A retrospective analysis to determine if exercise training-induced thermoregulatory adaptations are mediated by increased fitness or heat acclimation

Abstract

New findings: What is the central question of this study? Are fitness-related improvements in thermoregulatory responses during uncompensable heat stress mediated by aerobic capacity ${\dot{V}}_{O_2\max }$ or is it the partial heat acclimation associated with training? What is the main finding and its importance? During uncompensable heat stress, individuals with high and low ${\dot{V}}_{O_2\max }$ displayed similar sweating and core temperature responses whereas exercise training in previously untrained individuals resulted in a greater sweat rate and a smaller rise in core temperature. These observations suggest that it is training, not ${\dot{V}}_{O_2\max }$ per se, that mediates thermoregulatory improvements during uncompensable heat stress.

Abstract: It remains unclear whether aerobic fitness, as defined by the maximum rate of oxygen consumption ${\dot{V}}_{O_2\max }$ , independently improves heat dissipation in uncompensable environments, or whether the thermoregulatory adaptations associated with heat acclimation are due to repeated bouts of exercise-induced heat stress during regular aerobic training. The present analysis sought to determine if ${\dot{V}}_{O_2\max }$ independently influences thermoregulatory sweating, maximum skin wettedness (ω_max ) and the change in rectal temperature (ΔT_re ) during 60 min of exercise in an uncompensable environment (37.0 ± 0.8°C, 4.0 ± 0.2 kPa, 64 ± 3% relative humidity) at a fixed rate of heat production per unit mass (6 W kg^-1 ). Retrospective analyses were performed on 22 participants (3 groups), aerobically unfit (UF; n = 7; ${\dot{V}}_{O_2\max }$ : 41.7 ± 9.4 ml kg^-1 min^-1 ), aerobically fit (F; n = 7; ${\dot{V}}_{O_2\max }$ : 55.6 ± 4.3 ml kg^-1 min^-1 ; P < 0.01) and aerobically unfit (n = 8) individuals, before (pre; ${\dot{V}}_{O_2\max }$ : 45.8 ± 11.6 ml kg^-1 min^-1 ) and after (post; ${\dot{V}}_{O_2\max }$ : 52.0 ± 11.1 ml kg^-1 min^-1 ; P < 0.001) an 8-week training intervention. ω_max was similar between UF (0.74 ± 0.09) and F (0.78 ± 0.08, P = 0.22). However, ω_max was greater post- (0.84 ± 0.08) compared to pre- (0.72 ± 0.06, P = 0.02) training. During exercise, mean local sweat rate (forearm and upper-back) was greater post- (1.24 ± 0.20 mg cm^-2 min^-1 ) compared to pre- (1.04 ± 0.25 mg cm^-2 min^-1 , P < 0.01) training, but similar between UF (0.94 ± 0.31 mg cm^-2 min^-1 , P = 0.90) and F (1.02 ± 0.30 mg cm^-2 min^-1 ). The ΔT_re at 60 min of exercise was greater pre- (1.13 ± 0.16°C, P < 0.01) compared to post- (0.96 ± 0.14°C) training, but similar between UF (0.85 ± 0.29°C, P = 0.22) and F (0.95 ± 0.22°C). Taken together, aerobic training, not ${\dot{V}}_{O_2\max }$ per se, confers an increased ω_max , greater sweat rate, and smaller rise in core temperature during uncompensable heat stress in fit individuals.

Keywords: fitness; heat stress; sweating; thermoregulation; training.