Acute heat stress increases resting metabolic rate (Consolazio et al., 1961, 1963; Dimri et al., 1980), but the effect of heat stress on an individual's metabolic rate for performing a given submaximal exercise task is not so clear (see Table 3-1).
During exercise in the heat, the primary cardiovascular challenge is simultaneously to provide sufficient blood flow to exercising skeletal muscle to support metabolism and to provide sufficient blood flow to the skin to dissipate heat.
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.
To regulate body temperature, heat gain and loss are controlled by the autonomic nervous system's alteration of (a) heat flow from the core to the skin via the blood and (b) sweating.
Acclimatization state, aerobic fitness and hydration level are important factors influencing a person's ability to dissipate body heat to the environment. The higher the ambient temperature, the greater the dependence on evaporative heat loss to maintain body heat balance.
Humans often exercise strenuously in hot environments for reasons of recreation, vocation, and survival. The magnitude of physiological strain imposed by exercise-environmental stress depends on the individual's metabolic rate and capacity for heat exchange with the environment. Muscular exercise increases metabolism by 5 to 15 times ...
In conditions in which sweating occurs, the tendency of skin blood flow to warm the skin is approximately balanced by the tendency of sweating to cool the skin.
Muscular exercise increases metabolism by 5 to 15 times the resting rate to provide energy for skeletal muscle contraction. Depending on the type of exercise, 70 to 100 percent of the metabolism is released as heat and needs to be dissipated in order to maintain body heat balance.
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.
More importantly, the percentage of the total metabolic rate contributed by anaerobic metabolism also increased with the ambient temperature. The increase in anaerobic metabolic rate exceeded the increase of total metabolic rate during exercise at the elevated ambient temperatures.
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