The program is performed 2 times per week using resistance equipment in a physiotherapy clinic. Each session consists of on 2-legged loaded exercises for hamstring, quadriceps, gluteus maximun and core. The patients complete 3 or 4 sets in each exercise with a 2- to 3-minute rest between sets and a 5-minute rest period between the 4 exercises.

The number of repetitions decreases, and load gradually increases, every week. The repetitions and loads are as follows: 3 set of 12-repetition maximum (12RM), in week 1 and 2; 3 set of 10 RM, in week 3 and 4; 4 set of 10RM, in weeks 5 and 6; and 4 set of 8RM, in weeks 7 to 8.

Experimental: Inertial flywheel resistance training The program is performed 2 times per week using resistance equipment in a physiotherapy clinical. Inertial flywheel resistance is a type of strength training; which is based on the increasing demands on eccentric action (breaking) after a concentric action (acceleration), due to the inertial load caused during the return movement. The patients complete 16 repetition maximum (RM) with moment inertia 0.05 m² from week 1-2, 24 repetition maximum (RM) with moment inertia = 0.10 m² from week 3 to 4. 32 repetition maximum (RM) with moment inertia = 0.10 m² from week 5 to 6 and 32 repetition maximum (RM) with moment inertia = 0.13 m² in a flywheel hamstring curl devise.

To assess the effectiveness of the inertial flywheel resistance program in the architecture and function of the hamstrings post structural hamstring injury

To assess change in architecture and morphology of the hamstring at 8 week. It will be performed by a medical specialist in diagnostic imaging with a A GE Voluson 730 Expert model ultrasound machine , with 6-12 MHz linear transducer. Panoramic images of the long portion of the biceps femoris and semitendinosus will be taken from its proximal insertion, In each muscle, muscle thickness, fascicle length, penetration angle and physiological cross-sectional area will be evaluated.

To assess change in maximum isometric strength of hamstring in Newton (N). It will be measured in 3 repetitions with a hand dynamometer at two knee flexion angles.

To assess change in rate of force development of hamstring in N/seg at 8 week. it will be measured in 3 repetitions with a hand dynamometer at two knee flexion angles.

To assess the maximum eccentric strength of hamstring in Newton (N) from basiline. It will be measured in 3 repetitions with a hand held dynamometer.

Self-administered questionnaire. The hamstring injury detection test will be carried out to assess the previous injury, symptoms, function and quality of life of these players with the "Oslo hamstring injury screening questionnaire" questionnaire.This tool is divided in 23 items within 5 categories: symptoms, discomfort, pain depending on daily activity, sports and quality of life for injuries in right or left hamstrings