Sweat rate prediction equations for outdoor exercise with transient solar radiation

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.