• After 3 weeks of Nordic hamstring exercise training, maximal knee-flexion strength increased alongside higher motor unit discharge rates and recruitment thresholds, indicating that early strength gains are primarily driven by neural adaptations. • After 9 weeks, motor units were recruited at higher torque levels despite discharge rates returning to baseline, likely reflecting increased force-producing capacity of the muscle fibres within each motor unit. • These findings suggest a time-dependent shift from predominantly neural to structural mechanisms underpinning strength gains during eccentric hamstring training. 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. 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. 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). 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.
Andrews et al. (Wed,) studied this question.