Effect of time of day on aerobic power in exhaustive high-intensity exercise.

The purpose of this study was to determine if there is an effect of time of day on aerobic power elicited during exhaustive high-intensity constant-power cycle ergometer exercise. Twenty-four college students (12 women and 12 men) each performed three exhaustive exercise tests (against 3.5 W.kg-1 for women and 4.0 K.kg-1 for men). The first test served as a learning trial. The second and third tests were scheduled at about 08:00 hrs (AM) and 16:00 hrs (PM) with the order of testing randomly assigned. Results from these two tests were compared for evidence of a time of day effect using a two-way ANOVA with repeated measures over time of day and with subjects nested in sex. Time to exhaustion was 9% greater (p < 0.01) in the PM (214 +/- 43 sec) than in the AM (196 +/- 38 sec). Peak VO2 was 7% higher (p < 0.01) in the PM (3.34 +/- 1.00 1.min-1) in the AM (3.11 +/- 0.98 1.min-1). The aerobic system responded 6% faster (p = 0.04) in the PM than in the AM: the time constant describing VO2 kinetics was 33.5 +/- 5.7 sec in the PM and 35.5 +/- 5.5 sec in the AM. However, there were no significant correlations (p > or = 0.20) between the AM-PM differences in time to exhaustion, time constant of the response, or peak VO2. The findings confirm previous reports of a time of day effect on time to exhaustion in high-intensity exercise. It was also concluded that there is a higher peak VO2 in the PM than in the AM. In addition, there is a time of day effect on VO2 kinetics, with the aerobic system responding faster in the PM than in the AM.