Diurnal variation in the control of ventilation in response to rising body temperature during exercise in the heat

Abstract

We investigated whether heat-induced hyperventilation during exercise is affected by time of day, as diurnal variation leads to higher core temperatures in the evening. Nineteen male subjects were divided into two experiments (protocol 1, n = 10 and protocol 2, n = 9). In protocol 1, subjects performed cycle exercise at 50% peak oxygen uptake in the heat (37°C and 50% RH) in the morning (0600) and evening (1800). Results showed that baseline resting and exercising esophageal temperature (Tes) were significantly (0.5°C) higher in the evening than morning. Minute ventilation (V̇e) increased from 54.3 ± 7.9 and 54.9 ± 6.8 l/min at 10 min to 71.4 ± 8.1 and 76.5 ± 11.8 l/min at 48.5 min in the morning and evening, respectively (both P < 0.01). Time of day had no effect on V̇e (P = 0.44). When V̇e as the output response was plotted against Tes as thermal input, the Tes threshold for increases in V̇e was higher in the evening than morning (37.2 ± 0.7 vs. 36.6 ± 0.6°C, P = 0.009), indicating the ventilatory response to the same core temperature is smaller in the evening. In protocol 2, the circadian rhythm-related higher resting Tes seen in the evening was adjusted down to the same temperature seen in the morning by immersing the subject in cold water. Importantly, the time course of changes in V̇e during exercise were smaller in the evening, but the threshold for V̇e remained higher in the evening than morning (P < 0.001). Collectively, those results suggest that time of day has no effect on time course hyperventilation during exercise in the heat, despite the higher core temperatures in the evening. This is likely due to diurnal variation in the control of ventilation in response to rising core temperature.

Higher evening metabolic responses contribute to diurnal variation of self-paced cycling performance

“…The present findings are in accordance with other studies showing a higher [La] and V̇O 2 in the evening compared to the morning during a 1000 m cycling time trial [ 19 ], endurance exercise above the ventilatory threshold [ 38 , 42 ] or during the YoYo intermittent recovery test [ 7 , 11 ]. However, the present results are in contrast with several previous studies that failed to provide evidence for significant diurnal variation of V̇O 2 and/or [La] during submaximal [ 12 , 18 , 40 ], maximal [ 7 , 21 , 32 , 41 ] or self-paced [ 9 , 19 , 35 ] exercises. These results could be explained by an achieved V̇O 2max , [La] and RPE at different TOD by different athletes [ 21 ].…”