Original researchIdentifying risk factors for contact injury in professional rugby league players – Application of a frailty model for recurrent injury
Introduction
The development of effective injury prevention strategies is dependent on the identification of injury risk factors1 which then need to be demonstrated to work in the real world context of on-field sport.2 Despite the wealth of information documenting the incidence and cause of rugby league injuries, studies documenting specific risk factors for contact injuries in rugby league are limited.3 Furthermore, few studies have documented the risk factors for contact injury in any professional collision sport athletes.4 While the greater injury incidence with increases in playing standard have often been attributed to the higher playing intensity at the elite level of competition,3 very little is known about the role that physical qualities play (if any) in the risk of contact injury. It has been postulated that faster game speeds lead to higher impact forces in collisions, which subsequently lead to higher injury rates.5 It could therefore be assumed that better developed physical qualities (e.g. speed, strength, and power) could increase game speed and result in higher impact forces and injury rates. However, an equally compelling argument can be made to support the protective role that physical qualities may play against injury.3, 4 Watson4 investigated the intrinsic risk factors for injury in 86 high performance Gaelic football and hurling players, and found the number of days of injury sustained by players could be predicted by previous injury, posture, acceleration over 10 m, and the number of musculoskeletal clinical defects. In the only attempt to investigate risk factors for contact injury in rugby league players, Gabbett and Domrow3 reported that low body mass, and poorly developed speed and aerobic power were significant risk factors for injury. Players with poor aerobic power were six times more likely to sustain a contact injury than players with high aerobic power. Subsequently, Gabbett6 investigated the influence of progressive increases in exercise intensity and fatigue on tackling technique in rugby league players. High aerobic power was associated with smaller fatigue-related decrements in tackling technique. Collectively, these findings suggest that high aerobic fitness may reduce fatigue-induced decrements in tackling technique and consequently reduce fatigue-related tackling injuries.
The potential contribution of physical qualities as risk or protective factors for collision injuries in rugby league is not limited to aerobic fitness. Indeed, low body mass has been shown to be a significant injury risk factor.3 It has been suggested that higher percentage body fat may act as a means of protection against impact injuries, by providing an energy absorbing barrier,7 although, to date, no scientific evidence exists to support or refute this claim.8 The contribution of other physical qualities (e.g. anthropometric characteristics; muscular strength, power, and endurance; anaerobic and aerobic fitness qualities) to collision injury risk is yet to be determined for rugby league. With this in mind, the purpose of this study was to identify risk factors for contact injury in professional rugby league players.
Section snippets
Methods
Participants: Sixty-six professional rugby league players participated in this three-year prospective study. Of these, 38 (57.6%) played one season, 13 players (19.7%) played two seasons, while 15 players (22.7%) played all three seasons. The number of players participating in each season was 36, 38, and 35, respectively, giving a total of 109 player-seasons of observation. At the commencement of the study, all players were injury free, and were registered with the same professional rugby
Statistical analyses
Injury risk per total hours of exposure was determined by dividing the number of injuries by the total number of player-hours of exposure. Descriptive statistics for risk factors are expressed as means and standard deviations. A chi-square test was applied to the number of injuries and the exposure time to construct the 95% confidence intervals (CI) for injury incidence rates.
There are several extensions of the Cox proportional hazards regression model that can be applied to recurrent events,
Results
The physical characteristics of the participants were representative of professional rugby league players (Table 1).
Incidence of contact injury: A total of 154 contact injuries were recorded over the three seasons. Supplementary Table A shows the overall incidence of contact injury during the three seasons (92.5 [95% CI = 77.9–107.1] per 1000 h). The most common causes of the contact injuries were tackling (47.4 [95% CI = 38.0–59.1] per 1000 h) and being tackled (30.0 [95% CI = 22.6–39.7] per 1000 h),
Discussion
The present study is the first to report the statistical modelling of risk factors for contact injury in professional rugby league players, when injury recurrence is accounted for. We believe it also to be the first application of the frailty model for analysis of risk factors for recurrent injuries in any sport. The strength of the frailty model is that it considers individual hazards for different players and also is able to model time-varying covariates. The frailty model has fewer
Conclusion
In conclusion, this study investigated the risk factors for contact injury in professional rugby league players using a novel application of a frailty model to account for recurrent injury events in players within a season. Whilst the study is based on only 154 injuries in 66 players in one sport, it provides an indication of the value of this novel statistical analysis approach and serves as a demonstration of the application of the frailty model to injury risk factor data. It is hoped that
Practical implications
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The majority of rugby league injuries occur in tackles and physical collisions.
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Players with poor prolonged high-intensity intermittent running ability and chin-up strength had a higher injury incidence, and were at greater risk of sustaining a contact injury.
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Developing the physical qualities of players will likely reduce the risk of contact injury in professional rugby league.
Acknowledgements
The Australian Centre for Research into Injury in Sport and its Prevention (ACRISP) is one of the International Research Centres for Prevention of Injury and Protection of Athlete Health supported by the International Olympic Committee (IOC). Caroline Finch was supported by an NHMRC Principal Research Fellowship (NHMRC ID: 565900).
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