Evaluation of the limits to accurate sweat loss prediction during prolonged exercise

Abstract

Sweat prediction equations are often used outside their boundaries to estimate fluid requirements and generate guidance. The limitations associated with these generalized predictions have not been characterized. The purposes of this study were to: (1) evaluate the accuracy of a widely used sweat prediction equation (SHAP) when widening it's boundaries to include cooler environments (2 h) and very prolonged exercise (8 h), (2) determine the independent impact of holding skin temperature constant (SHAP36), and (3) describe how adjustments for non-sweat losses (NSL) and clothing saturation dynamics affect prediction accuracy. Water balance was measured in 39 volunteers during 15 trials that included intermittent treadmill walking for 2 h (300-600 W, 15-30 degrees C; n=21) or 8 h (300-420 W, 20-40 degrees C; n=18). Equation accuracy was assessed by comparing actual and predicted sweating rates (211 observations) using least-squares regression. Mean and 95% confidence intervals for group differences were compared against a zone of indifference (+/-0.125 l/h). Sweating rate variance accounted for by SHAP and SHAP36 was always high (r2>0.70), while the standard error of the estimate was small and uniform around the line of best fit. SHAP errors were >0.125 l/h during 2 and 8 h of exercise. SHAP36 errors were <0.125 l/h for 2 h conditions but were higher at 8 h in three of the six warmest trials. Adjustments for NSL and clothing saturation dynamics help explain SHAP errors at 2 and 8 h, respectively. These results provide a basis for future development of accurate algorithms with broader utility.

Non-evaporative effects of a wet mid layer on heat transfer through protective clothing

“…These relative reductions, especially those observed with the walking subjects, are somewhat higher than the values of 2-8% reported from manikin experiments (Chen et al 2003), but are in line with those used in a recent modelling approach (Cheuvront et al 2007). …”

“…This was accompanied by a lower increase in heart rate during walking, although metabolic rate was nearly the same as with the dry mid layer. A further consequence of the cooling effect of the wet clothing was the reduced sweat production, as it had been also reported with more permeable clothing (Cheuvront et al 2007;, which in this experiment, however, did not result in significant differences in moisture evaporation to the environment, the latter most likely due to the limiting effect of the impermeable clothing layers used.…”

“…These mechanisms occur simultaneously and their separate quantification in the framework of heat balance analysis is problematic as evaporative cooling efficiency, defined as the ratio of the observed evaporative cooling to the effect expected from clothed mass loss (Havenith et al 2007b), may deviate from unity with protective clothing (Nunneley 1989;McLellan et al 1996;Candas et al 2006;Havenith et al 2006Havenith et al , 2007a. This may lead to complications in the course of predicting unwanted cooling effects of sweat accumulated inside clothing while or after working in the cold (Meinander et al 2004) or when modelling the sweating response of persons performing heavy work with protective clothing (Cheuvront et al 2007). …”

“…Aoyagi et al (1996) adapted the skin-core temperature weighting used in the calculation of body heat storage, while assuming an unchanged clothing insulation, Chen et al (2003) calculated changes in dry heat loss with wet clothing from measurements with a sweating manikin assuming unity evaporative cooling efficiency, while Cheuvront et al (2007) showed that allowing for the clothing insulation to change during modelling the sweating response of clothed persons improved the predictive capabilities of the model. To provide data and models for the systematic assessment of the effects of moisture on the thermal properties of protective clothing was one major concern of the European research initiative THERMPROTECT (Havenith et al 2005).…”

Impact of a protective vest and spacer garment on exercise-heat strain

“…Sweating rates during P (0.75 ± 0.19 L/h) and S (0.77 ± 0.17 L/h) trials were higher than B (0.66 ± 0.13 L/h). Although the mean differences among trials were significant (B \ P and S; P \ 0.05), the magnitude of the differences was small and of questionable practical importance (\0.125 L/h) (Cheuvront et al 2007). …”