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 (1.34 m.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 (Tre) of 39.3 degrees C, a heart rate (fc) > or = 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 Tre, fc, 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.