Work Rate during Self-paced Exercise is not Mediated by the Rate of Heat Storage

PURPOSE: To date, there have been mixed findings on whether greater anticipatory reductions in self-paced exercise intensity in the heat are mediated by early differences in rate of body heat storage. The disparity may be due to an inability to accurately measure minute-to-minute changes in whole-body heat loss. Thus, we evaluated whether early differences in rate of heat storage can mediate exercise intensity during self-paced cycling at a fixed rate of perceived exertion (RPE of 16; hard-to-very-hard work effort) in COOL (15°C), NORMAL (25°C) and HOT (35°C) ambient conditions.

METHODS: On separate days, nine endurance-trained cyclists exercised in COOL, NORMAL and HOT conditions at a fixed RPE until work rate (measured after first 5-min of exercise) decreased to 70% of starting values. Whole-body heat loss and metabolic heat production were measured by direct and indirect calorimetry respectively.

RESULTS: Total exercise time was shorter in HOT (57±20 min) relative to both NORMAL (72±23 min, P=0.004) and COOL (70±26 min, P=0.045). Starting work rate was lower in HOT (153±31 W) compared to NORMAL (166±27 W, P=0.024) and COOL (170±33 W, P=0.037). Rate of heat storage was similar between conditions during the first 4 min of exercise (all P>0.05). Thereafter, rate of heat storage was lower in HOT relative to NORMAL and COOL until 30-min of exercise (last common time-point between conditions; all P<0.05). Further, rate of heat storage was significantly higher in COOL compared to NORMAL at 15- (P=0.026) and 20-min only (P=0.020). No differences were measured at end-exercise.

CONCLUSIONS: We show that rate of heat storage does not mediate exercise intensity during self-paced exercise at a fixed RPE in cool to hot ambient conditions.