Indices of thermoregulatory strain for moderate exercise in the heat

Abstract

The effect of varying humidity and dry bulb temperatures was studied on five normal male unclothed subjects while exercising (40-45 min) at 28% V(O)2(max). Air movement was 0.75 m.s-1. The initial test and the 16th test on each subject were both done at 50°C and 30 Torr (32% rh). Each subject did the intervening 14 experiments twice per day at varying ambient temperature (T(a)) and water vapor pressure (P(a)) levels, so selected to progressively increase skin wettedness levels. Mean skin temperature (T(sk)) and esophageal temperature (T(es)), heart rate (HR), skin evaporative heat loss (E(sk)), and warm discomfort were continuously observed. Skin wettedness (w) was evaluated as the ratio of the observed E(sk) to the maximum evaporative capacity of the environment. A rational effective temperature (ET) is defined as the dry bulb temperature at 50% rh in which the total heat exchange from the skin surface would be the same as in the test environment, described by the observed T(a) and P(a). The results showed that 1) during steady state both HR and T(es) were unaffected by T(a) from 26 to 41°C responding to the level of exercise intensity, when P(a) ≤ 20 Torr; 2) both mean body temperature, found by weighting T(sk:T(es) by 1:9, and ET were each significant indicators of physiological strain when P(a) > 20 Torr; 3) a level of strain, caused by skin wettedness values greater than 0.5, is suggested as a primary condition necessary for inducing heat acclimation.