Total Body Water and Fluid Distribution in Relation to Musculoskeletal Injury History: A Bayesian Analysis in Physically Active Young Adults

Background/Objectives: Body water status is relevant to physiological function during physical activity, yet its relationship with musculoskeletal injury occurrence remains insufficiently understood. This study examined whether total body water (TBW) and relative extracellular water distribution (ECW/TBW) were associated with injury occurrence in physically active young adults using Bayesian inference. Methods: Analysis included 418 physically active university students (199 men, 219 women). Body-water characteristics were measured at a single time point, whereas musculoskeletal injuries were retrospectively reported for the preceding 12 months. Bayesian logistic regression models examined associations of TBW and ECW/TBW with injury history, adjusting for sex, total physical activity, weekly training volume, and training age. Posterior probabilities were evaluated for meaningfully negative, practically negligible, and meaningfully positive associations using a predefined region of practical equivalence (ROPE; β = −0.10 to +0.10). Results: Overall, 51.4% of participants reported at least one injury. Higher TBW was associated with higher odds of reporting an injury in the adjusted model (OR = 1.38, 95% credible interval [CrI]: 1.11–1.73), with a 97.8% posterior probability of a meaningfully positive association. In contrast, higher ECW/TBW was associated with lower odds of injury occurrence (OR = 0.77, 95% CrI: 0.62–0.95), with a 93.6% posterior probability of a meaningfully negative association. The direction of both associations remained stable across unadjusted models and alternative ROPE definitions. Conclusions: TBW and relative extracellular water distribution showed distinct and opposing associations with retrospectively reported musculoskeletal injury history. These findings provide information about cross-sectional relationships but do not establish temporal direction or prospective injury risk. Longitudinal studies are needed to clarify their physiological and potential predictive relevance.