Participants sat in long-sitting with maximal knee extension and pelvic anteversion controlled by elastic straps over the pelvis and distal thigh. Two 5×5 cm self-adhesive electrodes were placed on the hamstrings: one two fingerbreadths below the gluteal fold (proximal) and one over the distal third of the muscle belly. Sham NMES delivered biphasic symmetric rectangular pulses (50 Hz, 250 µs) at the sensory threshold (minimal intensity eliciting only a slight tingling without visible muscle response). Each of three cycles comprised a 10 s maximal voluntary isometric hamstring contraction ("Squeeze…!") against the fixed table, immediately followed by a 30 s passive stretch to a moderate-strong but non-painful tension ("Stretch…!"). A primary researcher ensured consistent limb positioning and strap tension; a secondary researcher monitored timing and cued contractions by lightly supporting the heels.

Participants sat in long-sitting with maximal knee extension and pelvic anteversion controlled by elastic straps over the pelvis and distal thigh. Two 5×5 cm self-adhesive electrodes were placed on the hamstrings: one two fingerbreadths below the gluteal fold (proximal) and one over the distal third of the muscle belly. NMES delivered biphasic symmetric rectangular pulses (50 Hz, 250 µs) at each participant's maximal motor threshold without discomfort or pain; the resulting milliamperage was recorded for each leg and used during all contraction phases. Each of three cycles comprised a 10 s maximal voluntary isometric hamstring contraction ("Squeeze…!") with NMES applied at the recorded intensity, immediately followed by a 30 s passive stretch to a moderate-strong but non-painful tension ("Stretch…!"). A primary researcher ensured consistent limb positioning and strap tension; a secondary researcher monitored timing and cued contractions by lightly supporting the heels.

NMES is applied during each 10-second isometric hamstring contraction using an Enraf Nonius TensMed S82 stimulator delivering biphasic, symmetric rectangular pulses at 50 Hz with a 250 µs pulse width. Two 5 × 5 cm self-adhesive electrodes are placed on the hamstrings-one approximately two finger-breadths below the gluteal fold and the other over the distal third of the muscle belly. Stimulation intensity is individually titrated to each participant's maximal motor threshold-eliciting a strong, yet painless, contraction-and the exact milliamperage is recorded for each leg. Elastic straps over the pelvis and distal thigh maintain pelvic anteversion and knee extension. During each session, one researcher secures limb positioning while a second adjusts and logs NMES intensity and cues the timing of the 10 s contraction and ensuing 30 s passive stretch.

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. 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.