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

Brownstein, Callum G., Espeit, Loïc, Royer, Nicolas, Ansdell, Paul, Škarabot, Jakob, Souron, Robin, Lapole, Thomas and Millet, Guillaume Y. (2021) Reductions in motoneuron excitability during sustained isometric contractions are dependent on stimulus and contraction intensity. Journal of Neurophysiology. ISSN 0022-3077 (In Press)

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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<0.01) by up to −14% and −33% Mmax, respectively, and remained below baseline during all recovery measures (p≤0.02). TMS-CMEPs were unchanged relative to baseline during recovery following the low-intensity trial (p≥0.24). These results provide novel insight into motoneuron excitability during and following sustained contractions at different intensities, and suggest that contraction-induced reductions in motoneuron excitability depend on repetitive activation.

Item Type: Article
Uncontrolled Keywords: cervicomedullary evoked potentials, isometric exercise, motoneuron excitability
Subjects: B100 Anatomy, Physiology and Pathology
C600 Sports Science
Department: Faculties > Health and Life Sciences > Sport, Exercise and Rehabilitation
Depositing User: Elena Carlaw
Date Deposited: 06 Apr 2021 14:14
Last Modified: 06 Apr 2021 14:15

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