The effect of physical training on body heat regulation

Abstract

Purpose. We evaluated the effects of an eight-week aerobic exercise training program in previously sedentary individuals on whole-body heat balance. Whole-body evaporative (H˙E) and dry (H˙ D) heat loss as well as changes in body heat content (DeltaH b) were measured using simultaneous direct whole-body and indirect calorimetry. It was hypothesized that following the 8-week exercise training program, a more rapid increase in the rate of whole-body heat loss would occur during exercise resulting in a decrease in the change of body heat content by the end of exercise. It was further hypothesized that the rate of decay of whole-body heat loss during recovery would be greater following the exercise training program. Methods. Ten previously sedentary young adults (7 males, 3 females) underwent an 8-week exercise program. Participants exercised at a university based facilities 4-5 times per week, 30-90 minutes per session, supervised by personnel. Prior to, and after the 8-week training program, subjects underwent an incremental treadmill test to measure their maximal aerobic capacity (V˙O2max). On a subsequent day, they performed 60-90 minutes of cycling at a constant rate of heat production (∼450 W) followed by 60 minutes of recovery, in a calorimeter at 30°C and 15% relative humidity. Core temperature [esophageal; (Tes), rectal (Tre ) and aural canal (Tau)], mean skin temperature, skin blood flow (SkBF), local sweat rate (LSR), mean arterial pressure (MAP), and heart rate (HR) were measure at baseline and at 2 min, 5 min, 8 min, 12 min, 15 min, 30 min, 45 min, 60 min, and 90 min intervals for the exercise and post-exercise recovery periods. Results. No significant difference in the rate of total heat loss (H˙L=H˙E+H˙D) was observed during exercise. As a result, the average DeltaHb was similar for the pre- (+441+/-89 kJ) and post-training (+430+/-118 kJ) 60-min exercise bout (p=0.385). Although the absolute changes in Tes (p=0.060), Tre (p5≤0.05) and Tau (p≤0.05) were lower at rest post-training, no differences in the relative change from baseline was measured during exercise. Local sweat rate and SkBF were elevated during exercise however the relative changes from baseline were similar pre- to post-training. A 12% increase in V˙O2max was measured after the 8-week training program (p≤0.05). This was paralleled by a decrease in heart rate throughout exercise (p=0.004). Conclusion. Although physical training resulted in improvements in cardiorespiratory function as evidenced by increases in V˙O2max and reduced HR response during exercise, these adaptations did not result in an improvement in the capacity for heat dissipation during exercise.