Heat acclimation, physical fitness, and responses to exercise in temperate and hot environments

Three groups of men with different Vo₂ max (60.1, 47.7, and 35.6 ml.kg^-1 min^-1) were administered two submaximal tests at 23°C, at 41 and 82 W, before and after 8 days of heat acclimation (3-h work at 41 W at 39.4°C dry bulb, 30.3°C wet bulb). A control group with Vo₂max of 45.3 ml.kg^-1.min^-1 was tested at 23°C. and in heat before and after 6 days of exercise at 23°C. Trained subjects with the highest Vo₂max showed the best responses, and the lowest Vo₂max group showed the worst responses at 23°C and in heat (differences in heart rates and rectal temperatures but not in sweat rates and oxygen consumption responses). Heat acclimation resulted in substantial improvements in responses at 23°C and in heat of the acclimation groups, with very minor changes shown by the control group. Changes at 23°C were characterized by decreases in heart rate, rectal temperature (0.3-0.5°C), oxygen consumption, and sweat rate (25-30%), and increases of 13% and 23% in Vo₂max in the groups with average and low Vo₂max, respectively. Vo₂max correlated r=-0.62 and -0.65 with rectal temperatures at 23°C and in heat, respectively. It was shown that exercise rectal temperature at 23°C was mainly a function of heat acclimatization, as well as Vo₂max and surface area/mass ratio, that heat acclimation presented an effective method of physical training, and that Vo₂max was partially related to heat tolerance.