Impact of fluid replacement on heat storage while wearing protective clothing

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 a t 40 degreesC and 30% relative humidity while wearing nuclear, biological a nd chemical (NBC) protective clothing. Subjects received either no fluid (N F), or 200 or 250 ml of fluid (F) as warm water at similar to 35 degreesC i mmediately 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. To lerance 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 replacem ent 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). Howeve r, heat storage expressed per unit of mass was significantly increased duri ng 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 doe s not alter the rate of heat storage during uncompensable heat stress but d oes increase the heat storage capacity during light exercise when tolerance times are > 60 min.