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)degreesC [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 [43degreesC for 16.0 (3.2) min] prior to entering an environmental chamber [44.2 (0.8)degreesC 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)degreesC for 29.0 (1.9) min]. Subjects' core temperature prior to exercise was 38.1 (0.3)degreesC in the hot trial and 37.1 (0.3)degreesC 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) mumol l(-1) hot vs 70 (26) mumol l(-1) control (P<0.01)], immediately and at 2 min post-SpII [231 (76) mumol l(-1) hot vs 147 (72) mumol 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 1degreesC 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.