Time-of-day effects on motor unit firing and muscle contractile properties in humans

Abstract

Intrinsic factors related to neuromuscular function are time-of-day dependent, but diurnal rhythms in neural and muscular components of the human neuromuscular system remain unclear. The present study aimed to investigate the time-of-day effects on neural excitability and muscle contractile properties by assessing firing properties of tracked motor units and electrically evoked twitch muscle contraction. In fifteen young adults (22.9±4.7 years), neuromuscular function was measured in the morning (10:00), at noon (13:30), in the evening (17:00), and at night (20:30). Four measurements were completed within 24 hours. The measurements consisted of maximal voluntary contraction (MVC) strength of knee extension, recording of high-density surface electromyography (HDsEMG) from the vastus lateralis during ramp-up contraction to 50% of MVC, and evoked twitch torque of knee extensors by electrical stimulation. Recorded HDsEMG signals were decomposed to individual motor unit firing behaviors and the same motor units were tracked among the times of day, and recruitment thresholds and firing rates were calculated. The number of detected and tracked motor units was 127. Motor unit firing rates significantly increased from morning to noon, evening and night (p<0.01), but there were no significant differences in recruitment thresholds among the times of day (p>0.05). Also, there were no significant effects of time-of-day on evoked twitch torque (p>0.05). Changes in the motor unit firing rate and evoked twitch torque were not significantly correlated (p>0.05). These findings suggest that neural excitability may be affected by the time-of-day, but it did not accompany changes in peripheral contractile properties in a diurnal manner.