Differential Effects of Hip- and Knee-Based Stretches on Torque and Motor Unit Activity in the Hamstrings

Purpose: The purpose of the study was to compare the acute effects of hip- and knee-based static stretches on hamstrings torque and motor unit discharge characteristics. Methods: Fourteen healthy young adults (2 females) completed three experimental sessions that comprised hip- and knee-focused stretching exercises or a control condition. Participants performed maximal voluntary contractions as well as submaximal contractions at 10% and 40% of maximal torque during a hip-extension and knee-flexion task. High-density surface electromyography recordings were obtained from proximal and distal regions of semitendinosus and biceps femoris. Motor unit discharge times were identified and a subset was tracked before and after stretching using a filter reapplication approach. Mean discharge rate (MDR), coefficient of variation for interspike interval (variability in discharge times), and standard deviation of the filtered cumulative spike train (neural-drive variability) were analyzed. Results: Analysis of variance indicated hip-stretching resulted in greater reduction in maximal hip-extension torque compared with knee-stretching (~19% vs ~8%), whereas knee-stretching led to a larger reduction in maximal knee-flexion torque (~16% vs ~8%;p<0.001). Torque fluctuations increased (~28–32%) in the task most closely associated with the stretching modality. Linear mixed-effects models showed that both stretching conditions increased MDR (~5–20%), variability in discharge times (~10–25%), and neural drive variability (~20–45%), with no significant differences between muscles (p>0.05). Notably, hip-stretching induced greater increases in MDR in proximal regions (~10–18%), whereas knee-stretching produced larger increases distally (~12–25%). These regional patterns were also observed in the tracked motor-unit subset. Conclusions: The acute neuromuscular effects of prolonged static stretching depend on joint configuration and differ between muscle regions. Thus, hip- and knee-stretching exercises should not be treated as equivalent interventions.