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 (Vo(2max)), adiposity, DuBois body surface area (A(D)), surface to mass ratio (A(D): mass) and body mass, for the individual's reaction to humid heat stress. For this purpose 27 subjects (19 men: 8 women), with heterogeneous characteristics (Vo(2max) 1.86-5.28 1 . min(-1); fat% 8.0%-31.9%; mass 49.8-102.1 kg; A(D) 1.52-2.33 m(2)) first rested (30 min) and then exercised (60 W for 1 h) on a cycle ergometer in a warm humid climate (35 degrees 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 (T-re 37.5-39.0 degrees C), mean skin temperature (T-sk 35.7-37.5 degrees 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 . 100ml(-1). min(-1)) and forearm vascular conductance (FVC=FBF/BPa, 0.11-0.49 ml . 100ml(-1). min(-1). mmHg(-1)). The T-re, T-sk and S were (34%-65%) determined in the main by Vo(2max) or by exercise intensity expressed as a percentage of Vo(2max) (%Vo(2max)). For T-re, A(D):mass ratio also contributed to the variance explained, with about half the effect of Vo(2max). For T-sk, fat% contributed to the variance explained with about two-third the effect of Vo(2max). 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 T-re, was determined by Vo(2max) (48%-51%), with less than half the effect of A(D) or A(D):mass (20%). Other circulatory parameters (FBF, BPa, FVC) showed little relationship with individual characteristics (< 36% of variance explained). In general: the higher the Vo(2max) 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 as % Vo(2max) 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.