Sweat rate prediction equations for outdoor exercise with transient solar radiation

Abstract

We investigated the validity of employing a fuzzy piecewise prediction equation (PW) [Gonzalez et al. J Appl Physiol 107: 379–388, 2009] defined by sweat rate (m sw, g·m−2·h−1) = 147 + 1.527·(E req) − 0.87·(E max), which integrates evaporation required (E req) and the maximum evaporative capacity of the environment (E max). Heat exchange and physiological responses were determined throughout the trials. Environmental conditions were ambient temperature (T a) = 16–26°C, relative humidity (RH) = 51–55%, and wind speed ( V) = 0.5–1.5 m/s. Volunteers wore military fatigues [clothing evaporative potential (i m/clo) = 0.33] and carried loads (15–31 kg) while marching 14–37 km over variable terrains either at night ( N = 77, trials 1–5) or night with increasing daylight ( N = 33, trials 6 and 7). PW was modified ( Ṗw,sol) for transient solar radiation (R sol, W) determined from measured solar loads and verified in trials 6 and 7. PW provided a valid m sw prediction during night trials ( 1–5) matching previous laboratory values and verified by bootstrap correlation (r bs of 0.81, SE ± 0.014, SEE = ± 69.2 g·m−2·h−1). For trials 6 and 7, E req and E max components included R sol applying a modified equation Ṗw,sol, in which m sw = 147 + 1.527·(E req,sol) − 0.87·(E max). Linear prediction of m sw = 0.72· Ṗw,sol + 135 ( N = 33) was validated ( R 2 = 0.92; SEE = ±33.8 g·m−2·h−1) with PW β-coefficients unaltered during field marches between 16°C and 26°C T a for m sw ≤ 700 g·m−2·h−1. PW was additionally derived for cool laboratory/night conditions (T a < 20°C) in which E req is low but E max is high, as: PW,cool (g·m−2·h−1) = 350 + 1.527·E req − 0.87·E max. These sweat prediction equations allow valid tools for civilian, sports, and military medicine communities to predict water needs during a variety of heat stress/exercise conditions.