Circadian rhythm in the torque developed by elbow flexors during isometric contraction. Effect of sampling schedules

Time-dependent changes in elbow flexion torque have been documented according to two different sampling schedules. Seven physical education students took part in the first series of experiments, and 7 other similar subjects in the second. In both sets of experiments, the subjects performed isometric contractions: maximal and submaximal at 90 degrees in the first experiments and maximal at different angular positions in the second. After a 30-minute rest period, the torque developed was measured at 00:00, 06:00, 09:00, 12:00, 15:00, 18:00, and 21:00 h on the day of the experiment. These subjects remained in the laboratory for 24 h. In the second series of experiments, the torque developed was measured at 01:00, 05:00, 09:00, 13:00, 17:00, and 21:00 h over the subsequent 6 days with only one test session per day. In this case, there was an interval of 20 h between two successive test sessions. In the first experiment, a significant time-of-day effect was observed for the torque of the elbow flexors under isometric conditions with an acrophase at 17:58 h. The 24 h normalized mean score was 92.85% with an amplitude of 7.63% of the daily mean. In the second series of experiments, there was evidence of a circadian rhythm in the torque developed by the elbow flexors at every angle position, especially at 90 degrees, the angle investigated in the first set of experiments. The peak torque was calculated to have occurred at 17:55 h. The amplitude of the rhythm was equal to 6.99% of the daily mean. There were no statistically significant differences in the characteristics of the circadian rhythm observed between the two experimental designs. We concluded that an experiment extending over several days could be employed to evaluate circadian rhythms in muscular activity reliably.