INTRODUCTION: Absolute weekly workloads (1-4 weekly) and acute workloads relative to chronic workloads have been investigated. However, the association between workload and injury may vary from sport to sport and competition league. Therefore, the purpose of this study was to investigate the association between workload and injuries in elite Japanese rugby union players. METHODS: Fifty-two rugby union players (mean±SD; age, 27.7±3.2 years; height, 179.4±8.3 cm; body weight, 97.7±13.9 kg) from one elite Japanese rugby team were used as subjects during one in-season study. Training and match workloads were recorded as external loads (total distance, high-speed running (>5m/sec), acceleration and deceleration) using GPSPORTS-EVO (Catapult sports, Australia). All time-loss injuries were recorded during the period by a senior athletic trainer. Acute:chronic workload ratio (ACWR) was calculated by dividing acute workload (i.e. 1-week workload) by chronic workload (i.e. coupled and uncoupled rolling average 4-weekly workload). Players completed a Bronco test to assess injury risk in relation to fitness level. Generalized linear mixed model (GLMM) was used to analyze the relationship between ACWR and injury during the following week. These analyses can handle panel data (repeated individual measures). Model predictive accuracy was determined by area under the curve (AUC). RESULTS: Greater association was found between high-speed running and injury than total distance, acceleration, or deceleration. High ACWR (>1.5) at high-speed running exhibited the greatest injury risk (Odds ratio = 7.14, 95% CI = 1.91 - 26.65) in the subsequent week. Based upon the Bronco test performance, players with poor aerobic fitness had a greater risk of injury than players with better-developed aerobic fitness. Body fat percentage showed U-shape and was highly associated with an increased risk of injury for both high and low body fat percentage players. CONCLUSION: We recommend that strength and conditioning coaches, involved in planning training for performance and injury prevention, monitor the ACWR, and avoid rapid changes of high-speed running that approach or exceed 1.5 in elite Japanese rugby union players.
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