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Abstract
This study used partitional calorimetry to determine the influence of fluid replacement on heat storage during uncompensable heat stress. Eight males performed either light (L; level treadmill walking at 0.97 m·s-1 (3.5 km·h-1) or heavy (H; 1.33 m·s-1 (4.8 km·h-1) at a 4% grade) exercise at 40°C and 30% relative humidity while wearing nuclear, biological and chemical (NBC) protective clothing. Subjects received either no fluid (NF), or 200 or 250 ml of fluid (F) as warm water at ~ 35°C immediately before and every 15 min during the L and H trials respectively. Similar reductions in heart rate were observed at both metabolic rates with F but rectal temperature responses were not different between F and NF. Tolerance time was extended during L/F (106.5±22.1 min) compared with L/NF (93.1±20.8 min) but fluid replacement had no influence during H (59.8±9.5 min and 58.3±11.1 min for F and NF respectively). Fluid replacement also had no effect on the rate of heat storage during L (108.2±20.6 W·m-2 and 111.0±22.6 W·m-2 for F and NF respectively) and H (172.5±11.5 W·m-2 and 182.1±15.8 W·m-2 for F and NF respectively). However, heat storage expressed per unit of mass was significantly increased during L/F (18.5±4.0 kJ·kg-1) compared with the other trials (16.3±4.8 kJ·kg-1, 16.6±3.0 kJ·kg-1 and 16.7±4.0 kJ·kg-1 for L/NF, H/F and H/NF respectively). It was concluded that fluid replacement does not alter the rate of heat storage during uncompensable heat stress but does increase the heat storage capacity during light exercise when tolerance times are > 60 min.