BIORHYTHMIC INFLUENCES ON FUNCTIONAL-CAPACITY OF HUMAN MUSCLE AND PHYSIOLOGICAL-RESPONSES

Previously, this laboratory has demonstrated that exhaustive aerobic e xercise performance is not subject to significant chronobiological var iation between 0800 and 2000 h, but certain physiological responses to maximal aerobic effort do fluctuate significantly within that time fr ame. Purpose: The purpose of the present investigation was to determin e whether muscle performance, and selected physiological responses to resistance exercise, was significantly influenced by time of day effec ts. Methods: Ten healthy, but untrained, men(21.1 +/- 0.6 yr, mean +/- SE) volunteered to participate in the study. In a balanced and random ized study design, each subject performed resistance exercise protocol s on an isokinetic dynamometer with maximal effort at 0800 h, 1200 h, 1600 h, and 2000 h. Selected physiological and hormonal data were reco rded before and immediately following the exercise stimulus. Results: The data demonstrated significant chronobiological oscillation in peak torque, average power, maximal work in a single repetition, and total work per set. Interestingly, this oscillation was manifested only at the fastest velocities of limb movement utilized. Pre- and postexercis e rectal temperature exhibited significant rime of day fluctuation, as did postexercise blood pressure. Plasma levels of testosterone and co rtisol also displayed significant biorhythmicity under both pre- and p ostexercise conditions. However, exercise induced responses (pre- to p ostexercise differences) of those steroid hormones did not exhibit sig nificant biorythmic variation. Although plasma concentrations of both testosterone and cortisol were highest at 0800 h, testosterone to cort isol ratios were greatest at 2000 h. Conclusions: In summary, these da ta suggest that maximal muscle performance does vary within the segmen t of the day when exercise typically occurs (0800-2000 h) bur that thi s variation is specific to speed of movement.