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 (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.NEW & NOTEWORTHY 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.
摘要：
与神经肌肉功能相关的内在因素是一天中的时间依赖性的，但是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的运动单位的放电特性和电诱发的抽搐肌肉收缩来研究一天中对神经兴奋性和肌肉收缩特性的影响。在15名年轻人（22.9±4.7岁）中，早上(10:00)测量神经肌肉功能，中午(13:30)晚上（17:00），晚上（20:30）在24小时内完成四次测量。测量包括膝关节伸展的最大自愿收缩（MVC）强度，在收缩至MVC的50％期间，从股外侧肌记录高密度表面肌电图（HDsEMG），并通过电刺激引起膝关节伸肌的抽搐力矩。记录的HDsEMG信号被分解为单个电机单元点火行为，并且在一天中的时间跟踪相同的电机单元，并计算了招募阈值和射击率。检测和跟踪的电机单元数量为127。从早晨到中午，电机单元的点火率显著增加，晚上和晚上（p<0.01），但不同时间的招募阈值无显著差异(p>0.05)。此外，每日时间对诱发抽搐扭矩无显著影响(p>0.05)。运动单位激发率和诱发抽搐扭矩的变化没有显着相关（p>0.05）。这些发现表明，神经兴奋性可能会受到一天中时间的影响，但它并不伴随着外周收缩特性的昼夜变化。
