Background Hamstring strain injuries (HSI) are characterized by a very high recurrence rate. Despite indications from the field that deficits in strength endurance and fatigability play a role in (re-)injury, screening for these in Return To Sport (RTS) decision making is still not common and requires further investigation. Objective To justify hamstring strength endurance and fatigability at time of RTS as potential risk factors for re-injury. Design Observational. Setting Academic sports rehabilitation centre. Patients (or Participants) 13 recently injured HSI athletes (M/F;18-35yr) competitive in sprint sports, at time of RTS, and 17 healthy population-matched controls. Interventions (or Assessment of Risk Factors) All participants underwent a RTS test battery including clinical and functional tests. For this study, single leg hamstring bridge endurance test (SLHB) and maximal isometric hamstring strength test (MIHS) in inner, mid and outer range, using hand-held dynamometry, were investigated. MIHS was retested after a shuttle-run until exhaustion fatigue protocol. Differences between injured (INJ) and uninjured (UNINJ) limb, and differences between injured and healthy athletes (CON) were tested using Students` t-tests and mixed model ANOVA. Main Outcome Measurements SLHB [number of repetitions]; MIHS [Newton]. Results Concerning SLHB, injured athletes showed a clinically relevant deficit in hamstring strength endurance compared to their healthy peers, both in their injured and un-injured leg (INJ=19.0±5.0; UNINJ=18.9±5.4; CON=24.5±5.3; t=2.6&2.7; p=0.01&0.01respectively). For MIHS, a main group effect was found for outer range (INJ=347.5±67.3; CON=394.8±63.8; F=6.1;p=0.02), revealing that isometric strength was significantly worse in the injured leg compared to healthy controls. An overall fatigue effect was found (F=7.8; p=0.01), adding to the injured athletes` baseline deficit in MIHS performance. Conclusions Hamstring strength endurance and fatigability were both justified as potential risk factors of re-injury and should be integral parts of future rehabilitation programmes and prognostic screening.
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