Modulation of persistent inward currents in alpha motoneurons with joint angle depends on muscle length

During voluntary movement, activity of alpha motoneurons is modulated to account for changes in muscle force-generating capacity induced by variations in muscle length. To date, research has primarily focused on the modulation of ionotropic inputs, whereas the role of another key contributor to alpha motoneuron activity, persistent inward currents (PICs), has been largely overlooked. In this human study involving young male participants (n = 19), high-density surface electromyography signals were recorded from the gastrocnemius medialis and soleus muscles at different ankle positions, and subsequently decomposed into motor unit spiking activity. Metrics extracted from these spiking activities were used to estimate the respective contributions of PICs, neuromodulation, and inhibition to alpha motoneurons' activity. To differentiate the impact of muscle length from muscle tension, we compared voluntary contractions performed at both similar relative torque levels and similar absolute torque levels across different ankle positions. Our results revealed that PIC-induced prolongation of motor unit discharges was reduced at longer muscle lengths, accompanied by increased inhibition. This effect was consistent across both relative and absolute torque conditions, supporting the hypothesis that muscle length is the primary driver of PICs modulation with change of position. These findings suggest that PICs may serve as an additional mechanism-likely related to inhibitory inputs-to regulate alpha motoneuron activity, ensuring adaptation to changes in muscle length.NEW & NOTEWORTHY Little is known about how motoneuron activity is regulated to generate muscle contractions that adapt to variations in the muscle's force-generating capacity resulting from changes in muscle length. In this human study, we observed that the prolongation of discharge induced by persistent inward currents is modulated by muscle length, likely through the regulation of inhibitory inputs. Alongside the modulation of ionotropic inputs, this may serve as a regulatory mechanism to account for variations in muscle length.