Preseason isokinetic strength imbalance and hamstring injury in male professional rugby players: a prospective cohort study

Background Hamstring injuries are among the most common muscle injuries in team ball sports. Hamstring muscle strength deficits have been identified as potential intrinsic risk factors for hamstring injury. Objectives To determine the cumulative incidence of hamstring injuries in male professional “Top 14” rugby players during one season and to investigate the associations between preseason isokinetic strength imbalance and subsequent hamstring injury. Methods This prospective cohort study included 134 male professional rugby players from four French “Top 14” clubs recruited during the 2010–2011 and 2011–2012 seasons. The four participating clubs were selected from the 14 eligible “Top 14” clubs, and player participation was voluntary. Preseason isokinetic and muscle extensibility assessments were performed, and playing position and previous hamstring injury history were recorded. A preseason isokinetic strength imbalance was defined as the presence of at least one prespecified abnormality in bilateral hamstring symmetry, conventional ratio, or mixed/functional ratio. Players were followed prospectively for 12 months. Training and match exposure time were not collected. Associations with subsequent hamstring injury were assessed using multivariable logistic regression with adjustment for clustering by club. Results During the 12-month follow-up, 40 of 134 players sustained at least one hamstring injury, corresponding to a cumulative incidence of 29.9% (95% CI: 22.2–38.4). The cumulative incidence was 21.0% (13/62; 95% CI: 11.6–33.2) in players without a preseason isokinetic imbalance and 37.5% (27/72; 95% CI: 26.3–49.7) in those with an imbalance. In the single-predictor mixed-effects model accounting for club-level clustering, players who met at least one predefined criterion for preseason isokinetic imbalance had higher odds of hamstring injury than players who met none of these criteria, although the association did not reach statistical significance (OR 2.12, 95% CI 0.93–4.83, p = 0.073). After adjustment for previous hamstring injury and playing position, the estimated OR increased to 2.56 (95% CI: 1.07–6.12, p = 0.034). Conclusion In this cohort of male professional rugby players, preseason isokinetic strength imbalance was associated with higher odds of subsequent hamstring injury after adjustment for previous hamstring injury and playing position. However, the data were collected in 2011–2012, and the composite classification showed limited individual level discrimination. These findings should therefore be considered exploratory and require replication in contemporary professional rugby union cohorts using standardized injury definitions, exposure-adjusted analyses, and validated rugby-specific classification thresholds before their clinical relevance can be established.