Physiological effects of continuous or intermittent work in the heat

¹ Environmental Protection Research Division, Quartermaster Research and Engineering Command, U.S. Army Quartermaster Research and Engineering Center, Natick, Massachusetts

To define thermal limits for everyday work, a possible criterion which has been put forward described the climates ("prescriptive" climates) in which the level of bodily thermoregulation remained steady for a given rate of work. The present experiments were intended to determine the effect on results, obtained from brief exposures involving continuous work, of extending the exposure to a period of 8 hr and of presenting a given total energy expenditure (2,100 kcal) in different patterns of work and rest. During 8-hr exposures to three climates with dry and wet bulb temperatures of 29.4 and 23.9, 36.7 and 25.6, and 41.1 and 28.3 C, respectively, two subjects expended approximately 2,100 kcal in both 1) a nearly continuous level of activity and 2) intermittent bouts of much harder work interspaced by longer periods of rest. Evaluation and comparison of the results show that extension of exposures to periods of up to 8 hr per se did not demonstrably change the levels of rectal temperature, pulse rate, or weight loss found by the 2nd hr of exposure in the climates examined. Further, in prescriptive climates (in which the level of thermoregulation depends on the rate of work rather than on the environment), when the energy expenditure was 2,100 kcal in 8 hr, either continuously at a moderate rate, or intermittently at a high rate with compensatory rest pauses, the physiological cost was similar, as judged by rectal temperature, pulse rate, and weight loss.

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