The Influence Of Aerobic Training On Maximum Skin Wettedness And Its Effects During Uncompensable Heat Stress

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PURPOSE: The purpose of the present experiment was to quantify how maximum skin wettedness (ωmax) is altered by aerobic training, and compare it to what is achieved following heat acclimation (HA). METHODS: Eight sedentary individuals (6 males, 2 females) participated in an 8-week aerobic training regime followed by a 9-day heat acclimation (HA) protocol. Participants completed on separate days, i) a treadmill humidity ramp protocol trial to assess ωmax; and ii) a 60-min treadmill march (450 W of heat production) in an uncompensable environment: 38°C, 60% RH, on three separate occasions: pre-training (PRE-T), post-training (POST-T), and post-heat acclimation (POST-HA); The change in rectal (ΔTre), and mean skin temperature (Tsk) were recorded. Whole body sweat loss (WBSL) was calculated as the change in nude body mass and sweating efficiency (Seff) was derived by dividing the sweating required to achieve ωmax (with 100% evaporation) by the actual whole-body sweat rate between the 30th and 60th minute of exercise. RESULTS: Aerobic training increased aerobic capacity by ∼14% (PRE-T: 45.8±11.8 ml/kg/min; POST-T: 52.0±11.1 ml/kg/min, P<0.001). In the humidity ramp trial, ωmax was lower PRE-T (0.75±0.07) compared to POST-T (0.87±0.12, P=0.01) and POST-HA (1.00±0.00, P=0.001), and POST-T was lower than POST-HA (P=0.04). In the UC trial, ΔTre was greater PRE-T (1.13±0.16°C) compared to POST-T (0.96±0.13°C, P<0.001) and POST-HA (0.96±0.20°C, P<0.001). PRE-T Tsk was higher after 60-min (38.0±0.4°C) compared to POST-T (37.2±0.9°C, P<0.001) and POST-HA (37.1±0.4°C, P<0.001). WBSL was significantly greater POST-HA (913±126 g) compared to POST-T (794±78 g; P=0.002) and PRE-T (671±83 g, P<0.001), however Seff was similar throughout (PRE-T: 67±10%; POST-T: 68±11%; POST-HA: 66±8%). CONCLUSIONS: Aerobic training and HA independently increase ωmax without altering Seff. A graded reduction in thermal strain during uncompensable heat stress is observed from PRE-T to POST-T, and to POST-HA.