Subsequent Injury Risk After Return-to-Play From Lower-Extremity Muscle Injuries in Professional Male Football (Soccer).

Background: Injuries to the upper extremity account for nearly half of the overall injury burden in professional football (soccer), with incidence rates differing across major muscle groups under both acute and overuse conditions. Importantly, they are commonly associated with an increased susceptibility to subsequent injury, especially in the weeks after return to play (RTP). However, the specific post-RTP risk trajectories after different lower-extremity muscle injuries remain insufficiently characterized. Purpose: To investigate the risk trajectory for noncontact subsequent injury after returning from major lower-extremity muscle injuries in professional football players, differentiating acute from overuse index injuries. Study Design: Descriptive epidemiology study. Methods: Prospectively collected injury data from clubs in the first and second German Bundesliga over 3 seasons (2022-2023 to 2024-2025) were used for time-to-event analysis. Index injuries (ie, injuries from which players returned to play during the study period) of interest included hamstring, quadriceps, adductor, and calf injuries. Acute and overuse index muscle injuries were analyzed separately across these locations. Noncontact time-loss subsequent injuries occurring in the same season as RTP were considered the event of interest. Built upon previously published methodology, the Kaplan-Meier survival function was used to estimate the cumulative hazard function, from which the continuous hazard function was derived. Results: During the 3 analyzed seasons, 374 acute and 304 overuse index injuries were identified at the 4 lower-extremity muscle groups. Observed noncontact subsequent-injury rates were 27.9%, 26.2%, 39.3%, and 21.2% after acute hamstring, quadriceps, adductor, and calf index injuries, respectively, and 28.1%, 27.7%, 27.2%, and 27.4% for overuse index injuries. While the time course of noncontact subsequent injury risk globally aligned with previous findings, differences between muscle groups were observed. Players returning from acute hamstring and adductor injuries showed elevated risk, which diminished within approximately 12 and 4 weeks, respectively, and leveled off thereafter. By contrast, a delayed risk peak around 3 weeks after RTP was found for overuse calf index injuries. Conclusion: This descriptive study further confirms the time-varying nature of subsequent injury risk for specific lower-extremity muscle injuries. The observed differences in risk trajectories across muscle groups highlighted the need for careful RTP decision-making based on each index injury.