Background The hamstring muscles are especially vulnerable during the late swing phase of sprinting, where the simultaneous actions of hip flexion and knee extension actively stretch the hamstrings to greater lengths.

Objective To assess whether a functional strengthening program (i.e. eccentric, plyometric and acceleration exercises) increased optimum lengths of the knee flexors in soccer players.

Design Longitudinal, unblinded randomized controlled study.

Setting Amateur male soccer players from nine Spanish teams of the same competitive level.

Participants 52 players were assigned to an exercise group (EG) or a control group (CG). Over the study, 8 athletes (6 from CG and 2 from EG) were dropped due to personal reasons, contracting by other clubs or injuries (non-hamstring strain injuries). After these exclusions, 25 players constituted the EG (22.7±4.8 years; 71.2±8.4 kg; 174.5±5.6 cm) and 19 the CG (21.8±2.5 years; 71.1±6.5 kg; 176.8±6.2 cm).

Interventions Both groups performed their habitual soccer training throughout the study (with a similar physical load profile). Additionally, the EG performed a functional strengthening preventive program twice per week for seven weeks.

Main outcome measurements The optimum angle of peak torque (and therefore optimum length) was calculated through of a 4th order polynomial curve. Previously, each subject performed 6 maximum concentric repetitions on an isokinetic dynamometer at angular velocity of 60°/s.

Results After the seven-week period, the optimum length of the EG during knee flexion was significantly greater (i.e. the optimum angle of peak torque was smaller; 39.85±8.42 pre-test vs 35.26±7.50 pos-test). Similarly, the EG (35.26±7.50) showed a significantly great optimum length in the pos-test than the CG (40.89±8.71).

Conclusions The prevention program proposed increases the optimum length of the knee flexors, promoting the development of strength at longer muscle lengths.