Differences in persistent inward current contribution to spinal motor neuron behavior between early morning and late afternoon

Despite the critical role of persistent inward currents (PICs) in modulating motor neuron output, and thus neuromuscular performance, it remains unknown whether their contribution to motor neuron discharge behavior varies throughout the day. This study aimed to determine whether PIC-related effects on motor neuron activity during submaximal dorsiflexion tasks differ between the early morning and late afternoon. Eighteen healthy adults (4 females; 27.4 ± 5.6 yr) performed triangular isometric contractions at two randomized time-points on separate days: early morning (7:00-8:30 AM) and late afternoon (5:00-7:30 PM). Two conditions were tested: <i>1</i>) a relative condition, where the target force corresponded to 40% of the maximal voluntary force (MVF) measured during that session, and <i>2</i>) an absolute condition, where the target force was 40% MVF recorded during the first session. High-density surface electromyography signals were recorded from the tibialis anterior and decomposed into motor unit spike trains. The prolongation effect of PICs, estimated via Δ<i>F</i>, was significantly greater in the late afternoon in both the relative-absolute force conditions. The amplification effect of PICs, estimated by the acceleration phase of the discharge trajectory, was higher in the late afternoon, but only in the relative force condition. Brace height did not differ between morning and afternoon, but attenuation was lower in the late afternoon during the relative force condition. Collectively, these findings suggest a <i>time-of-day</i> modulation of PIC contribution to motor neuron discharge behavior, likely mediated by a change in inhibitory-excitatory balance between early morning and late afternoon rather than by changes in neuromodulatory drive.<b>NEW & NOTEWORTHY</b> Although human neuromuscular performance often peaks in the late afternoon, this pattern may not arise solely from peripheral mechanisms. This study provides evidence for a modulation in the contribution of persistent inward currents (PICs) to motor neuron discharge behavior between early morning and late afternoon. PIC-related estimates indicate a modest enhancement of motor neuron excitability in the late afternoon, likely explained by a change in the inhibitory-excitatory balance rather than by changes in neuromodulatory drive.