Acute Effects of Contract-Relax PNF With and Without Neuromuscular Electrical Stimulation on Hamstring Flexibility and Jump Performance in Adolescent Soccer Players: A Randomized Controlled Trial.

Forty-seven adolescent soccer players (ages 11–18) from a single youth club were randomly assigned, stratified by baseline Popliteal Angle, sex, and age, to one of two groups. Both groups performed the same PNF-CR hamstring stretch: three cycles of a 10-second isometric contraction (heels pressed against a table) followed by a 30-second passive forward lean, done bilaterally with knees extended and hips/knees strapped. The only difference was the electrical stimulation applied during the contraction phase: the experimental group received motor-threshold NMES (strong, visible, pain-free contraction; mean ~37 mA), while the placebo group received sensory-threshold NMES (tingling only, no visible contraction; mean ~8 mA). The same device, electrodes, and placement were used in both arms. A 10-minute cycle-ergometer warm-up preceded testing. Outcomes — Sit-and-Reach, Popliteal Angle, and Countermovement Jump — were measured before and immediately after the session by a physiotherapist blinded to group allocation. Single session, single centre, registered at ClinicalTrials.gov (NCT07075497).

Primary outcome (between-group difference in hamstring flexibility): no significant time-by-group interaction for the Sit-and-Reach (p = 0.942) or the Popliteal Angle (p = 0.618). Both groups improved within-group: Sit-and-Reach improved significantly in both (p < 0.001 each); Popliteal Angle improved in the placebo group (p = 0.018) and the experimental group (p = 0.003). Secondary outcome (jump performance): no significant time effect (p = 0.498), no group effect (p = 0.793), and no interaction (p = 0.793) for Countermovement Jump height. NMES intensity was confirmed to differ substantially between groups (36.9/36.5 mA vs 8.5/8.0 mA, right/left), supporting the validity of the placebo condition. All three outcome measures showed excellent test reliability (ICC 0.930–0.981).

If you are considering adding NMES to a PNF-CR stretching session for an adolescent athlete to get an extra flexibility boost, this trial does not support that step: the motor-level stimulation produced no more range-of-motion gain than a placebo-level tingle on top of the same stretch. The PNF-CR protocol itself did produce a small, statistically significant acute increase in hamstring extensibility in both groups, so the stretch is doing the work. However, because there was no PNF-CR-only arm, you cannot conclude NMES is useless in isolation — only that it did not add to this specific protocol in a single acute session. For pre-training warm-ups in youth soccer, a standard PNF-CR routine appears sufficient without the added cost and complexity of an NMES unit.

No PNF-CR-only group was included, so the specific contribution of NMES (as opposed to no stimulation at all) cannot be isolated; the authors explicitly note this limits interpretation. The age range spanned 11 to 18 years and biological maturation was not assessed, so maturation-related differences in neuromuscular responsiveness could not be examined. The sample was too small to perform sex-specific analyses, even though both males and females were included. The study measured only acute, single-session effects; there is no information on whether repeated applications produce cumulative or sustained flexibility gains. The sensory-threshold placebo is not completely physiologically inert — low-intensity afferent input may still influence stretch tolerance — so the true 'no stimulation' effect remains unknown. Single centre, single club, single session; generalisability to other populations or repeated-use protocols is limited.

No competing interests declared. Funded by a grant from Universidad CEU-Cardenal Herrera (GIR25-41), the institution where the study was conducted.

Do not read the within-group flexibility improvements as evidence that NMES is the active ingredient. Both groups, including the one receiving only a placebo-level tingle, improved, and the between-group difference was null. The research question was whether motor-level NMES added benefit on top of PNF-CR, and the answer was no. Conversely, do not read the null as proof that NMES is useless in all contexts: without a no-stimulation group, you cannot separate the effect of the stretch from the effect of any electrical input, and the authors themselves flag this as a key interpretive limitation.