These data confirmed the finding of a peak knee flexion moment and a peak of hip extension moment immediately after the foot strike by Mann and Sprague.43 and 44 These data, however, also demonstrated that knee and hip joint resultant powers were all positive when the peak knee flexion moment and peak hip extension moment occurred immediately after the foot strike. This suggests that the hamstring FG-4592 cell line muscle group is in a concentric contraction after the foot strike, in which a hamstring muscle strain injury is not likely to occur.45 The hamstring muscle length and EMG data demonstrated that hamstring muscles were in

eccentric contractions during the late swing phase before foot strike and late stance phase before takeoff.45 These data suggest that hamstring muscle strain injury may occur before foot strike and before takeoff. Two recent studies confirmed find more the data in the previous study.45 Thelen et al.46

also found a hamstring muscle eccentric contraction during the late swing phase of treadmill sprinting, and suggested that the potential for hamstring muscle strain injury existed during the late swing phase. Their results, however, did not show a hamstring muscle eccentric contraction during the stance phase as Wood45 did. Yu et al.47 analyzed the biomechanics of ground sprinting, and also found that the hamstring was in eccentric contraction during the late swing phase as well as during the late stance phase as reported by Wood. Yu et al.47 suggested that hamstring muscles were at the risk for strain injury during the late stance phase as well as during the late swing phase. However, hamstrings may have higher risk for strain injury during the late swing phase than during the late stance phase because the lengths of the hamstring muscles were significantly longer during the late swing phase than

during the late stance phase.47 Understanding risk factors for hamstring strain injury is critical for developing prevention and rehabilitation strategies. Many risk factors for hamstring muscle strain injury have been identified in the literature, however, only a few of these are evidence-based while the majority are theory-based. These risk factors can be categorized as modifiable Etomidate factors and non-modifiable factors.48 Modifiable risk factors include shortened optimum muscle length, lack of muscle flexibility, strength imbalance, insufficient warm-up, fatigue, low back injury, and increased muscle neural tension (Table 1). Non-modifiable risk factors include muscle compositions, age, race, and previous injuries (Table 1). Optimum muscle length is defined as the muscle length at which the muscle contractile element generates maximum force, which is similar to the muscle resting length.49 and 50 Brocket et al.