(Rowell, 1986) during maximal vasodilation, the contracting musculature could receive less perfusion at a given cardiac output level. Rowell et al. (1966) reported that during high-intensity exercise in the heat, cardiac output can be reduced by 1.2 liters per minute below control levels. A reduction in maximal cardiac output by 1.2 liters per minute could account for a 0.25-liter-per-minute decrement in with heat exposure, because each liter of blood could deliver about 0.2 liter of oxygen (1.34 ml oxygen per g hemoglobin × l5 g hemoglobin per 100 ml of blood).
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). Such an effect would influence the calculation of the heat balance and might have implications for the nutritional requirements of individuals exposed to hot environments. Many investigators report that to perform a given submaximal exercise task, the metabolic rate is greater in a hot than temperate environment (Consolazio et al., 1961, 1963; Dimri et al., 1980; Fink et al., 1975). Some investigators, however, report lower metabolic rates in the heat (Brouha et al., 1960; Petersen and Vejby-Christensen, 1973; Williams et al., 1962; Young et al., 1985). Heat acclimation state does not account for whether individuals demonstrate an increased or decreased metabolic rate during submaximal exercise in the heat. However, other mechanisms can explain this discrepancy. Most investigators have only calculated the aerobic metabolic rate during submaximal exercise, ignoring the contribution of anaerobic metabolism to total metabolic rate.
Dimri et al. (1980) had six subjects exercise at three intensities in each of three environments. Figure 3-4 presents their subjects' total metabolic rate (bottom) and the percentage of this metabolic rate that was contributed by aerobic and anaerobic metabolic pathways. The anaerobic metabolism was calculated by measuring the postexercise oxygen uptake that was in excess of resting baseline levels. Although there are limitations to this methodology, the study provides useful information. Note that to perform exercise at a given power output, the total metabolic rate increased with the elevated ambient temperature. 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. Therefore, if only the aerobic metabolic rate had been quantified, Dimri et al. (1980) would probably have reported a decreased metabolic rate in the heat for performing exercise at a given power output. Investigations that report a lower metabolic rate during exercise in the heat also report increased plasma or muscle lactate levels (Petersen and Vejby-Christensen, 1973; Williams et al., 1962; Young et al., 1985) or an increased respiratory exchange ratio (Brouha et al., 1960), which also suggests an