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FIGURE 3-1 Heat exchange data averaged over 1 hour for one subject performing constant intensity exercise in a variety of ambient temperatures. The difference between metabolic rate and total heat loss is the sum of mechanical power (147 watts) and mean rate of heat storage. SOURCE: Sawka and Wenget (1988), used with permission. Redrawn from Nielsen (1938).

independent of environmental conditions is inconsistent with the personal experience of most athletes. For example, a runner will experience greater hyperthermia if he or she competes in a 35°C environment (Robinson, 1963). Lind (1963) showed that the magnitude of core temperature elevation during exercise is independent of the environment only within a certain range of conditions or a ''prescriptive zone.'' Figure 3-2 presents a subject's steady-state core temperature responses during exercise performed at three metabolic intensities in a broad range of environmental conditions. The environmental conditions are represented by the "old" effective temperature, which is an index that combines the effects of dry-bulb temperature, humidity, and air motion. Note that during exercise the greater the metabolic rate, the lower the upper limit of the prescriptive zone. In addition, Lind found that even within the prescriptive zone there was a small but significant positive relationship between the steady-state core temperature and the "old" effective temperature. It seems fair to conclude that throughout a wide range of environmental conditions, the magnitude of core temperature elevation during exercise is largely, but not entirely, independent of



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