RESUMEN
BACKGROUND: Hamstring muscle injuries are common in the sports field, with lack of hamstring flexibility being a risk factor. Stretching the hamstring muscles is an important part of the training and rehabilitation programs used to prevent or treat injury and improve performance. We aim to compare the immediate and follow-up effect on hamstring muscle flexibility between 2 different stretching protocols, proprioceptive neuromuscular facilitation (PNF) stretching and PNF stretching combined with electrostimulation. A second aim is to determine whether physical activity level is related to flexibility. METHODS: Design of study: Single-blind, randomized controlled trial; Participants: 30 healthy volunteers (30 men, 18-39 years old); Intervention: A 6-session intervention program spanning over 2 weeks and 1-week and 3-week follow-up. The control group underwent PNF stretching with a voluntary contraction. The experimental group underwent PNF stretching but the muscle contraction was triggered with an electrical stimulation device (TENS); Main outcome measu: Hamstring muscles flexibility was assessed using the Modified Sit and Reach (MSR) and the Back Saver Sit and Reach (BSSR) tests. Physical activity level was evaluated with the short form of the International Physical Activity Questionnaire (IPAQ). RESULTS: Both groups showed an overall increase in hamstring muscle flexibility. However, when comparing flexibility gain between baseline and the end of the stretching program (session 6), and at the 1-week and 3-week follow-ups, only the experimental group showed significant differences. No correlation between the IPAQ score and the degree of hamstring muscle flexibility was observed. CONCLUSIONS: Hamstring muscle PNF stretching combined with electrical stimulation to trigger muscle contraction during the muscle contraction phase of the stretch achieves better flexibility results when compared to PNF alone.