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 (Formula presented.) 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 (Formula presented.) 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 (Formula presented.) 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 (Formula presented.), 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 (Formula presented.) independently influences thermoregulatory sweating, maximum skin wettedness (ωmax) and the change in rectal temperature (ΔTre) 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; (Formula presented.) : 41.7 ± 9.4 ml kg−1 min−1), aerobically fit (F; n = 7; (Formula presented.) : 55.6 ± 4.3 ml kg−1 min−1; P < 0.01) and aerobically unfit (n = 8) individuals, before (pre; (Formula presented.) : 45.8 ± 11.6 ml kg−1 min−1) and after (post; (Formula presented.) : 52.0 ± 11.1 ml kg−1 min−1; P < 0.001) an 8-week training intervention. ωmax was similar between UF (0.74 ± 0.09) and F (0.78 ± 0.08, P = 0.22). However, ωmax 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 ΔTre 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 (Formula presented.) per se, confers an increased ωmax, greater sweat rate, and smaller rise in core temperature during uncompensable heat stress in fit individuals.