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

Background Injury 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. Objectives To 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. Methods Media-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. Results 1623 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. Conclusions The 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. ### Competing Interest Statement Tim Meyer is the chairman of the medical committees of the German FA (DFB) and the European Football Confederation (UEFA). ### Funding Statement Guangze Zhang is supported by a PhD scholarship from the German Football Federation (DFB). No further external funding was used in this work. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study used (or will use) ONLY openly available human data that were originally located at: 1) https://www.kicker.de/; 2) ligainsider.de, transfermarkt.de; 3) https://www.transfermarkt.de I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study may be available upon reasonable request to the authors