Human motor unit discharge patterns reveal differences in neuromodulatory and inhibitory drive to motoneurons across contraction levels

Transformation of motor commands via a motor unit (MU) into mechanical actions of muscle fibres is a non-linear function influenced by ionotropic excitatory/inhibitory and neuromodulatory inputs. Neuromodulatory inputs facilitate dendritic persistent inward currents on motoneurons, which introduce non-linearities in MU discharge patterns allowing estimation of the structure of motor commands. We investigated the relative contribution of neuromodulation and the pattern of inhibition to human MU discharge patterns with increasing contraction force. In Experiment 1, we identified MU discharges in tibialis anterior, and vastus lateralis and medialis during isometric triangular dorsiflexion and knee extension contractions, respectively, up to 70% of maximal voluntary force (MVF). We quantified the onset-offset hysteresis (ΔF) and performed quasi-geometric analyses of MU discharge patterns to quantify the magnitude of non-linearity, and slopes of MU discharge patterns during the acceleration and rate attenuation regions. We show that ΔF increased, whereas discharge patterns became more linear and had lower slopes at greater contraction forces. Experiment 2 required participants to dorsiflex up to 70% MVF with either matched duration or rate of force increase to determine if these factors were confounding the modulation in MU discharge patterns across contraction forces. Though ΔF and the magnitude of non-linearity were influenced by contraction duration, the relative changes in these variables across contraction forces were similar to Experiment 1. The results suggest that neuromodulatory input and patterns of inhibition are uniquely shaped to support force increases across a large proportion of the motor pool’s recruitment range in three human lower limb muscles. ### Competing Interest Statement The authors have declared no competing interest.