Exercise in the Heat: the Ultimate Challenge

Abstract

Upon transition from rest to exercise, heat production (HP) increases instantaneously, while the heat-loss mechanisms are still operating at resting levels. Thus HP exceeds heat loss (HL) during the initial stages of exercise. The difference between HP and HL is stored and causes core temperature (T_core) to rise. The increase in core temperature activates heat-loss mechanisms such as panting, sweating and higher skin blood flow. With increasing HL, the rate of heat storage is attenuated and T_core rises more slowly. Finally, as exercise continues, HL matches HP, and Tcore stabilizes at an elevated level. Thus, on the one hand, the increase in T_core is the result of HL lagging behind HP. On the other hand, it links HL to HP: the increase in T_core is an indispensable prerequisite for achieving and maintaining a rate of HL which is proportional to the rate of heat production. This is not to neglect the input provided by mean skin temperature, and the modulation of sweating and skin blood flow by local skin temperature (Chaps. 10 and 11). During exercise, however, the role of rising T_core in activating the heat loss mechanisms is dominant.