The relative influence of body characteristics on humid heat stress response

The present study was designed to determine the relative importance of individual characteristics such as maximal oxygen uptake (V_O2 max), adiposity, DuBois body surface area (AD), surface to mass ratio (AD:mass) and body mass, for the individ-ual's reaction to humid heat stress. For this purpose 27 subjects (19 men, 8 women),eterogeneous characteristics (V_O2 max 1.86-5.28 l×min-1; fat% 8.0-31.9; mass 49.8-102.1 kg; AD 1.52-2.33 m²) first rested (30 min) and then exercised (60 W for 1 h) on a cycle ergometer in a warm humid climate (35°C, 80% relative humidity). Their physiological responses at the end of exercise were analysed to assess their relationship with individual characteristics using a stepwise multiple regression technique. Dependent variables (with ranges) included final values of rectal temperature (Tre 37.5-39.0°C), mean skin temper-ature (Tsk 35.7-37.5°C), body heat storage (S 3.2-8.1 J×g-1), heart rate (HR 100-172 beat×min-1), sweat loss (397-1403 g), mean arterial blood pressure (BPa 68-96 mmHg), forearm blood flow (FBF 10.1-33.9 ml×100 ml-1×min-1) and forearm vascular conductance (FVC=FBF/BPa 0.11-0.49 ml×100 ml-1× min-1×mmHg-1). The Tre, Tsk and S were (34-65%) determined in the main by V_O2 max or by exercise intensity expressed as a percentage of V_O2 max (%V_O2 max). For Tre, AD:mass ratio also contributed to the variance explained, with about half the effect of V_O2 max. For Tsk, fat% contributed to the variance explained with about two-third the effect of V_O2 max. Total body sweat loss was highly dependent (50%) on body size (AD or mass) with regular activity level having a quarter of the effect of body size on sweat loss. The HR, similar to Tre, was determined by V_O2 max (48-51%), with less than half the effect of AD or AD:mass (20%). Other circulatory parameters (FBF, BPa, FVC) showed little relationship with individual characteristics (<36% of variance explained). In general, the higher the V_O2 max and/or the bigger the subject, the lower the heat strain observed. The widely accepted concept, that body core temperature is determined by exercise intensity expressed in %V_O2 max and sweat loss by absolute heat load, was only partially supported by the results. For both variables, other individual characteristics were also shown to contribute.