Physiological and perceptual responses to cyclic heat stress variations.

The effect of the time presentation of a given external heat load was examined on five subjects exercising at a constant work load (50 W). The subjects, dressed in briefs, were exposed to cyclic variations for 120 min in air temperatures between 51 degrees C and 23 degrees C, under three different schedules involving heat pulses of 10-min, 20-min and 30-min duration, respectively. The strain induced by each of these conditions was compared in terms of both physiological and perceptual criteria. Results showed that between conditions, there were significant differences in skin temperature levels but not in core temperature levels, body heat storage, or body weight loss. Perception of effort and thermal sensation ratings both exhibited similar changes in all three conditions. Due to the time constant of the sweating response, sweating rates and skin wettednesses at the end of the heat pulses were lower for 10-min heat pulses than for those of 20- and 30-min duration, and these differences were perceived by the subjects. Lower perceived skin wettedness ratings are thus suggested as the main factor to explain why all subjects rated the 10-min heating-cooling cycle as the least strenuous and uncomfortable condition. It is concluded that under the conditions of this study, perceptual criteria associated with physiological criteria represent a useful means of discriminating slight differences in strain.