Association Between Body Composition and Non-Contact Muscle Injury Risk in Elite Soccer Players: A Cox Proportional Hazards Model Analysis

The study investigated the association between body composition and non-contact muscle injury risk in elite soccer players. Thirty-three elite male outfield players from one Russian Premier League club were monitored across three seasons (2021/22-2023/24). Anthropometry (body-fat %, muscle-mass %, sum of 8 skinfolds, body mass, body mass index) was performed at six standardized time points per season by anthropometrist with The International Society for the Advancement of Kinanthropometry certificate. Non-contact muscle injuries were recorded to International Olympic Committee standards. Time-varying Andersen-Gill Cox models were fit on counting-process intervals with multiple imputation and player-clustered inference; covariates included age, position, seasonal period, exposure minutes, body-fat %, and fat-adjusted muscle residuals. Discrimination was summarized by Harrell's C-index. A total of 54 non-contact muscle injuries were observed in 23 players. Across endpoints, no covariate reached p < 0.05. The body-fat % effect (per 1%-point) was directionally positive for Minimal/Mild injuries (HR = 1.61; 95% CI 0.88-2.93), and closer to null for All (HR = 1.15; 95% CI 0.66-2.01) and Moderate/Severe (HR = 0.62; 95% CI 0.18-2.12). Discrimination was moderate: C-index 0.686 (All), 0.692 (Minimal/Mild), 0.789 (Moderate/Severe). In this cohort of elite soccer players, a higher body fat percentage demonstrated an associational signal with the hazard of minimal and mild muscle injuries, whereas no association was observed for moderate or severe events. Given the observational study design and the potential for residual confounding, these findings should be interpreted as associative rather than causal.