Determination of body heat storage: how to select the weighting of rectal and skin temperatures for clothed subjects.

Two methods of estimating body heat storage were compared under differing conditions of clothing and acclimation to heat. Sixteen male subjects underwent 6 consecutive days or two 6-day periods, separated by a 1-day rest period of heat acclimation, exercising 60 min.day-1 at 45%-55% of maximal aerobic power in a hot, dry environment (dry bulb temperature 40 degrees C; relative humidity 30%; and wind speed 0.3 m.s-1). Before and after acclimation, the subjects entered the same environment, wearing either normal light combat clothing or clothing protective against nuclear, biological, and chemical agents; they walked on a treadmill at 1.34 m.s-1, 0% slope continuously (n = 11 for normal clothing) or as repeated 15-min bouts of exercise followed by 15-min sitting rest (n = 5 for normal clothing and n = 16 for protective clothing). Average exposure times were 147 min (preacclimation) and 150 min (postacclimation) for continuous exercise and 150 min (both pre- and postacclimation) for intermittent exercise while wearing normal clothing, and 103 min (preacclimation) and 116 min (postacclimation) for intermittent exercise while wearing protective clothing. Heat storage was determined calorimetrically (from heat gains and heat losses) and thermometrically [using various weightings of rectal temperature (Tre) and mean skin temperature (Tsk)]. There were only minor (<5%) differences in estimated heat storage, whether calculations used a single specific heat (3.47 kJ.kg-1.degree C-1) or a value computed according to the subject's body composition. When wearing normal clothing, a formula with an invariant relative weighting for Tre to Tsk of 4:1 provided the best thermometric estimate of heat storage. When wearing protective clothing, the invariant relative weighting of 4:1 underestimated heat storage by 2%-12%; underestimation was attenuated by using respective relative weightings for a thermoneutral and hot environment of 2:1 and 2:1 or 4:1 and 9:1 before acclimation and 4:1 and 9:1 after acclimation. We conclude that the accuracy of thermometric estimates of heat storage can be improved by modifying the weighting factors according to environment, acclimation, and type of clothing.