Effects of passive heat adaptation and moderate sweatless conditioning on responses to cold and heat

Summary

Two series of experiments were performed in physically untrained subjects. In series A (heat adaptation, HA), seven male subjects were adapted to dry heat (five consecutive days at 55‡ C ambient air temperature (T_a) for 1 h · day⁻¹) under resting conditions. Before and after HA, the subjects' shivering responses were determined in a cold test (T_a+10 to 0‡ C). In series B, eight male subjects underwent mild exercise training (five consecutive days at a heart rate, HR, of 120b · min⁻¹) under T_a conditions individually adjusted (T_a + 15 to +5‡ C) to prevent both sweating and cold sensations. Before and after “sweatless training≓, the subjects were subjected to a combined cold and heat test. During HA the thresholds for shivering, cutaneous vasodilatation (thumb and forearm) and sweating were shifted significantly (p<0.05) towards lower mean body temperatures ($\bar T_b$). The mean decrease in threshold$\bar T_b$ was 0.36‡ C. “Sweatless training≓ resulted in a mean increase in work rate (at HR 120b · min⁻¹) and oxygen pulse of 13 and 8%, respectively. However, “sweatless training≓ did not change the threshold$\bar T_b$ for shivering or sweating. Neither HA nor “sweatless training≓ changed the slopes of the relationships of shivering and sweating to$\bar T_b$. It is concluded that the previously reported lowering of shivering and sweating threshold$\bar T_b$ in long-distance runners is not due to an increased fitness level, but is essentially identical with HA. The decreased shivering threshold following HA is interpreted as “cross adaptation≓ produced by the Stressors cold and heat.