Is there evidence for nonthermal modulation of whole body heat loss during intermittent exercise?

Abstract

This study compared the effect of active, passive, and inactive recoveries on whole body evaporative and dry heat loss responses during intermittent exercise at an air temperature of 30°C and a relative humidity of 20%. Nine males performed three 15-min bouts of upright seated cycling at a fixed external workload of 150 W. The exercise bouts were separated by three 15-min recoveries during which participants 1) performed loadless pedaling (active recovery), 2) had their lower limbs passively compressed with inflatable sleeves (passive recovery), or 3) remained upright seated on the cycle ergometer (inactive recovery). Combined direct and indirect calorimetry was employed to measure rates of whole body evaporative heat loss (EHL) and metabolic heat production (M-W). Mean body temperature (Tb) was calculated from esophageal and mean skin temperatures, and mean arterial pressure (MAP) was measured continuously. Active and passive recoveries both reversed the reduction in MAP associated with inactive recovery ( P ≤ 0.05). This response was paralleled by greater levels of EHL during active (207 ± 53 W) and passive recoveries (203 ± 55 W) compared with the inactive condition (168 ± 53 W, P ≤ 0.05). However, the greater rate of EHL during active recovery was paralleled by a greater M-W (194 ± 16 W) compared with inactive recovery (149 ± 27 W, P ≤ 0.001). In contrast, M-W during passive recovery (139 ± 20 W) was not significantly different from the inactive condition ( P = 0.468). Furthermore, there were no differences in Tb between inactive and passive conditions during the recovery periods ( P = 0.820). As such, passive recovery resulted in greater levels of EHL for a given change in Tb compared with inactive recovery ( P ≤ 0.05). These results strongly suggest that the progressive increase in core temperature during successive exercise/rest cycles is primarily the result of a baroreflex-mediated attenuation of postexercise whole body evaporative heat loss.