Identifying risk factors for contact injury in professional rugby league players – Application of a frailty model for recurrent injury

Abstract

Objectives

Well-developed physical qualities may protect against contact injuries. However, the potential contribution of physical qualities as risk or protective factors to contact injury risk is yet to be determined for rugby league. This study applied a frailty survival model that accounts for recurrent injury to identify risk factors for all physiotherapist-reported contact injury in professional rugby league players.

Design

Prospective cohort study.

Methods

Sixty-six professional rugby league players participated in this three successive year prospective study. At the start of each season, all players underwent measurements of standard anthropometry (height, body mass, and sum of seven skinfolds), speed (10 m and 40 m sprint), muscular strength (1 repetition maximum [RM] bench press, 1RM squat, 1RM weighted chin-ups), power (vertical jump, bench throw, 1RM power clean, jump squat), and endurance (maximum repetition bench press with 60 kg resistance), repeated-sprint ability (12 × 20 m sprints performed on a 20 s cycle), prolonged high-intensity intermittent running ability (8 × 12 s maximal effort shuttles performed on a 48 s cycle), and maximal aerobic power (multi-stage fitness test). Data was used to demonstrate the application of the frailty model extension of the Cox proportional regression model for recurrent events to identify factors associated with a high hazard ratio (HR) of injury.

Results

Heavier (body mass, HR = 2.6, 95% CI = 1.2–5.7), and faster (40 m sprint, HR = 2.1, 95% CI = 1.0–4.2) players, and those with poorly developed prolonged high-intensity intermittent running ability (HR = 2.9, 95% CI = 1.7–5.0) and upper-body strength (chin-up, HR = 2.2, 95% CI = 1.3–3.7) had a higher incidence of contact injuries.

Conclusions

This study demonstrates application of a novel statistical approach for the analysis of injury data that is recurrent in nature. This approach identified that the greater impact forces generated from heavier players with faster speed may result in an increase in recurrent contact injury rates. However, the development of prolonged high-intensity intermittent running ability and upper-body strength and power may assist to reduce the risk of contact injury in professional rugby league players.

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.