The main objective of this study is to compare the effect and the duration of the Neuromuscular Electrical Elongation (NMEE ) technique versus the PNF technique and versus a control group, on the extensibility of the hamstrings muscles.

Hypothesis:

NMEE of shortened hamstrings muscles in healthy subjects, compared with PNF and control group produces a significant increase in hamstrings extensibility.

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.

Inclusion Criteria:

• Healthy Volunteers
• Older than 18 years
• Bilateral shortened hamstrings muscles with a knee extension under 60º (TEAR, Neto et al., 2015) and a ROM under 80º in the hip (straight leg raise test angle, Espejo-Antúnez et al., 2015)

Exclusion Criteria:

• Participation in a muscular stretching program during the study
• Previous history of surgical intervention in the hip, tight or knee
• Muscle-skeletal pain or injury in the low back or inferior limbs
• Abdominal o spine surgery within three month before the study beginning.

Exit Criteria:

• Patient´s petition
• Change in the regular physical activity level during the study period
• Appearance of an injury that would contraindicate the use of stretching techniques