During April 1963, five distance runners from the Indiana University track team and one champion swimmer performed 85-min walks on a treadmill at 5.6 km/hr up a 5.6% grade in a hot environment (40 C DB, 25% relative humidity). Although none had been exposed to the heat since the preceding summer, the runners made responses typical of heat-acclimatized men. Untrained subjects exposed to the same stresses and the swimmer failed to regulate body temperature effectively. Although sweat rate was less in the runners than in the untrained men, it was 2.4 times greater per degree rise of rectal temperature for the runners. The runners produced 8% less metabolic heat per square meter of body surface than did the untrained men, and they also had much higher tissue heat conductance values. The swimmer's difficulty in adjusting to the heat stress was largely due to his relatively high metabolic cost in walking on the treadmill. It is thought that the preacclimatized state of the trained men probably resulted from the daily elevations of central temperature in their strenuous workouts during the preceding months.

acclimatization to heat; heat stress; hyperthermia; sweat rate; metabolic rate; tissue heat conductance; temperature regulation

Submitted on June 11, 1964