MAXIMAL MUSCLE PERFORMANCE AND SELECTED PHYSIOLOGICAL VARIABLES ARE SENSITIVE TO BIORHYTHMIC INFLUENCES

Abstract

372 The purpose of this study was to determine whether chronobiological variation exists in maximal muscle performance and/or physiological variables during the segment of the day when exercise is typically performed. Ten healthy, but untrained males (21.1 ± 0.6 yrs, 181.7 ± 2.0 cm, 81.6 ± 3.3 kg; mean ± SE) served as subjects. In a randomized manner, each subject completed a regimen of alternating, maximal concentric muscle actions of knee extensors and flexors at 0800, 1200, 1600, and 2000 h on an isokinetic dynamometer. Each leg performed sets of 5 repetitions at 1.05, 1.57, and 2.09 rads/sec, followed by a 50 repetition set at 3.14 rads/sec. Performance variables quantified were peak torque, total work, maximal work for 1 repetition, average power, and fatigue. The physiological parameters of rectal temperature, heart rate, blood pressure, as well as plasma lactate, testosterone, and cortisol concentrations, were assessed pre- and post-exercise. One way, repeated measures ANOVA was performed on each variable. With the exception of fatigue, each muscle performance parameter demonstrated time of day fluctuation whereby improvements were noted from 0800 h to 2000 h. However, significant (p<0.05) differences were detected only at greater velocities of movement. Among physiological variables, post-exercise blood pressure displayed significant (p<0.05) oscillation(lowest at 0800 h), as did pre- and post-exercise rectal temperature (greatest at 2000 h) and plasma testosterone and cortisol (lowest at 2000 h). Time of day variation in muscle performance was highly correlated ( r=0.88) with that of rectal temperature. These data suggest that during the portion of the day that exercise generally occurs, chronobiological effects alter muscle performance and selected physiological variables.