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

J Neurophysiol

Laboratory of Neuromuscular Biomechanics, School of Health and Sport Sciences, Chukyo University, Toyota, Japan.

Published: March 2024

AI Article Synopsis

  • The study explored how the time of day affects neuromuscular function by measuring neural excitability and muscle contraction properties in 15 young adults at four different times: morning, noon, evening, and night.
  • Results showed that motor unit firing rates significantly increased throughout the day, peaking at noon, but there were no changes in the recruitment thresholds or the evoked twitch torque across different times.
  • The findings indicate that, while neural excitability fluctuates with the time of day, these changes do not impact the muscle's contractile properties, suggesting a dissociation between neural and muscular responses within a 24-hour period.

Article 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 the firing properties of tracked motor units and electrically evoked twitch muscle contraction. In 15 young adults (22.9 ± 4.7 yr), 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 h. 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 ( < 0.01), but there were no significant differences in recruitment thresholds among the times of day ( > 0.05). Also, there were no significant effects of time of day on evoked twitch torque ( > 0.05). Changes in the motor unit firing rate and evoked twitch torque were not significantly correlated ( > 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. We investigated the variations of tracked motor unit firing properties and electrically evoked twitch contraction during the day within 24 h. The variation of motor unit firing rate was observed, and tracked motor unit firing rate increased at noon, in the evening, and at night compared with that in the morning. The variation in motor unit firing rate was independent of changes in twitch contraction. Motor unit firing rate may be affected by diurnal rhythms.

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Source
http://dx.doi.org/10.1152/jn.00368.2023DOI Listing

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