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

Abstract

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 degrees C ambient air temperature (Ta) for 1 h X day-1) under resting conditions. Before and after HA, the subjects' shivering responses were determined in a cold test (Ta + 10 to 0 degrees C). In series B, eight male subjects underwent mild exercise training (five consecutive days at a heart rate, HR, of 120 b X min-1) under Ta conditions individually adjusted (Ta + 15 to +5 degrees 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 less than 0.05) towards lower mean body temperatures (Tb). The mean decrease in threshold Tb was 0.36 degrees C. "Sweatless training" resulted in a mean increase in work rate (at HR 120 b X min-1) and oxygen pulse of 13 and 8%, respectively. However, "sweatless training" did not change the threshold Tb for shivering or sweating. Neither HA nor "sweatless training" changed the slopes of the relationships of shivering and sweating to Tb. It is concluded that the previously reported lowering of shivering and sweating threshold Tb 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.

Humid heat acclimation does not elicit a preferential sweat redistribution toward the limbs

“…Indeed, Armstrong et al (1) and Cotter et al (6) failed to observe significant ṁ sw elevations after short-duration heat acclimation, while others have reported a more rapid sudomotor adaptation (9,14,38). Our long-term, controlledhyperthermia design was well suited to an evaluation of this adaptation pattern, and the current data show that only six exercise-heat exposures (controlled hyperthermia) were needed to elicit local sweat elevations from 33 to 69% (thigh-forearm; Fig.…”

Challenges to Temperature Regulation When Working in Hot Environments

“…(ii) Exercise-induced heat adaptation: Endurance exercise elevates muscle and deep tissue temperatures and, if this thermal load is applied regularly, heat adaptation will ensue 8) . This method may be used in three different ways, each of which can modify the resultant thermal strain: · Exercise under cool and temperate states: However, exercise in the absence of a T c elevation will not elicit heat adaptation 33) . · Exercise with a significant solar load.…”

Shifts of thermoeffector thresholds in heat‐acclimated rats.

“…Our previous study has shown that, in rats wvhich had been exposed to heat given for about 5 h in the last half of the dark phase for mnore than 10 days, T'or fell for 3-4 h during the period corresponding to the previous heat exposure time Schwennicke & Briick, 1976;Henane, 1981;Hessemer et al 1986) and animals (Bruiek et al 1970;Shido & Nagasaka, 1990). Shido & Nagasaka (1990) reported that, after subjection to heat for 5 h daily for 15 consecutive days, threshold T'or for tail skin vasodilatation tended to be lower in heat-acclimated rats than in non-acclimated animals.…”

“…In their experiments, the rats were exposed daily to heat in the light phase and the determination of threshold T,or was made during the same period of the heat exposure. Similarly, thermoeffector thresholds in humans were usually determined during the previous heat exposure periods (Schwennicke et al 1976;Henane, 1981;Hessemer et al 1986). Thus, previous reports are essentially consistent with the present observation which shows downward shifts in thermnoeffector thresholds during the period when the subjects had previously been exposed to heat.…”

“…In humans, threshold Tor values for sweating and shivering shift to low levels after subjection to heat for several hours daily (Schwennicke & Briick, 1976; Henane, 1981;Hessemer et al 1986). Downward shifts in T'or and threshold Tlor for non-evaporative heat loss are likely to occur in rats after acclimation to heat loaded intermittently (Shido & Nagasaka, 1990).…”