Effectiveness of Two Stretching Techniques on Healthy Volunteers With Shortened Hamstrings

It is a double-blinded randomized clinical trial where subjects are healthy volunteers from the city of Zaragoza older than 18 years. All subjects will be injury free at the time of the study and must not have a history of knee or hip surgery. They must be physically active, but without engaging in competition-level sports, and must have flexibility deficits, as measured in the AKE test position and SLK test position (\<60º AKE; \< 80º SLK). There will be an NMEE Group (Neuromuscular Electrical Elongation), an PNF Group (Proprioceptive Neuromuscular Facilitation) and a Control Group (No Invervention). Intervention: NMEE technique will be performed by stretching the muscle till resistance and applying a bipolar interferential current to produce an involuntary contraction of the muscle which will be maintained 3 seconds. After that, the subject will contract the antagonist muscle (quadriceps) actively until he feels resistance again . Then, intensity of the electrical current is increased again. This cycle will be repeated 4 times. PNF technique will be performed by stretching the muscle till resistance is felt. Then the patient will have to make an isometric contraction of the muscle (against resistance) during 3 seconds. After that, the subject will contract the antagonist muscle (quadriceps) actively until he feels resistance again. This contract-stretch cycle will be repeated 4 times. Control group: This group will not receive a training program. In case of shortening, the assigned intervention will be performed until the values considered normal with a maximum of 8 stretching sessions (2 per week). Otherwise, if there is no shortening, the intervention will end at that moment. Assessment: To assess the effectiveness of these techniques several tests will be performed on the subjects. For hamstrings extensibility: * Active-Knee-Extension Test (AKE) results will bring information of the knee extension range of motion. * Straight-Leg-Raise Test (SLR) results will bring information of the hip flexion range of motion. For quadriceps extensibility: * Goniometric measure of knee flexion. * Distance from heel to gluteus using a tape measure. Viscoelasticity properties of hamstrings and quadriceps muscles: \- Myotonometric measurement using a myotonometer to obtain frequency, decrement (elasticity) and stiffness. For neural assessment: \- The Slump Test is a neural tension test used to detect altered neurodynamics or neural tissue sensitivity.