Participants in this group performed a stretching protocol using contract-relax proprioceptive neuromuscular facilitation (crPNF). The participants are placed in a long sitting position with maximum knee extension possible until a moderate-strong stretch sensation is felt, without pain. The stretch lasts for 20 seconds, followed by a maximal voluntary isometric contraction of the hamstrings for 5 seconds. Three stretch-contraction cycles are completed. One researcher maintains the stretch position, while a second researcher controls the stretching and contraction times.

The participants follow the same stretching protocol but with neuromuscular electrical stimulation (NMES) added. Electrical stimulation (50Hz, 300 µs pulse width) is applied using an Enraf Nonius TensMed S82 device. Two 5x9 cm electrodes are placed on the hamstrings, and participants adjust the current to a moderate-strong, but painless, contraction. One researcher maintains the stretch, while another adjusts the current intensity and controls the timing of the stretching and contraction.

Neuromuscular electrical stimulation (NMES) is applied during an isometric contraction. The NMES uses a symmetrical biphasic rectangular pulse (50Hz frequency and 300 µs phase width) delivered through an Enraf Nonius TensMed S82 electrostimulator. Two 5x9 cm electrodes are placed on the hamstrings, and participants adjust the current to a moderate-strong, yet painless, contraction level. During the intervention, one researcher maintains the stretch position while another researcher controls the current intensity and monitors the timing of both stretching and contractions.

The Popliteal Angle Test is validated for measuring hamstring extensibility. Participants are placed in the supine position. A researcher holds the hip at a 90° flexion and passively extends the knee until the participant feels a strong stretch without pain. The examiner records the knee extension degrees at that moment using an inclinometer. The test is repeated three times, and the average is obtained. A full knee extension corresponds to a value of 0 degrees, and a higher number of degrees indicates greater hamstring shortening.

The Sit & Reach Test has high intra-examiner reliability and is validated for measuring hamstring extensibility. For its execution, participants are placed in a long sitting position with the soles of their feet against the base of the measurement box. Keeping their knees extended, they reach for the maximum possible distance in the box with their fingers. The test is repeated three times, and the average is obtained. Values are recorded in centimeters, where a greater distance indicates a higher degree of hamstring extensibility.

The Counter Movement Jump is validated and standardized in adolescents for measuring jumping ability. The instructions by Petrigna et al. are followed to perform the jump (Figure 2C). The My Jump App is used, which has high reliability and validity for measuring vertical jump height in centimeters. A greater height indicates a greater jumping ability.