Diurnal Modulation of Persistent Inward Current Contribution to Spinal Motor Neuron Behaviour

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 behaviour 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 years) performed triangular isometric contractions at two time-points: early morning (7:00 – 8:30 a.m.) and late afternoon (5:00 – 7:30 p.m.). Two conditions were tested: (1) a relative condition, where the target force corresponded to 40% of the maximal voluntary force (MVF) measured during that session, and (2) an absolute condition, where the target force was 40% of the 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 ΔF, was significantly greater in the late afternoon in both the relative and absolute force conditions. The amplification effect of PICs, estimated by the acceleration phase of the discharge trajectory, was also higher in the late afternoon, but only in the relative force condition. No time-of-day differences were found for brace height, while attenuation was reduced in the late afternoon in the relative force condition. Collectively, these findings provide evidence for a diurnal modulation of the influence of PICs on motor neuron discharge behaviour, likely mediated by reduced inhibitory input in the late afternoon rather than by changes in neuromodulatory drive.