CHRONOBIOLOGICAL EFFECTS ON EXERCISE PERFORMANCE AND SELECTED PHYSIOLOGICAL-RESPONSES

Previous studies investigating the impact of circadian rhythms on phys iological variables during exercise have yielded conflicting results. The purpose of the present investigation was to examine maximal aerobi c exercise performance, as well as the physiological and psychophysiol ogical responses to exercise, at four different intervals (0800 hours, 1200 hours, 1600 hours, and 2000 hours) within the segment of the 24- h day in which strenuous physical activity is typically performed. Ten physically fit, but untrained, male university students served as sub jects. The results revealed that exercise performance was unaffected b y chronobiological effects, Similarly, oxygen uptake, minute ventilati on and heart rate showed no time of day influences under pre-, submaxi mal, and maximal exercise conditions. Ratings of perceived exertion we re unaffected by time of day effects during submaximal and maximal exe rcise. In contrast, rectal temperature exhibited a significant chronob iological rhythm under all three conditions. Under pre-and submaximal exercise conditions, significant time of day effects were noted for re spiratory exchange ratio, while a significant rhythmicity of blood pre ssure was evident during maximal exercise. However, none of these phys iological variables exhibited significant differential responses (perc ent change from preexercise values) to the exercise stimulus at any of the four time points selected for study. Conversely, resting plasma l actate levels and lactate responses to maximal exercise were found to be significantly sensitive to chronobiological influences. Absolute po st-exercise plasma norepinephrine values, and norepinephrine responses to exercise (percent change from pre-exercise values), also fluctuate d significantly among the time points studied. In summary, these data suggest that aerobic exercise performance does not vary during the tim e frame within which exercise is normally conducted, despite the fact that some important physiological responses to exercise do fluctuate w ithin that time period.