Reductions in motoneuron excitability during sustained isometric contractions are dependent on stimulus and contraction intensity

Cervicomedullary stimulation provides a means of assessing motoneuron excitability. Previous studies demonstrated that during low-intensity sustained contractions, small cervicomedullary evoked potentials (CMEPs) conditioned using transcranial magnetic stimulation (TMS-CMEPs) are reduced, whilst large TMS-CMEPs are less affected. Since small TMS-CMEPs recruit motoneurons most active during low-intensity contractions while large TMS-CMEPs recruit a high proportion of motoneurons inactive during the task, these results suggest that reductions in motoneuron excitability could be dependent on repetitive activation. To further test this hypothesis, this study assessed changes in small and large TMS-CMEPs across low- and high-intensity contractions. Twelve participants performed a sustained isometric contraction of the elbow flexor for 4.5 min at the electromyography (EMG) level associated with 20% maximal voluntary contraction force (MVC; low-intensity) and 70% MVC (high-intensity). Small and large TMS-CMEPs with amplitudes of ~15 and ~50% Mmax at baseline, respectively, were delivered every minute throughout the tasks. Recovery measures were taken at 1, 2.5 and 4-min post-exercise. During the low-intensity trial, small TMS-CMEPs were reduced at 2-4 min (p≤0.049) by up to −10% Mmax, while large TMS-CMEPs remained unchanged (p≥0.16). During the high-intensity trial, small and large TMS-CMEPs were reduced at all time-points (p