Effect of circadian rhythm of neuromuscular properties on muscle fatigue during concentric and eccentric isokinetic actions

This study was conducted to describe the time-of-day effect on muscle fatigue during both intermittent concentric (CE) and eccentric (EE) exercises. A second aim was to examine whether the time of day at which the exercise was performed influenced recovery of muscle strength. Ten male subjects performed CE and EE consisting of 5 sets of 10 maximal voluntary contractions (MVC) of the elbow flexors (angular velocity: 60°/s) at two different times (06: 00 h and 18: 00 h). Torque and Electromyographic activities (EMG) of the biceps brachii and triceps brachii were recorded before (Pre), during and immediately after (Post) each fatiguing task. Furthermore, strength recovery was measured 2 days (Post ₂₃), 3 days (Post ₃₃) and 7 days (Post ₇₃) after each fatiguing task. Results showed that during CE, torque variations were greater at 18: 00 h than at 06: 00 h. No time-of-day effect occurred during EE. Recovery time was faster in the morning than in the evening for isometric MVC only. To conclude, unlike the case in eccentric mode, mechanisms involved in concentric muscle action appear to be time-of-day dependent, with a greater muscle capacity in the late afternoon. Also, strength recovery is greater when fatiguing exercises are performed in the early morning than in the late afternoon.