal women. Furthermore, short-term (6–15 weeks) PLY can change the stiffness of various elastic components of the muscle-tendon complex of plantar flexors in both athletes and non-athletes; however, due to conflicting results in the literature, it is difficult to arrive at a definitive conclusion on this issue. Regarding neuromuscular adaptation to short-term PLY, the results generally show positive increases in lower-extremity strength, power and SSC muscle function in healthy individuals. These adaptive changes in neuromuscular function are likely to be the result of (i) an increased neural drive to the agonist muscles; (ii) changes in the muscle activation strategies (i.e. improved intermuscular coordination); (iii) changes in the mechanical characteristics of the muscle-tendon complex of plantar flexors; (iv) changes in muscle ª 2010 Adis Data Information BV. All rights reserved.

size and/or architecture; and (v) changes in singlefibre mechanics. Our results also show that PLY, either alone or in combination with other training modalities, has the potential to (i) enhance a wide range of athletic performance (i.e. jumping, sprinting, agility, and endurance performance) in children and young adults of both sexes; and (ii) reduce the risk of lower-extremity injuries in female athletes. Finally, available evidence suggest that short-term PLY on non-rigid surfaces (i.e. aquatic-based or sand-based PLY) could elicit similar increases in jumping and sprinting performance as traditional PLY, but with substantially less muscle soreness. Although many issues related to PLY remain to be resolved, the results of the present review allow us to recommend the use of PLY as a safe and effective training modality for improving lower-extremity muscle function and functional performance of healthy individuals. For performance enhancement and injury prevention in sports, we recommend an implementation of PLY into a well designed, sportspecific physical conditioning programme. Acknowledgements Goran Markovic was supported by the Croatian, Ministry of Science, Education and Sport Grant (no. 034-03426072623). The authors have no conflicts of interest that are directly relevant to the content of this review.

References 1. Chmielewski TL, Myer GD, Kauffman D, et al. Plyometric exercise in the rehabilitation of athletes: physiological responses and clinical application. J Orthop Sports Phys Ther 2006 May; 36 (5): 308-19

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153. Young WB, James R, Montgomery I. Is muscle power related to running speed with changes of direction? J Sports Med Phys Fitness 2002 Sep; 42 (3): 282-8 154. Jung AP. The impact of resistance training on distance running performance. Sports Med 2003; 33 (7): 539-52 155. Noakes TD. Implications of exercise testing for prediction of athletic performance: a contemporary perspective. Med Sci Sports Exerc 1988 Aug; 20 (4): 319-30 156. Jamurtas AZ, Fatouros IG, Buckenmeyer P, et al. Effects of plyometric exercise on muscle soreness and plasma creatine kinase levels and its comparison with eccentric and concentric exercise. J Strength Cond Res 2000 Feb; 14 (1): 68-74 157. Miyama M, Nosaka K. Influence of surface on muscle damage and soreness induced by consecutive drop jumps. J Strength Cond Res 2004 May; 18 (2): 206-11 158. Saxton JM, Clarkson PM, James R, et al. Neuromuscular dysfunction following eccentric exercise. Med Sci Sports Exerc 1995 Aug; 27 (8): 1185-93 159. Almeida SA, Williams KM, Shaffer RA, et al. Epidemiological patterns of musculoskeletal injuries and physical training. Med Sci Sports Exerc 1999 Aug; 31 (8): 1176-82 160. Kyrolainen H, Takala TE, Komi PV. Muscle damage induced by stretch-shortening cycle exercise. Med Sci Sports Exerc 1998 Mar; 30 (3): 415-20 161. Nosaka K, Kuramata T. Muscle soreness and serum enzyme activity following consecutive drop jumps. J Sports Sci 1991 Summer; 9 (2): 213-20 162. Impellizzeri FM, Rampinini E, Castagna C, et al. Effect of plyometric training on sand versus grass on muscle soreness and jumping and sprinting ability in soccer players. Br J Sports Med 2008 Jan; 42 (1): 42-6 163. Martel GF, Harmer ML, Logan JM, et al. Aquatic plyometric training increases vertical jump in female volleyball players. Med Sci Sports Exerc 2005 Oct; 37 (10): 1814-9 164. Alentorn-Geli E, Myer GD, Silvers HJ, et al. Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 2: a review of prevention programs aimed to modify risk factors and to reduce injury rates. Knee Surg Sports Traumatol Arthrosc 2009 Aug; 17 (8): 859-79 165. Hewett TE, Ford KR, Myer GD. Anterior cruciate ligament injuries in female athletes. Part 2: a meta-analysis of neuromuscular interventions aimed at injury prevention. Am J Sports Med 2006 Mar; 34 (3): 490-8 166. Hewett TE, Myer GD, Ford KR. Reducing knee and anterior cruciate ligament injuries among female athletes: a systematic review of neuromuscular training interventions. J Knee Surg 2005 Jan; 18 (1): 82-8 167. Steele JR. Biomechanical factors affecting performance in netball: implications for improving performance and injury reduction. Sports Med 1990 Aug; 10 (2): 88-102 168. Hewett TE, Lindenfeld TN, Riccobene JV, et al. The effect of neuromuscular training on the incidence of knee injury in female athletes: a prospective study. Am J Sports Med 1999 Nov-Dec; 27 (6): 699-706 169. Heidt Jr RS, Sweeterman LM, Carlonas RL, et al. Avoidance of soccer injuries with preseason conditioning. Am J Sports Med 2000 Sep-Oct; 28 (5): 659-62 170. Pfeiffer RP, Shea KG, Roberts D, et al. Lack of effect of a knee ligament injury prevention program on the incidence

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of noncontact anterior cruciate ligament injury. J Bone Joint Surg Am 2006 Aug; 88 (8): 1769-74 Gilchrist J, Mandelbaum BR, Melancon H, et al. A randomized controlled trial to prevent noncontact anterior cruciate ligament injury in female collegiate soccer players. Am J Sports Med 2008 Aug; 36 (8): 1476-83 Steffen K, Myklebust G, Olsen OE, et al. Preventing injuries in female youth football: a cluster-randomized controlled trial. Scand J Med Sci Sports 2008 Oct; 18 (5): 605-14 Soligard T, Myklebust G, Steffen K, et al. Comprehensive warm-up programme to prevent injuries in young female footballers: cluster randomised controlled trial. BMJ 2008 Dec; 337: a2469 Pasanen K, Parkkari J, Pasanen M, et al. Neuromuscular training and the risk of leg injuries in female floorball players: cluster randomised controlled study. BMJ 2008 Jul; 337 (7661): 96-9

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175. Pollard CD, Sigward SM, Ota S, et al. The influence of inseason injury prevention training on lower-extremity kinematics during landing in female soccer players. Clin J Sport Med 2006 May; 16 (3): 223-7 176. Myer GD, Ford KR, Brent JL, et al. Differential neuromuscular training effects on ACL injury risk factors in ‘‘high-risk’’ versus ‘‘low-risk’’ athletes. BMC Musculoskelet Disord 2007; 8: 39 177. Zebis MK, Bencke J, Andersen LL, et al. The effects of neuromuscular training on knee joint motor control during sidecutting in female elite soccer and handball players. Clin J Sport Med 2008 Jul; 18 (4): 329-37

Correspondence: Dr Goran Markovic, University of Zagreb, School of Kinesiology, Horvacanski zavoj 15, 10 000 Zagreb, Croatia. E-mail: [email protected]

Sports Med 2010; 40 (10)

CORRESPONDENCE

Sports Med 2010; 40 (10): 897-898 0112-1642/10/0010-0897/$49.95/0

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Therapeutic Hypoxia Overdue Naming Convention In the recently published author’s reply ‘‘Hypoxic Training Terminology: Time for Consensus’’ to our letter to the editor[1] Dr Millet calls for a ‘‘consensus statement on terminology.’’ We fully agree with this suggestion, but the issue of hypoxic training extends beyond the confusing terminology and the need for proper naming. Actually, terminology change is already happening. An exposure to hypoxia is now being considered by the Australian Therapeutic Goods Administration (TGA) as an officially recognized medical therapy. Recently, the TGA released a consultation paper in which they have named this intervention ‘‘hypoxic therapy,’’ based upon its ‘‘modification of physiological process.’’ In this paper,[2] TGA refers to all devices used for altitude training and hypoxic therapy as ‘‘hypoxicators.’’ They suggest mandatory regulation of these devices and systems based upon their concerns that the hypoxicators are ‘‘potentially hazardous, regardless of the manufacturer’s intended purpose’’ and that ‘‘y their use could result in the user experiencing adverse side effects, such as hypoxaemia.’’ The TGA’s concerns also applied to healthy individuals (e.g. athletes) for altitude training and the TGA clearly proposed that hypoxicators must be regulated as medical devices. Naturally, hypoxicators must meet essential requirements for medical devices (e.g. electrical medical safety and electromagnetic compatibility[3]) and must conform with the Australian and European Union Essential Principles for quality, safety and performance, based upon standard conformity assessment procedures.[4-6] Furthermore, we agree with Dr Millet that the physiological effects of exposure to hypoxia either at rest or during exercise and also various combinations with the change in barometric pressure or time of the day etc., may vary. However, in all of these situations the underlying basic physiological mechanisms operate, which are triggered by oxygen sensing pathways at the systemic and cellular level.

We are glad that Dr Millet mentions the fact that we used the term ‘‘intermittent hypoxic training (IHT)’’ in our letter,[1] whilst referring to hypoxia exposure at rest. Such use of the term IHT was deliberate because we have been using it in this context for over 20 years and it has been widely accepted.[7,8] We therefore consider that this precedence allows us to continue using this terminology in the same way. The recent suggestion to use different terms for exercise hypoxia (and call it IHT) and at rest hypoxia (to be called intermittent hypoxia exposure or IHE) simply describes different protocols of the same therapy. It appears that the word ‘exposure’ is redundant as it fails to explain what the therapy does or is used for. Indeed, we do not normally say ‘‘exposure to weight lifting’’ or ‘‘exposure to running’’ or ‘‘steam room exposure.’’ As the main purpose of the hypoxia exposure, at rest or during workout, is to deliver a certain training stress, we believe that the term ‘training’ was appropriate for both these training modalities. The very fact that Dr Millet uses the words ‘‘Hypoxic Training’’ in the title of his reply letter demonstrates that he is in agreement with the ‘training’ definition for all hypoxia applications. In view of the recent TGA consultation paper[2] and proposed medical device regulations, we suggest that the terminology for hypoxic training should now be refined and become more precise and concise. Therefore, we propose using ‘therapeutic hypoxia’ (TH) as a generic term for all types of hypoxic training, followed by a brief specific description of the protocol used. The word ‘therapeutic’ in the term TH delivers an important message that such hypoxia treatment has demonstrable therapeutic benefits, such as human performance enhancement, whilst the associated risks are reduced to the lowest practically achievable level.[9] By convention, it should be accepted that the term TH covers only mild, non-damaging hypoxic training protocols. An additional implication of the TGA paper is the requirement for appropriately supporting clinical evidence, to demonstrate the benefits of the therapy.

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Letter to the Editor

Unfortunately, the quantity of various research studies on this topic does not automatically provide the required quality of such evidence. In the world of clinical research, more stringent standards are used for validation of new therapies and, in contrast to much sport physiology research, studies on a very small group of patients can only be used for pilot studies with non-exhaustive conclusions and recommendations. An important point was made by Gore et al.[10] that to demonstrate statistically significant 1% performance enhancement in the group of highly trained athletes, the number of subjects should be at least 64. Consensus on the TH terminology is more easily reached, but to establish the optimal uses of TH, we all need more hard data from carefully designed studies. This would help us to bridge many years of scientific and technological research with the practical needs of athletes and patients, and thus achieve the goal to make TH more affordable, safe and accessible. Oleg Bassovitch Biomedtech Australia (GO2Altitude) Pty Ltd, Melbourne, Victoria, Australia

Acknowledgements The author is grateful for valuable comments and critique by Dr Rod Westerman (Associate Professor, MB, BS, PhD, MD, FRACGP, Postgraduate Medicine, Edith Cowan University, Joondalup, WA, Australia) during drafting of this letter. No sources of funding were used to assist in the preparation of the letter. The author has no conflicts of interest that are directly relevant to the content of this letter.

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References 1. Bassovitch O. ‘Combining hypoxic methods for peak performance’: a biomedical engineering perspective [letter]. Sports Med 2010; 40 (6): 519-21; author reply 521-3 2. Therapeutic Goods Administration (TGA). Consultation paper on the regulation of hypoxic therapy and altitude training devices (hypoxicators) in Australia [online]. Available from URL: http://www.tga.gov.au/devices/drhypoxic. htm [Accessed 2010 Aug 30] 3. International Electrotechnical Commission. IEC 60601-1-6 ed3.0. Medical electrical equipment – part 1–6: general requirements for basic safety and essential performance-collateral standard [online]. Available from URL: http://webstore. iec.ch/webstore/webstore.nsf/artnum/043725 [Accessed 2010 Aug 16] 4. Department of Health and Ageing Therapeutic Goods Administration. Australian regulatory guidelines for medical devices: version 1.0 April 2010 [online]. Available from URL: http://www.tga.gov.au/devices/argmd.htm [Accessed 2010 Aug 16] 5. Council Directive 93/42/EEC of 14 Jun 1993 concerning medical devices. Official Journal of the European Communities 1993 Jul 12; 36: L169 6. International Organization for Standardization. ISO 14971: 2007 medical devices – application of risk management to medical devices [online]. Available from URL: http://www. iso.org/iso/catalogue_detail.htm?csnumber=38193) [Accessed 2010 Aug 16] 7. Serebrovskaya TV. Intermittent hypoxia research in the former Soviet Union and the Commonwealth of Independent States: history and review of the concept and selected applications. High Alt Med Biol 2002 Summer; 3 (2): 205-21 8. Bassovitch O, Serebrovskaya TV. Equipment and regimes for intermittent hypoxia therapy. In: Xi L, Serebrovskaya TV, editors. Intermittent hypoxia: from molecular mechanisms to clinical applications. New York: Nova Science Publishers, 2009 9. Bassovitch O. Intermittent hypoxic training: risks versus benefits. A biomedical engineering point of view. Eur J Appl Physiol. Epub 2010 Jun 5 10. Gore CJ, Clark SA, Saunders PU. Nonhematological mechanisms of improved sea-level performance after hypoxic exposure. Med Sci Sports Exerc 2007 Sep; 39 (9): 1600-9

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