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 (Pw,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 Pw,sol, in which m(sw) = 147 + 1.527·(E(req,sol)) - 0.87·(E(max)). Linear prediction of m(sw) = 0.72·Pw,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.

National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active

“…30,31 Appropriate hydration is similarly important for physical activity at altitude, for individuals with chronic disease, and for thermoregulation in cold environments. 32,33 Conversely, excessive fluid consumption can lead to lifethreatening exercise-associated hyponatremia (EAH), in which extracellular body water enters the cells and causes organ and tissue swelling. 34,35 Although EAH is uncommon in team-sport athletes, it has been identified in 10% to 20% of distance athletes after events.…”

“…Hydration safety for physically active individuals can be influenced by body fluid status, but significant variability exists in factors such as habitual daily water intake, 1,40 sweat rate, 32,53,201 and gastrointestinal tolerance of consuming a large fluid volume during exercise, 202 which makes universal recommendations impossible. Ideally, individual sweat-rate assessments should be performed several times under realistic environmental conditions and at varying exercise intensities while wearing standard clothing and equipment specific to the sport (Table 2).…”

“…For example, sweat rate can vary according to body size, environmental conditions, exercise intensity, acclimatization status, and other factors. 206 Reliable estimates of sweat losses based on some of these variables exist 32 ; however, they are not valid for every person and should only be used as an estimate when specific calculations cannot be conducted. Individual recommendations on fluid replacement should be based on individual data and not population estimates.…”

“…Sweat rates for adults (.18 years old) during exercise can range from 0.5 to 4.0 L/h, 32,43,53,201,207,208 and measured sodium losses during exercise range between 0.2 and 7.3 g/h.…”

Implementation and comparison of a suite of heat stress metrics within the Community Land Model version 4.5

“…Numerous metrics are based upon direct physiological responses within humans and animals; however, almost all of them are complicated algorithms (e.g., Moran et al, 2001; Berglund and Yokota, 2005;Gribok et al, 2008;Maloney and Forbes, 2011;Havenith et al, 2011;Gonzalez et al, 2012;Chan et al, 2012). Most metrics require radiation measurements or heart rates, and/or even sweat rates.…”