ACCLIMATION TO HUMID HEAT REDUCES RESTING CORE TEMPERATURE BUT NOT HEAT STORAGE 560

Abstract

The purpose of this study was to test the hypothesis that a reduction in resting temperature, not in heat storage, is responsible for the reduction in core temperature during heat exposure following acclimation to humid heat. The subjects were 12 healthy, male volunteers with a mean age of 22 years and a body surface area (BSA) of 1.93 m-2. Each subject completed an eight day heat acclimation protocol, during which they exercised at a workload that elicited an oxygen uptake of 1.2 L/min for 2-hours each day in a hot, humid environment (35°C, 75% RH). Rectal temperature (Tre) and mean skin temperature (Tmsk, shoulder, chest, calf, thigh) were obtained in a resting state prior to entering the heat chamber and upon completing the 2-hour bout of exercise in the heat. The change in Tre(ΔTre °C) and Tmsk (Δ Tmsk°C) derived from ending value minus beginning value and heat storage calculated as watts·m-2 (0.965 × body weight × (0.8 ×ΔTre) + (0.2 × ΔTmsk))/BSA were measured each day of the acclimation protocol. Mean ending core temperature decreased significantly (p<0.05) from 38.9°C on day 1 to 38.2°C on day 8. However, neither ΔTre, ΔTmsk nor heat storage significantly (p>0.05) changed over the 8-day heat acclimation protocol. These results suggest that a reduction in resting temperature, not in heat storage, is the physiological mechanism responsible for the attenuation in ending core temperature following acclimation to humid heat.