can be attributed, at least in part, to a redistribution of blood flow away from the splanchnic tissues.
Lactate accumulation in blood and muscle during submaximal exercise is generally found to be reduced following heat acclimatization (Young, 1990). Figure 3-5 shows that heat acclimatization resulted in lower postexercise muscle lactate concentrations. Muscle lactate concentrations were still higher in the heat than in the cool, and changes in blood lactate concentrations followed exactly the same patterns (Young et al., 1985). King et al. (1985) and Kirwan et al. (1987) observed that heat acclimatization reduced muscle glycogen utilization during exercise in the heat by 40 to 50 percent compared to before acclimatization. Young et al. (1985) also observed a statistically significant glycogen sparing effect due to heat acclimatization, but the reduction in glycogen utilization was small and apparent only during exercise in the cool conditions. Glycogen utilization during exercise in the heat was negligibly affected. The mechanism(s) for the reduction in lactate accumulation during exercise associated with heat acclimatization remains unidentified.
Figure 3-1 illustrates that when ambient temperature increases, there is a greater dependence on insensible (evaporative) heat loss to defend core temperature during exercise. In contrast to most animals, respiratory evaporative cooling is small in humans when compared to total skin evaporative cooling. The use of skin provides the advantage of having a greater surface area available for evaporation. The eccrine glands secrete sweat on the skin surface, which is cooled when the sweat evaporates. The rate of evaporation depends on the wetted area, air movement, and the water vapor pressure gradient between the skin and the surrounding air; the wider the gradient, the greater the rate of evaporation.
For a given person, sweating rate is highly variable and depends on environmental conditions (ambient temperature, dew point temperature, radiant load, and air velocity); clothing (insulation and moisture permeability); and physical activity level (Shapiro et al., 1982). Adolph et al. (1947) reported that for 91 men studied during diverse military activities in the desert, the average sweating rate was 4.1 liters every 24 hours, but values ranged from 1 to 11 liters every 24 hours. The water requirements of soldiers on the modern battlefield may be even greater. The threat of chemical warfare may require military personnel to wear nuclear-biological-chemical (NBC) protective clothing, which prevents noxious agents from reaching the skin. Characterized by low moisture permeability and high insulating properties, NBC clothing prevents the normal dissipation of body heat. As a