Diurnal variations in ventilatory and cardiorespiratory responses to submaximal treadmill exercise in females

Diurnal variations in ventilatory and cardiorespiratory responses to submaximal treadmill exercise were analysed in 11 eumenorrhoeic women and in 10 women using monophasic oral contraceptives. Subjects performed submaximal treadmill exercise at three intensities averaging 7, 8, and 9 km . h(-1), each for 4 min at 0800, 1300 and 1700 hours, assigned randomly on 3 separate days. Rectal temperature was measured before (T-recb) and after (T-reca) exercise. Cardiac frequency (f(c)), ventilation ((V) over dot E), oxygen uptake ((V) over dot O-2), carbon dioxide output ((V) over dot CO2), and respiratory exchange ratio (R) were assessed in the last minute of each stage of the exercise. Both T-recb and T-reca increased from 0800 to 1700 hours (P < 0.001), For a given submaximal work rate, (V) over dot O-2 and (V) over dot CO2 were higher in the afternoon compared to the morning. Similarly, R was increased at 1700 hours compared to 0800 hours during the recovery period following exercise (P < 0.05). However, (V) over dot E did not vary significantly during the day at any of the running intensities. No significant interactions (group x time of day) were observed in any of the studied parameters. In contrast to ventilation, the (V) over dot O-2 and (V) over dot CO2 of the females during submaximal exercise were both affected by the time of day, without ally differences between eumenorrhoeic women and users of oral contraceptives.