Background: Hamstring strain injury (HSI) is the single most commonly (~12%) sustained injury in professional football. It is suggested HSIs extending into the intramuscular tendon are more prone to reinjury, resulting in greater time-loss and delayed time to return to play (TtRtP). Currently, there is a lack of evidence regarding the effects of HSI with and without intramuscular involvement as well as the impact of factors such as removal of a player and grade. Objectives: 1) Describe the number of HSIs using the British Athletics Muscle Injury Classification (BAMIC); 2) Determine if intramuscular tendon HSI results in extended TtRtP and higher reinjury risk; 3) Determine the predictors of TtRtP and reinjury. Methods: A retrospective study in one English Premier League football club over four seasons. Players included underwent an MRI within 7 days of injury and were graded by a Radiologist using the BAMIC (0a-4). TtRtP and injury recurrence rates were recorded along with information on whether the players were removed from play. Data were analysed using a Kruskal-Wallis test and linear regression. Results: Twenty-nine HSI across 24 players (age = 26.1 ± 3.8 years) were recorded over 4 seasons. There was a significant difference in TtRtP between grades 1a and 2c (P = 0.027). No significant difference was observed in TtRtP between 2b and 2c and no greater risk of reinjury. Grade of HSI (P = < 0.000) and removal of the player (P = 0.001) were significant predictors of TtRtP. An increase in grade of HSI resulted in an additional 4 days TtRtP and removal from the field of play (P = 0.001) resulted in an additional 10 days. Conclusion: HSIs extending into the intramuscular tendon (2b cf. 2c) do not influence TtRtP or reinjury, however TtRtP is affected by the removal of a player and overall grade
