Eccentric Hamstring Strength Monitoring to Predict Injury Risk in Men’s Non-League Professional Football: An Exploratory Cox Regression Study

Hamstring-strain injuries (HSIs) are the most prevalent time-loss injuries in professional football. While player monitoring of muscular strength is ubiquitous in professional football, the utility of in-season testing for predicting HSI in non-league football (NLF) settings is unclear. This study aimed to investigate if short-term, in-season changes in eccentric hamstring strength are associated with HSI risk and compare the predictive performance to a baseline model. This was a single-season prospective cohort study (36 weeks) in 20 male professional NLF players (nine HSI events). Eccentric hamstring strength was measured twice weekly during Nordic hamstring exercise (NHE) using a NordBord device. Cox proportional hazard models (Andersen–Gill) evaluated the association of HSI with bilateral peak force and inter-limb asymmetry as time-varying and baseline predictors. Nine HSIs occurred (29% of all time-loss injuries; n = 31). The predictive analysis revealed that the baseline model with hazard ratio (HR) of 0.20 (95% CI: 0.09–0.46; C-index = 0.824) outperformed the time-varying model (HR 0.29; 95% CI: 0.15–0.56; C-index = 0.776), with higher bilateral peak force protective across both models. Conversely, inter-limb asymmetry showed no association with HSI risk (HR 1.10; 95% CI: 0.95–1.27; C-index = 0.527). A key related finding was that while single test inter-limb asymmetry measurements were unreliable, stability across the season was good (ICC(1,k) = 0.895). In this cohort, a greater bilateral peak force was protective against HSI, with baseline testing more effective than twice-weekly in-season testing. Inter-limb asymmetry did not predict HSI, and the utility of its isolated use remains unclear despite the stability of players’ season-long profiles. These exploratory findings require confirmation in larger cohorts.