Effects of hyperthermia on the metabolic responses to repeated high-intensity exercise

In this study, we investigated the metabolic and performance responses to hyperthermia during high-intensity exercise. Seven males completed two 30-s cycle sprints (SpI and SpII) at an environmental temperature of 20.6 (0.3)°C [mean (SD)] with 4 min recovery between sprints. A hot or control treatment preceded the sprint exercise. For the hot trial, subjects were immersed up to the neck in hot water [43°C for 16.0 (3.2) min] prior to entering an environmental chamber [44.2 (0.8)°C for 30.7 (7.1) min]. For the control trial, subjects were seated in an empty bath (15 min) and thereafter in a normal environment [20.2 (0.6)°C for 29.0 (1.9) min]. Subjects' core temperature prior to exercise was 38.1 (0.3)°C in the hot trial and 37.1 (0.3)°C in the control trial. Mean power output (MPO) was significantly higher in the hot condition for SpI [683 (130) W hot vs 646 (119) W control (P < 0.025)]. Peak power output (PPO) tended to be higher in the hot trial compared with the control trial for SpI [1057 (260) W hot vs 990 (245) W control (P = 0.03, NS)]. These differences in power output were a consequence of a faster pedal cadence in the hot trial (P < 0.025). There were no differences in sprint performance in SpII in the hot trial compared to the control trial; however, MPO was significantly reduced from SpI to SpII in the hot condition but not in the control condition (P < 0.025). Plasma ammonia was higher in the hot trial at 2 min post-SpI [169 (65) μmol l^-1 hot vs 70 (26) μmol^-1 control (P < 0.01)], immediately and at 2 min post-SpII [231 (76) μmol l^-1 hot vs 147 (72) μmol l^-1 control (P < 0.01)]. Blood lactate was higher in the hot trial compared with the control trial at 5 min post-SpII (P < 0.025). The results of this study suggest that an elevation in core body temperature by 1°C can improve performance during an initial bout of high-intensity cycle exercise but has no further beneficial effect on subsequent power production following a 4-min recovery period.