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TABLE 6-9 Mean Changes in Aerobic-Anaerobic Fractions of Oxygen Utilization During Fixed Submaximal Exercise for 5 Minutes in Comfortable and Hot Conditions

 

Mean Change in O2 Utilization (± SD)

 

Comfortable Environment (Tdb 29.0°C, Twb 21.3°C)

Very Hot Environment (Tdb 36.5°C, Twb 28.5°C)

Level of Significance

Total (liters)

7.60 ± 1.21

8.06 ± 0.87

n.s.

Aerobic (liters)

5.60 ± 6.50

5.27 ± 0.68

p < 0.05

Percentage of total

73.97 ± 6.50

65.40 ± 2.99

p < 0.02

Anaerobic (liters)

2.00 ± 0.62

2.78 ± 0.33

p < 0.01

Percentage of total

26.03 ± 6.50

34.60 ± 2.99

p < 0.02

NOTE: Exercise intensity = 600 kg per min, cycle ergometer; Tdb = dry-bulb temperature; Twb = wet-bulb temperature.

SOURCE: Adapted from Sen Gupta (1977).

this hypothesis might apply to short periods of exercise such as the 5-minute bouts used by Sen Gupta et al. (1977), it would undoubtedly not apply to longer bouts of exercise, for example, 1 to 8 hours or more when a balance in muscle and skin blood flow would be necessary to sustain the exercise. A relatively elevated anaerobic metabolism and higher blood lactate concentrations would not be present. Thus, the conclusion of Sen Gupta et al. "that during submaximal work in heat, the metabolism becomes more anaerobic and there is reduction in in submaximal and maximal workloads as the heat stress increases" must be qualified at least with respect to duration of submaximal exercise.

Overall, whether energy expenditure is modified during exercise in the heat depends on the circumstances and conditions. Brief intense exercise in a hot environment may elevate energy expenditure by evoking anaerobic processes, but the increment in daily energy expenditure is likely to be negligible. Thus, the earlier investigators posed the problem, but in terms of meeting the daily kcal needs of troops working in a hot environment, the submaximal exercise they perform has no greater impact than if they performed the same tasks in a more comfortable environment.

The possible reasons for either an increase or a decrease in metabolic rate in hot environments are listed in Tables 6-10 and 6-11. A careful appraisal of each military situation would reveal which factors are most important to consider while also bearing in mind the possible causes of different responses previously set forth in Table 6-4.



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