Heat storage regulation in exercise during thermal transients

Abstract

Rate of heat storage (S) was measured by using direct and indirect calorimetry simultaneously in 11 subjects during rest and exercise at three ambient temperatures (Ta of 20, 25, and 30 degrees C), and at two work intensities (40 and 90 W). At rest, the mean value of S was -64.9 W at 20 degrees C, -26.1 W at 25 degrees C, and +9.9 W at 30 degrees C. After 50 min of exercise at 40 or 90 W, S tended toward zero at the three ambient temperatures. This indicates that thermal equilibrium was reached. In addition, at the end of the exercise periods total heat losses (R + C + E) measured at a Ta of 20, 25, and 30 degrees C were similar, i.e., independent of Ta. During the thermal transients and the steady state of exercise, the calorimetric method allows immediate measurement of S to be made, since all the physical terms of the body heat balance equation are determined. The changes in mean body temperature (delta Tb) measured by thermometry showed a delay of 5-10 min when compared with delta Tb measured by calorimetry. Thus, determination of delta Tb by thermometry is not directly applicable during thermal transients, unless the observed delay is taken into account. Our results also support the concept that Tb may be the regulated variable of the thermoregulatory system, since we obtained a very significant and uniform correlation between Esk and delta Tb at the three Ta and the two work intensities which were studied.

Physiological factors associated with the onset of sweating.

“…Nevertheless, when the differences in subjects' thermal state prior to exercise were taken into account, the variations between subjects and even within subjects remained large. Moreover, the average "threshold" heat storage at the onset varied from one study to another: from 109 (HoUDAS et al, 1973) to 62 kJ/m2 (HENANE and BITTEL, 1975) in resting subjects, and from 64 (CHAPPUIS et a!.,1976) to 37 kJ/m2 (present study) in exercising subjects. This large range of values does not rule out the body heat content as a determining factor in the initiation of sweating but at least suggests that its threshold level may partly depend on the exposure conditions.…”

“…The initiation of sweating thus takes place, in terms of regulation, some time before a sweat onset could be identified on the basis of an increase in skin evaporative loss (OGAWA and BULLARD,1972). Second, the temperature measurements taken as representative of core and skin temperature changes may not reflect, during thermal transients, the actual variations in heat content in the cutaneous and central compartments of the body as was shown by CHAPPUIS et al (1976). Hence the physiological variations associated with the sweating onset may not correspond to the inputs constituting the actual drive for sweating.…”

“…The sweating delay was inversely related to the external heat load (COLIN and HOUDAS, 1965;SALTIN et al, 1970;DAVIES, 1980) and the core temperature at sweat onset was independent of the conditions of exposure (CHAPPUIS et al, 1976;DAVIES, 1980) but exhibited a circadian variation (CROCKFORD et al, 1970). If no relationship was found between delay and work rate, this may be ascribed to the limited range of metabolic rates investigated in comparison to those used in previous studies (SALTIN et al, 1970;DAVIES, 1980).…”

“…Available data suggest that the possible range of variation in time constant is less than the range of sweating delay (TIMBAL et a!.,1970;HENANE and BITTEL,1975;CHAPPUIS et al, 1976). Sweating can be initiated in a few seconds in a pre-heated subject ( VAN BEAUMONT and BULLARD, 1963), but is often delayed when the subject is at or below thermoneutrality (HARDY and STOLwui, 1966;DAVIES, 1980).…”

“…Some studies (HoUDAS et al, 1973;CHAPPUIS et al, 1976) supported the concept that the onset of sweating is related to a critical level of mean body temperature or heat storage, but these results were not always confirmed (LIBERT et a!., 1978). In other studies (NADEL et al, 1971a;LIBERT et a!., 1978), the initiation of sweating was analyzed in terms of the interaction between central and peripheral inputs to the thermoregulatory system.…”