| The purpose of this study was to determine the separate and combined importance of aerobic fitness and body fatness to uncompensable heat stress. Twenty-four subjects (16 men; 8 women) matched for aerobic fitness and body fatness, performed light exercise to exhaustion at 40°C, 30% relative humidity while wearing nuclear, biological and chemical protective clothing. Thermoregulatory responses were compared between four matched groups; endurance trained (T) or untrained (UT) with high and low levels of body fatness. The change in core temperature from the beginning to end of the heat stress exposure was significantly greater in T compared to UT due to the higher core temperature tolerated at exhaustion. Tolerance time was significantly longer for T Low (116.2 +/- 6.5 min) compared to UTLow (69.5 +/- 3.6 min) and THigh (82.2 +/- 3.9 min), indicating an effect of both fitness and fatness, respectively. However, similar effects were not evident between THigh and UTHigh (73.7 +/- 4.1 min) or between the UT groups because of the lower average rate of heat storage for UTHigh compared with the other matched groups. In conclusion, the present data suggest that fitness is a key factor for enhanced tolerance to uncompensable heat stress because it enables continued and sustained performance even at higher Tre, through mechanisms which remain to be elucidated. |
