Heat acclimation and its relation to resting core temperature and heart rate.

Acclimation as an adaptive response of the human body to repeatedly occurring heat stress causes a reduction of core temperature (Tco) and heart rate (HR) at the end of heat exposure. The analysis of three acclimation series (WBGT = 33.5 ◦ C) showed that the lowering of Tco and HR occurred already in the resting period preceding heat stress. The lowered resting values accounted for a substantial part of the beneficial effects of acclimation and may be mainly induced by the physical exercise, as a similar reduction of resting values was also observed under thermally neutral conditions. Expanding the database with short-term acclimation series revealed that the resting values were less reduced for females compared to males, but that the same relations between resting and final Tco and HR existed. The results further suggest that the reduction of resting Tco reflects long term effects of adaptation whereby the resting HR also depends on unspecific situational influences. The lowering of the initial values might be a suitable instrument when considering the effects of acclimation in thermoregulatory models for the assessment of heat stress at the workplace.