PURPOSE: There is some evidence that both aerobic and anaerobic exercise performances have a diurnal variation, with morning lows and evening peaks in exercise performance a common finding in many studies. It has been demonstrated that hyperthermia before exercise attenuates subsequent exercise performance in the heat. However, it has not been investigated whether hyperthermia before exercise influence the time-of-day effect on exercise performance in the heat. Therefore, the aim of this study was to examine the effects of passive hyperthermia on the relationship between circadian rhythm and both aerobic and anaerobic exercise performances in the heat.
METHODS: Eight male volunteers completed four trials, including anaerobic and aerobic cycling performance tests, in a climatic chamber (30°C, 50% RH) at two different times-of-day: 08:00 (a.m.) and 17:00 (p.m.) h. An anaerobic performance test consisted of 10 sec maximal sprints at 5 kp to determine the maximal anaerobic power. An aerobic performance test consisted of cycling at 80% maximum oxygen uptake until exhaustion to determine exercise time to exhaustion. Participants cycled after: a 30 min seated rest at a.m. (AR) and p.m. (PR), and a 30 min water immersion at 40°C to the upper chest at a.m. (AH) and p.m. (PH). Rectal temperature, skin temperature, heart rate, skin blood flow and blood pressure were recorded at rest and during exercise.
RESULTS: Rectal temperature at the start of exercise was higher in AH and PH than in AR and PR (AR 36.8 ± 0.4°C; AH 37.9 ± 0.2°C; PR 37.3 ± 0.3°C; PH 38.0 ± 0.2°C; p < .0001). Maximal anaerobic power was not different between trials (AR 11.5 ± 1.7 W/kg; AH 12.5 ± 1.9 W/kg; PR 11.7 ± 1.4 W/kg; PH 12.0 ± 1.9 W/kg; p = .24). Exercise time to exhaustion was reduced in AH (15 ± 8 min) and PH (24 ± 9 min) than in AR (39 ± 16min; p < .05), and in AH than in PR (43 ± 24 min; p < .05). At the point of exhaustion, rectal temperature, mean skin temperature, heart rate and cutaneous vascular conductance were not different between trials.
CONCLUSIONS: This study demonstrates that passive hyperthermia before exercise elicits significant reductions in aerobic exercise performance in the heat at a.m., but this response was not observed at p.m. However, no statistically difference in anaerobic exercise performance is apparent at both a.m. and p.m.