Journal of Sport and Health Science · Published 2026-04-27 · DOI 10.1016/j.jshs.2026.101142
Background: Neural adaptations play a key role in early strength gains from Nordic hamstring exercise (NHE) training, while architectural adaptations also contribute over time. The specific neural adaptations in biceps femoris long-head (BFlh) motor unit firing properties remain unexplored. This study aimed to determine changes in BFlh motor unit discharge rates, recruitment, and de-recruitment thresholds during submaximal isometric contractions after 3 and 9 weeks of NHE training. Methods: Nineteen recreationally active male participants completed a 9-week NHE training program with assessments at pre-training, 3 weeks, and post-training. Motor units were decomposed from high-density surface electromyography signals recorded during submaximal isometric knee flexion contractions. Motor units were tracked between time points by matching action potential waveforms. Changes in discharge rate, recruitment threshold, and de-recruitment threshold of matched units were compared across sessions using linear mixed-effects models. Results: NHE training significantly increased knee flexor strength by 6% at Week 3 (p = 0.015) and 17% at post-training (p < 0.001). Motor unit discharge rate was increased by 6% after 3 weeks (p = 0.015), but this was no longer elevated after 9 weeks of training (p = 0.365). The knee flexion torque at which motor units were recruited increased by 12% after 3 weeks (p < 0.001) and by 16% at post-training (p < 0.001). The torque at which motor units were de-recruited showed no significant change after 3 weeks (p = 0.515) but increased by 12% at post-training (p = 0.006). Conclusion: NHE training increased motor unit discharge rate and recruitment threshold after 3 weeks, indicating early neural adaptations. After 9 weeks of training, discharge rate adaptations plateaued, while motor units were both recruited and de-recruited at higher torque levels. These findings are consistent with a shift toward structural adaptations contributing to continued strength improvement.
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