The relative influence of body characteristics on humid heat stress response

Abstract

The present study was designed to determine the relative importance of individual characteristics such as maximal oxygen uptake ($\dot V$O_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 ($\dot V$O_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 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 (T _re 37.5–39.0°C), mean skin temperature (T _sk 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 (397–1403g), mean arterial blood pressure (BP_a, 68–96 mmHg), forearm blood flow (FBF, 10.1–33.9 ml · 100ml⁻¹ · min⁻¹) and forearm vascular conductance (FVC = FBF/BP_a, 0.11–0.49 ml · 100 ml⁻¹ · min⁻¹ · mmHg⁻¹). The T _re, T _sk and S were (34%–65%) determined in the: main by ($\dot V$O_2max), or by exercise intensity expressed as a percent age of $\dot V$O_2max (%$\dot V$O_2max). For T _re, A _D: mass ratio also contributed to the variance explained, with about half the effect of ($\dot V$O_2max), For T _sk, fat% contributed to the variance explained with about two-third the effect of $\dot V$O_2max. Total body sweat loss was highly dependent (50%) on body size (A _D 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 $\dot V$O_2max (48%–51%), with less than half the effect of A _D or A _D :mass (20%). Other circulatory parameters (FBF, BP_a, FVC) showed little relationship with individual characteristics ( < 36% of variance explained). In general, the higher the ($\dot V$O_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 % $\dot V$O_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.