PURPOSE: To quantify how maximum skin wettedness (ωmax); that is, the determinant of the boundary between compensable and uncompensable heat stress, is altered by aerobic training in previously unfit individuals and further augmented by heat acclimation.

METHODS: Eight untrained individuals completed an 8-wk aerobic training program immediately followed by 8 d of hot/humid (38°C, 65%RH) heat acclimation. Participants completed a humidity ramp protocol pretraining (PRE-TRN), posttraining (POST-TRN), and after heat acclimation (POST-HA), involving treadmill marching at a heat production of 450 W for 105 min in 37.5°C, 2.0 kPa (35%RH). After attaining a steady-state esophageal temperature (Tes), humidity increased 0.04 kPa·min. An upward inflection in Tes indicated the upper limit of physiological compensability (Pcrit), which was then used to quantify ωmax. Local sweat rate, activated sweat gland density, and sweat gland output on the back and arm were simultaneously measured throughout.

RESULTS: Peak aerobic capacity increased POST-TRN by approximately 14% (PRE-TRN: 45.8 ± 11.8 mL·kg·min; POST-TRN: 52.0 ± 11.1 mL·kg·min, P < 0.001). ωmax values became progressively greater from PRE-TRN (0.72 ± 0.06) to POST-TRN (0.84 ± 0.08; P = 0.02), to POST-HA (0.95 ± 0.05; P = 0.03). These shifts in ωmax were facilitated by a progressively greater local sweat rate and activated sweat gland density from PRE-TRN (0.84 ± 0.21 mg·cm·min; 67 ± 20 glands per square centimeter) to POST-TRN (0.96 ± 0.21 mg·cm·min, P = 0.03; 86 ± 27 glands per square centimeter; P = 0.009), to POST-HA (1.15 ± 0.21 mg·cm·min; P < 0.001; 98 ± 35 glands per square centimeter; P < 0.001). No differences in sweat gland output were observed.

CONCLUSIONS: A greater ωmax occurred after 8 wk of aerobic training, but ωmax was further augmented with heat acclimation, indicating only a partially increased heat loss capacity with training. These ωmax values may assist future predictions of heat stress risk in untrained/trained unacclimated individuals and trained heat-acclimated individuals.