Laboratory of Neuromuscular Biomechanics, School of Health and Sport Sciences, Chukyo University, Toyota, Japan.
Human Health Sciences, Graduate School of Medicine, Kyoto University, Kyoto, Japan.
J Neurophysiol. 2024 Mar 1;131(3):472-479. doi: 10.1152/jn.00368.2023. Epub 2024 Jan 24.
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.
与神经肌肉功能相关的内在因素随时间变化，但人类神经肌肉系统的神经和肌肉成分的昼夜节律仍不清楚。本研究旨在通过评估跟踪运动单位的放电特性和电诱发的抽搐肌肉收缩来研究时间对神经兴奋性和肌肉收缩特性的影响。在 15 名年轻成年人（22.9±4.7 岁）中，在早上（10:00）、中午（13:30）、晚上（17:00）和晚上（20:30）测量神经肌肉功能。在 24 小时内完成了四项测量。测量包括膝关节伸展的最大自主收缩（MVC）强度、股外侧肌在 ramp-up 收缩至 MVC 的 50%时的高密度表面肌电图（HDsEMG）记录，以及电刺激引起的膝关节伸肌的诱发抽搐扭矩。记录的 HDsEMG 信号被分解为单个运动单位的放电行为，并在一天中的不同时间跟踪相同的运动单位，计算募集阈值和放电率。检测到和跟踪的运动单位数量为 127 个。运动单位放电率从早上到中午、晚上和晚上显著增加（<0.01），但募集阈值在一天中的不同时间没有显著差异（>0.05）。此外，在诱发抽搐扭矩方面，时间对时间没有显著影响（>0.05）。运动单位放电率和诱发抽搐扭矩的变化没有显著相关性（>0.05）。这些发现表明，神经兴奋性可能受时间的影响，但它不会以昼夜方式伴随外周收缩特性的变化。我们在 24 小时内研究了一天中跟踪运动单位放电特性和电诱发抽搐收缩的变化。观察到运动单位放电率的变化，与早上相比，中午、晚上和晚上跟踪的运动单位放电率增加。运动单位放电率的变化与抽搐收缩的变化无关。运动单位放电率可能受昼夜节律的影响。
