The time course of injury-risk after return-to-play in professional football

BackgroundInjury risk in professional football is increased in the weeks following return-to-play (RTP). However, the time course of injury risk after RTP (the hazard curve) as well as its influencing factors are largely unknown. This knowledge gap, which is arguably due to the volatility of instantaneous risk when calculated for short time intervals, impedes on informed RTP decision-making and post-RTP player management. ObjectivesTo characterize the hazard curve for non-contact, time-loss injuries after RTP in male professional football and investigate the influence of the severity of the index injury and playing position. MethodsMedia-based injury records from the first German football league were collected over four seasons as previously published. Time-to-event analysis was employed for non-contact, time-loss injury after RTP. The Kaplan-Meier survival function was used to calculate the cumulative hazard function, from which the continuous hazard function was retrieved by derivation. Results1623 observed and 1520 censored events from 646 players were analyzed. The overall shape of the hazard curve was compatible with an exponential decline of injury risk, from an approximately two-fold level shortly after RTP towards baseline, with a half-time of about four weeks. Interestingly, the peak of the hazard curve was slightly delayed for moderate and more clearly for severe index injuries. ConclusionsThe time course of injury risk after RTP (the hazard curve) can be characterized based on the Kaplan-Meier model. The shape of the hazard curve and its influencing factors are of practical as well as methodological relevance and warrant further investigation. Summary boxWhat is already known on this topic - As football players return to play after an injury, the risk of incurring a subsequent injury is high. With (event-free) time, this elevated risk returns to baseline. However, the shape of the risk-trajectory over time as well as its influencing factors are unknown. What this study adds - This study characterizes the time course of injury risk after RTP by providing a continuous hazard curve. Moreover, differences in risk trajectories across severities of index injury and playing positions were investigated. How this study might affect research, practice or policy - An evidence-informed estimate of the excess injury risk still remaining at a certain time-point after RTP is of obvious use for RTP decision making and post-RTP player management. Moreover, the continuous hazard curves enable informed specification of follow-up period in epidemiological studies and verification of the proportional hazard assumption in data analysis.