EFFECTS OF 6 VERSUS 12 DAYS OF HEAT ACCLIMATION ON HEAT TOLERANCE IN LIGHTLY EXERCISING MEN WEARING PROTECTIVE CLOTHING

This study investigated the influence of 6 versus 12 days of heat acclimation on the tolerance of low-intensity exercise in the heat while wearing protective clothing. Sixteen young men were acclimated by treadmill walking (50% of each subject's maximal aerobic power for 60 min . day(-1)) in a climatic chamber [40 degrees C dry bulb (db), 30% relative humidity] for either 6 consecutive days or two 6-day periods, separated by a 1-day rest. Before and after heat acclimation, the subjects performed a heat-exercise test (1.34m . s(-1), 0% grade; 40 degrees C db, 30% relative humidity), either under control conditions [wearing normal light combat clothing (continuous exercise; n = 5)] or when wearing protective clothing resistant against nuclear, biological, and chemical (NBC) agents (repeated bouts of 15-min walk + 15-min rest; n = 8). Criteria for halting the test exercise were a rectal temperature (T-re) of 39.3 degrees C, a heart rate (f(c)) greater than or equal to 95% of the subject's observed maximum, unwillingness of the subject to continue, or the elapse of 150 min. Heat acclimation decreased overall test values of T-re, f(c), and mean skin temperature for both control and protective clothing conditions. When wearing normal combat clothing, acclimation responses were about twice as large after 12 than after 6 days, but the response was not increased by longer acclimation when wearing NBC protective clothing. Both 6 and 12 days of acclimation increased tolerance times in NBC protective clothing by about 15 min [from 97 (4) to 112(6) min and from 108 (10) to 120 (10) min for 6 and 12 days, respectively]. We conclude that the physiological strain and limitation of heat-exercise tolerance imposed by wearing NBC protective clothing are not reduced if heat acclimation is prolonged from 6 to 12 days.