Background Hamstring strain injuries (HSI) are the most prevalent injuries in team sports and occur more frequently in the fatigued phases of a match. In the search for interindividual factors determining increased muscle fatigue-and thus HSI risk-the muscle fibre type composition (fast-twitch vs slow-twitch muscle typology) is a likely candidate.

Objective The objective of this study was to identify whether the muscle fibre typology is a risk factor for HSI.

Design Prospective cohort study over 3 consecutive seasons

Setting First division Belgian (discovery cohort) and UK (replication cohort) soccer.

Patients (or Participants) Players had to be adult out-field players, who did not sustain a HSI in the 6 months before the scan. In total, 96 participants finished the study.

Interventions (or Assessment of Risk Factors) The muscle fibre typology was non-invasively estimated using proton magnetic resonance spectroscopy. During the 1–3 years follow-up period, all sustained injuries were provided by the medical team.

Main Outcome Measurements A cox regression model was used to identify if muscle fibre typology is a risk factor to sustain a HSI. Receiver operating characteristic (ROC) curves were analysed.

Results Belgian players with a fast typology display a 6.7-fold higher risk than slow typology players in experiencing a new HSI (14 HSI; p=0.024). This was independently confirmed in the UK cohort (5.0-fold risk; 13 HSI; p=0.023). ROC curve analysis revealed an AUC of 0.63 in the total cohort and 0.76 in the discovery cohort, indicating poor to fair sensitivity and specificity.

Conclusions The muscle fibre typology is discovered as a novel and important risk factor to sustain a HSI in professional soccer. This opens a new perspective on the mechanism of HSI, possibly related to more pronounced muscle fatigue in fast typology players. Future personalised prevention strategies could take these interindividual muscle composition differences into account.