Aviation, Space, and Environmental Medicine
Effect of daily versus intermittent exposure on heat acclimation
Anyone who has ever participated in an outdoor activity on a hot humid day understands the benefits of heat acclimation. Many studies that define the physiological changes that take place during this process have been reported. There is less literature, however, describing the best method with which to become heat acclimated. The authors of this study test the hypothesis that daily exposure to heat is more efficacious for heat acclimation than intermittent exposure.
Fourteen healthy athletes (mean age 23.5 years) were randomized into 2 groups of 7 subjects. Each group contained 5 females and 2 males, and there were no significant differences between the groups. Before the sessions started, the maximum exercise capacity of each subject was determined by measuring peak oxygen consumption, VO2, while exercising on a rowing ergometer. Each subject then exercised on the rowing ergometer at a power output equal to 70% of their maximum ability. One group exercised for 30 minutes per day for 10 consecutive days. The other group exercised for 30 minutes per day every Monday, Wednesday, and Friday for a total of 10 sessions. All sessions took place in an environmental chamber set at 38°C and 70% relative humidity. Rectal temperature, skin temperature, heart rate, ratings of perceived exertion, and sweat rate were all recorded.
Results show consecutive exposure to be superior to intermittent exposure in nearly every category. Rectal temperature decreased significantly in both groups by day 5, but fell to 96% of its predicted plateau value in the consecutive exposure group and to only 26% of the predicted plateau in the intermittent exposure group. Heart rate decreased to 63% of the predicted plateau in the consecutive exposure group compared to only 16% in the intermittent exposure group. Skin temperature was also significantly lower in the continuous exposure group. Ratings of perceived exertion and sweat rate were not significantly different between the groups.
The main limitation of this study is the small sample size. Otherwise, the methods are clear and consistent, and the statistical analysis seems sound. In addition, the authors of this article present a very thorough review of relevant literature, which adds to the validity of the design and the results. Despite the study's small size, the results of this study agree with the results of previous similar studies. Larger studies should be done to confirm these results.
(Aviat Space Environ Med. 2000;71:385–390) N. Gill, B. Phed, and G. Sleivert.
© 2001 Wilderness Medical Society. Published by Elsevier Inc. All rights reserved.