Adaptation to heat: differences induced isothermal strain accompanying exercise in hot and neutral environments

Acclimatisation induces physiological changes which reduce thermal strain and may occur in response to artificial heat exposure (acclimation), and endurance exercise which elevates body core temperature. This project sought to evaluate the importance of elevated skin temperature in the thermal environment during the acclimation process, using an isothermal strain technique which kept core temperature constant between conditions. Two groups of seven healthy males, matched for peak aerobic power (Vo2pcJ and sum of six skinfolds, participated (1 hr per day for 10 consecutive days) in one of two conditions: (i) neutral physical training (NEUT: 22.4°C {s.d. 0.7}, RH 41.0% {s.d. 0.9}) or (ii) combined physical training and heat acclimation (HEAT: 38.2°C {s.d. 0.7}, RH 39.7% {s.d. 1.3}). Isothermal strain was induced by rapidly elevating rectal temperature (TJ to 1°C above its initial resting level (cycling), then holding it constant by manipulating the external work. Subjects completed heat stress tests (39.8°C {s.d. 0.1}, RH 38.6% {s.d. 1.2}), before and after acclimation, consisting of: 20 min seated rest, 20 min cycling at 30% Vo2pcdc, and 20 min cycling at 45% Vo2pcak- Cto the basis of pre- versus post-acclimation differences in T^, forehead rfi, and SkBF, and perceived exertion, the HEAT condition elicited greater improvements in acclimation state, even though central thermal strain was equated between conditions. While the elevation in body core temperature is critical to acclimation, it appears necessary to expose subjects to an exogenous external thermal stress. This verifies the importance of peripheral thermoreceptors in the acclimation process, altiiough this observation has not been demonstrated previously under conditions of isothermal strain.