Modulation of Hamstrings Neural Drive During Isometric Contractions of the Hip Extensors and Knee Flexors

The aim of the study was to compare motor-unit discharge characteristics across regions of the hamstring muscles during hip-extension and knee-flexion. Fourteen healthy adults (2 females) performed isometric contractions with the hip extensors and knee flexors at 10% and 40% of maximal voluntary contraction (MVC) torque. High-density surface electromyography was recorded from proximal and distal regions of biceps femoris and semitendinosus. Motor units were decomposed and tracked between tasks using a filter reapplication approach. The outcome measures included mean discharge rate (MDR), coefficient of variation for interspike interval (CoV_ISI), and standard deviation of the filtered cumulative spike train (SD of fCST). Task differences in common synaptic input were assessed at 10% MVC using within-muscle coherence derived from CSTs in the delta band. Maximal torque was ~24% higher in hip-extension than knee-flexion, accompanied by a ~31% increase in torque variability. Across all the identified motor units and those that were tracked, MDR (~5%-14%), CoV_ISI (~7%-11%) and SD of fCST (~10%-16%) were greater during hip-extension, whereas delta-band coherence was greater across muscles and regions during knee-flexion (p = 0.02-p < 0.001; d = 1.11-1.94). Moreover, proximal regions generally exhibited greater values for MDR (~11%) during knee-flexion, whereas modulation was greater in distal regions during hip-extension (~4%, p < 0.01). The neural-drive variability was greater proximally than distally for both tasks (~16%, p = 0.01). The neural control of motor units differs between proximal and distal regions of the hamstrings during hip-extension and knee-flexion. Hamstring testing and exercise selection need to consider the task and the muscle region being examined.