Circadian Variations in Anaerobic Threshold

This study aimed to determine whether certain respiratory and cardiovascular parameters associated with anaerobic threshold (AT), measured during graded exercise testing, occur at lower intensities in the morning than in the evening. Ten healthy Japanese men volunteered to participate in this study, which involved two conditions that were performed at different times of day: morning (M) exercise was performed between 9:00 and 10:00 a.m., and afternoon (A) exercise was performed between 4:00–6:00 p.m. After resting supine for 30 minutes, each subject performed graded cycle ergometer exercise testing comprising 90-second stages. Exercise intensity was initially 10 W and was increased by 10 W for each stage. Heart rate (HR), blood pressure (BP), absolute double product (DP), cardiac autonomic nervous system modulation, and ventilatory volume (VE) were measured during each exercise stage. Ventilatory threshold (VT), the double product breaking point (DPBP), and breaking point of the natural log of high frequency (ln HF) (HFBP) were reached at a lower exercise intensity in the M condition than in the A condition (p<.05). Values for VE at VT intensity, DP, HR, and systolic blood pressure (SBP) at DPBP were significantly lower in the M condition than in the A condition (p<.05). These data suggest that AT is reached at a lower intensity in the morning than in the afternoon, and that relative burden, as indicated by HR and SBP, is greater in the morning than in the afternoon. Exercise prescriptions that incorporate awareness of the circadian rhythms may prevent cardiac or cerebrovascular accidents during exercise.