Calorimetric Evidence for an Exercise Intensity Dependent Increase in the Level of Postexercise Hyperthermia

Abstract

Studies show that, in parallel to the rapid decrease in metabolic heat production following cessation of exercise, there is a rapid decline in local skin blood flow and sweating to pre-exercise levels during the early stages of recovery. Consequently, a prolonged sustained elevation of core and muscle temperature relative to pre-exercise rest, which is dependent upon the intensity of exercise, is evidenced. It is unclear, however, if this pattern of response is due to a greater thermal inertia (i.e., the time taken to balance the differential rates of heat production and heat loss) during exercise as opposed to during recovery. PURPOSE: We investigated heat balance during and following exercise of different intensities using a whole-body calorimeter regulated to an ambient air temperature of 30°C. METHODS: Six physically active males performed 60-min of cycling at a constant rate of metabolic heat production of either: 1) 200 (L); 2) 350 (M) or; 3) 500 (H) Watts followed by 60-min of seated rest. RESULTS: Exercise resulted in a change in body heat content of +227±37, +308±40 and +396±31 kJ for the L, M and H exercise conditions respectively. Values were significantly different between L and H only (p<0.05). This was paralleled by a corresponding increase in rectal temperature of 0.18±07, 0.56±11 and 0.84±06°C for L, M and H respectively. Postexercise, there were no differences in the rates of metabolic heat production and whole-body heat loss between the three conditions. Consequently, the magnitude of change of body heat content was similar between conditions (i.e., -46±6, -56±8, and -70±8 kJ for L, M and H respectively). Only ∼21, 19 and 18% of the change in body heat content during exercise was dissipated postexercise for L, M and H respectively. This was paralleled by a sustained elevation in rectal temperature above baseline resting equal to 0.15±0.04, 0.30±0.06 and 0.41±0.09°C for the L, M and H conditions respectively. CONCLUSIONS: The exercise intensity dependent increase in the postexercise elevation in core temperature is likely associated with a greater thermal inertia during recovery and not exercise. Funding support from the Natural Sciences and Engineering Research Council of Canada.