Relationships among training, heat acclimation, and heat tolerance in men and women: the controversy revisited.

For more than a decade there has been a controversy over the beneficial effects of physical training in a cool environment on exercise-heat tolerance. The issues include (a) inadequate controls regarding the physical characteristics of subjects, (b) marked variability in the conditions of the standard heat tolerance tests and (c) differences in the intensity and duration of the training programs employed. Intense training in a cool environment cannot serve as a substitute for exercise in the heat if acclimation is desired within a 2 week period. However, a substantial improvement (50%) in heat tolerance can be derived from 8-11 weeks of training under temperate conditions (21 degrees C) and thermal equilibrium can be maintained for at least 4 hours during mild work (200 W/m2) in dry or wet heat by endurance runners. These adjustments occur in both men and women and appear to be independent of aerobic capacity. The key to improved thermal tolerance with training in a cool environment is maintenance of an elevated core temperature for a sufficient duration of time to produce an adaptive response.