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

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The additional value of neuromuscular electrical stimulation (NMES) during contract-relax proprioceptive neuromuscular facilitation (PNF-CR) stretching remains unclear, particularly in adolescent athletes. It is also unknown whether combining PNF-CR with motor-threshold NMES improves hamstring flexibility more than a sensory-threshold comparator without impairing vertical jump performance. # Purpose To compare the acute effects of PNF-CR combined with motor-threshold NMES versus sensory-threshold NMES on hamstring flexibility and to assess the impact of both interventions on vertical jump performance. # Study Design Randomized controlled trial. # Methods Forty-seven youth soccer players (mean age: 13.2 ± 1.8 years) were randomly assigned to one of two groups. Both groups received the same single-session PNF-CR stretching protocol. The experimental group (n = 23) received motor-level NMES during the contraction phases, whereas the placebo group (n = 24) received sensory-level NMES during the same PNF-CR protocol. Hamstring flexibility was assessed using the Popliteal Angle (PA) and Sit-and-Reach (SR) tests, while vertical jump performance was evaluated via the Countermovement Jump (CMJ) test. Analyses followed an intention-to-treat approach. Time, group, and time × group effects were examined using two-way repeated-measures ANOVA with Bonferroni-adjusted post hoc comparisons; CMJ was additionally analyzed using ANCOVA adjusted for baseline values. # Results Both groups showed statistically significant pre-post improvements in SR and PA scores. Specifically, PA decreased in the experimental group by 4.0° (95% CI: 1.4 to 6.5; p = 0.003) and by 3.1° (95% CI: 0.5 to 6.0; p = 0.018) in the placebo group. SR increased by 2.7 cm (95% CI: 1.9 to 3.6; p < 0.001) in the experimental group and 2.8 cm (95% CI: 1.9 to 3.7; p < 0.001) in the placebo group. No significant between-group differences were found for either flexibility outcome. Regarding CMJ, no significant intra- or intergroup changes were observed (p > 0.05). # Conclusion The addition of motor-threshold NMES to a PNF-CR stretching protocol does not yield superior acute flexibility gains. Moreover, neither intervention negatively affected jump performance in this athletic population, supporting their safe and potentially beneficial application in pre-training contexts for adolescent soccer players. # Level of Evidence 1b.

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