Motor unit discharge rate modulation during isometric contractions to failure is intensity and task dependent

Abstract

The nature of neuromuscular decrements associated with contractions to task failure is known to dependent on task demands. Task-specificity of the associated adjustments in motor unit discharge rate (MUDR) behaviour, however, remains unclear. This study examined MUDR adjustments during different submaximal isometric knee-extension tasks to failure. Participants performed a sustained and an intermittent task at 20 and 50% of maximal voluntary torque (MVT), respectively (Experiment 1). High-density surface electromyography signals were recorded from vastus lateralis (VL) and medialis (VM) and decomposed into individual MU discharge timings, with the identified MUs tracked from recruitment to task failure. MUDR was quantified and normalised to intervals of 10% of contraction time (CT). MUDR of both muscles exhibited distinct modulation patterns in each task. During 20% MVT sustained task, MUDR decreased until [~]50% CT, after which it gradually returned to baseline. Conversely, during the 50% MVT intermittent task, MUDR remained stable until [~]40-50% CT, after which it started to continually increase until task failure. To explore the effect of contraction intensity on the observed patterns, VL and VM MUDR was quantified during sustained contractions at 30 and 50% MVT (Experiment 2). During the 30% MVT sustained task, MUDR remained stable until [~]80-90% CT in both muscles, after which it continually increased until task failure. During the 50% MVT sustained task the increase in MUDR occurred earlier, after [~]70-80% CT. Our results suggest that adjustments in MUDR during submaximal isometric contractions to failure are task- and intensity-dependent.