DIURNAL EFFECTS ON THERMOREGULATION AND TOLERANCE OF UNCOMPENSABLE HEAT STRESS

Abstract

1537 This study examined whether the lower rectal temperature (Tre) in the morning hours would increase tolerance to uncompensable heat stress compared with trials conducted in the early afternoon. Nine males (34 y, 82 kg, 2.0 m2 surface area) performed a familiarisation trial and then 2 randomly ordered trials in the morning (0930h) and afternoon (1330h). The heat stress test consisted of intermittent walking at 3.5 km/h and seated rest at 40°C and 30% relative humidity while wearing nuclear, biological and chemical protective clothing. Initial Tre was significantly higher during the afternoon (37.1 ± 0.2°C) compared with the morning (36.8 ± 0.2°C) and these differences remained throughout the uncompensable heat stress such that final Tre was also significantly higher for the afternoon (39.2 ± 0.2°C) versus the morning (39.0 ± 0.3°C) trials. Metabolic rate, heart rate and the sweat production rate were similar during the morning and afternoon exposures. Since the rate of heat storage (100.6 ± 24 W/m2 and 96.5 ± 21.2 W/m2 for the morning and afternoon trials, respectively) and tolerance time (111 ± 22 and 107 ± 19 min for the morning and afternoon trials, respectively) were similar, calculated total heat storage also was similar for the morning (16.1 ± 2.9 kJ/kg) and afternoon heat stress exposures (15.1 ± 3.0 kJ/kg). It was concluded that trials conducted in the early afternoon were associated with an increased rectal temperature tolerated at exhaustion that offset the circadian influence on resting rectal temperature, and thus, maintained tolerance times similar to trials conducted in the morning.