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

Abstract

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-tominute 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 T 20 min) relative to both NORMAL (72 T 23 min, P = 0.004) and COOL (70 T 26 min, P = 0.045). Starting work rate was lower in HOT (153 T 31 W) compared with NORMAL (166 T 27 W, P = 0.024) and COOL (170 T 33 W, P = 0.037). Rate of heat storage was similar between conditions during the first 4 min of exercise (all P 9 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 G 0.05). Further, rate of heat storage was significantly higher in COOL compared with NORMAL at 15 min (P = 0.026) and 20 min (P = 0.020) of exercise. 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.