INFLUENCE OF AEROBIC FITNESS AND BODY FATNESS ON TOLERANCE TO UNCOMPENSABLE HEAT STRESS

This study examined the independent and interactive effects of aerobic fitness and body fatness on heat tolerance while wearing a protective clothing ensemble. Twenty-four men and women were defined as being either trained (T) (regularly active with a VO2max of 65 ml·kg−1·min−1) with a low (11%) or high (20%) body fatness. Subjects were matched for fitness and fatness into one of 4 groups (4 men and 2 women in each group). The heat-stress exposure involved wearing a semipermeable protective clothing ensemble and light treadmill exercise at 0.97 m·s−1 at 40°C and 30% relative humidity. Tolerance times were significantly greater for Tlow (116 ± 16 min) compared with their matched sedentary (UT low 70 ± 9 min) of fatness (Thigh 82 ± 10 min) counterparts indicating an advantage for both a high fitness and low body fatness. However, the same advantages were not evident between Thigh and UThigh (74 ± 10 min) and between UThigh and UTlow. Partitional calorimetric estimates revealed that rates of heat storage (S) were significantly lower for UThigh (2.87 ± 0.19 W ·kg−1) compared with Thigh (3.41 ± 0.16 W ·kg−1) and UTlow (3.52 ± 0.19 W ·kg−1) due to a lower oxygen cost of walking and therefore lower rate of heat production. The major advantage attributed to a higher aerobic fitness was the ability to tolerate a higher core temperature at exhaustion (the difference being as great as 0.9°C) whereas both body fatness and S affected the rate of increase in core temperature as independent factors.