Arm locomotor behaviour affects motor unit discharge characteristics in the stationary leg

Persistent inward currents (PICs), which are facilitated by monoaminergic inputs such as serotonin (5-HT), amplify synaptic drive and strongly influence motoneuron excitability. Although rhythmic locomotor activity increases serotonergic drive in animal models, its effects on intrinsic motoneuron properties in humans remain unclear. We examined whether rhythmic arm cycling alters motoneuron excitability of the non-exercising tibialis anterior during submaximal contractions. Twelve healthy adults (8 males, 4 females) performed triangular isometric dorsiflexion contractions at 25% and 50% MVC under four conditions: resting arm (CONTROL), finger tapping (TAP), arm cycling at 50-60 RPM (LOW), and arm cycling at 80-90 RPM (HIGH). Motor unit activity was identified from high-density surface electromyography that was decomposed into spike trains. Recruitment thresholds of identified and tracked motor units were consistent across conditions, but {Delta}F (i.e., an estimate of the PIC-related contributions to motor unit discharge) decreased during high-cadence arm cycling at stronger contraction intensities, which may reflect either reduced neuromodulation and/or increased or altered patterns of inhibition. In contrast, ascending discharge rate modulation deviated from linearity to a greater extent (i.e., brace height was larger) during both low- and high-cadence cycling, indicating greater neuromodulatory influence on the ascending discharge rate pattern. Self-sustained discharge was also elevated during cycling tasks, reflecting prolonged motor unit discharge. Taken together, these findings suggest that rhythmic activity of the arms modulates the discharge characteristics of motoneuron pools in the legs via unique combinations of excitatory, neuromodulatory and inhibitory inputs, which advances our understanding on the mechanisms of interlimb neural coupling.