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

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.