Short-Term, Low-Volume Training Improves Heat Acclimatization in an Operational Context

Abstract

Personnel who travel to areas with a hot climate (WBGT > 27°C) may suffer from the heat (physiological strain, thermal discomfort, increased probability of heat illness), making them partially or fully inoperative. Performing physical activities during heat acclimatization is known to improve this process (i.e., improve measures of acclimatization for the same duration of acclimation). However, it is unknown whether such training would be efficient in an operative context, characterized by a high volume of work-related physical activity. Thirty French soldiers (Training group, T) performed a short (5 days), progressive, moderate (from three to five 8-min running sets at 50% of the speed at VO_2max for 32-56 min) aerobic training program upon arriving at their base in United Arab Emirates (~40°C and 12% RH). A control group (30 soldiers; No Training, NT) continued to perform their usual outdoor military activities (~6 h.d^-1). A field heat stress test (HST; three 8-min running sets at 50% of the speed at VO_2max) was performed, before and after the heat acclimatization period, to assess physiological and subjective changes. Rectal temperature, heart rate (HR), thermal discomfort at rest and at the end of exercise, rates of perceived exertion (RPE), and sweat loss and osmolality decreased following heat acclimatization in both groups. However, the decreases in the T group were larger than those in the NT group for HR at the end of exercise (-20 ± 13 vs. -13 ± 6 bpm, respectively, p = 0.044), thermal discomfort at rest (-2.6 ± 2.7 vs. -1.4 ± 2.1 cm, respectively, p = 0.013) and at the end of exercise (-2.6 ± 1.9 vs. -1.6 ± 1.7 cm, respectively, p = 0.037) and RPE (-2.3 ± 1.8 vs. -1.3 ± 1.7, respectively, p = 0.035). Thus, we showed that adding short (<60 min), daily, moderate-intensity training sessions during a professional mission in a hot and dry environment accelerated several heat-acclimatization-induced changes at rest and during exercise in only 5 days.

Keywords: exercise; heat acclimation; heat acclimatization; military; rectal temperature; sweat rate; training.

Figure 1

Protocol description. (1) The day after arrival, participants remained at rest in air-conditioned buildings. On D1 and D7, they performed a heat stress test (HST). Between D2 and D6, they either trained (Training [T] group) or did not (No Training [NT] group). From D1 to D7, all participants performed military tasks, mostly outdoors. These activities were not controlled. (2) The HST test consisted of three 8 min periods of running at 50% of the speed at VO_2max with 4 min recovery periods in between. Rectal temperature, nude and dry body mass, heart rate, and thermal discomfort were assessed before and after the HST test. During the recovery periods, heart rate, thermal discomfort, and rates of perceived exertion were measured. At the end of the tests, the sweat of some participants was recovered from sweat collectors for sweat osmolality measurements. (3) Training for the T group consisted of 3, 4, or 5 sets of 8-min periods running at 50% of the speed at VO_2max with 4-min recovery periods in between.

Figure 2

Box-and-whisker plots for rectal temperatures of the No Training and Training groups before (D1) and after (D7) heat acclimatization, at rest and at the end of exercise. The box encompasses the 25–75% quartiles, and the median is represented by the horizontal line within the box. The whiskers extend to the highest and lowest values. Values shown above the boxes, at the top of the plot, correspond to the differences (mean ± SD) between D7 and D1. Significantly different from D1, ^**p < 0.01; ^***p < 0.001.

Figure 3

Box-and-whisker plots for heart rates of the No Training and Training groups before (D1) and after (D7) heat acclimatization, at rest and at the end of exercise. The box encompasses the 25–75% quartiles, and the median is represented by the horizontal line within the box. The whiskers extend to the highest and lowest values. Values shown above the boxes, at the top of the plot, correspond to the differences (mean ± SD) between D7 and D1. Significantly different from D1, ^***p < 0.001; ΔD7-D1 significantly different from NT group, ^#p < 0.05.

Figure 4

Box-and-whisker plots for sweat loss of the No Training and Training groups before (D1) and after (D7) heat acclimatization. The box encompasses the 25–75% quartiles, and the median is represented by the horizontal line within the box. The whiskers extend to the highest and lowest values. Values above the boxes, at the top of the plot, correspond to the differences (mean ± SD) between D7 and D1. Significantly different from D1, ^*p < 0.05.

Figure 5

Box-and-whisker plots for sweat osmolality of the No Training and Training groups before (D1) and after (D7) heat acclimatization. The box encompasses the 25–75% quartiles, and the median is represented by the horizontal line within the box. The whiskers extend to the highest and lowest values. Values shown above the boxes, at the top of the plot, correspond to the differences (mean ± SD) between D7 and D1. Significantly different from D1, ^***p < 0.001.

Figure 6

Box-and-whisker plots for thermal discomfort of the No Training and Training groups before (D1) and after (D7) heat acclimatization, at rest and at the end of exercise. The box encompasses the 25–75% quartiles, and the median is represented by the horizontal line within the box. The whiskers extend to the highest and lowest values. Values shown above the boxes, at the top of the plot, correspond to the differences (mean ± SD) between D7 and D1. Significantly different from D1, ^***p < 0.001; significantly different from NT group, ^$p < 0.05; ΔD7-D1 significantly different from NT group, ^#p < 0.05.

Figure 7

Box-and-whisker plots for the rates of perceived exertion of the No Training and Training groups before (D1) and after (D7) heat acclimatization at the end of exercise. The box encompasses the 25–75% quartiles, and the median is represented by the horizontal line within the box. The whiskers extend to the highest and lowest values. Values shown above the boxes, at the top of the plot correspond to the differences (mean ± SD) between D7 and D1. Significantly different from D1, ^***p < 0.001; significantly different from NT group, ^$p < 0.05; ΔD7-D1 significantly different from NT group, ^#p < 0.05.