Quantifying the Time-course of Changes in Maximal Skin Wettedness with 7 days of Heat Acclimation.

The aim of the present study was to quantify the time-course of changes in maximum skin wettedness (ω_max) - i.e., the proportion of skin surface area covered in sweat at the point of uncompensable heat stress, throughout 7 consecutive days of heat acclimation. Nine adults (6M, 3F) completed a humidity-ramp protocol (RAMP) on days 1, 3, 5 and 7 of seven consecutive days of heat acclimation. In each RAMP trial, participants cycled continuously at 275 W·m^-2 for 120 min at 37°C: 60-min at a vapour pressure of 2.05 kPa followed by 60-min with vapour pressure increased by 0.045 kPa·min^-1. An upward inflection in esophageal temperature (T_eso) signaled a transition to uncompensable heat stress with the critical water vapour pressure at that point used to calculate ω_max. In days between RAMP assessments participants cycled for 90-min at 75% HRmax at 37°C, 60% RH. T_eso, whole-body sweat rate (WBSR), local sweat rate (LSR_back, LSR_arm) and activated sweat gland density (AGSD) were measured throughout. ω_max was progressively and significantly greater from Day 1 (0.68±0.10) to Day 3 (0.75±0.10;P=0.002), to Day 5 (0.79±0.10;P=0.004), to Day 7 (0.87±0.06;P=0.009). WBSR was higher on Day 5 (1.11±0.30 L·h^-1;P=0.01) and Day 7 (1.12±0.19 L·h^-1;P<0.001) compared to Day 1 (0.94±0.21 L·h^-1). ASGD was higher on Day 5 (78±15 glands·cm^-2;P<0.001), and Day 7 (81±17 glands·cm^-2;P=0.001) compared to Day 1 (65±12 glands·cm^-2). There were no observed differences in sweat gland output (P=0.21). In conclusion, ω_max significantly increased throughout 7 days of heat acclimation. These progressive increases in ω_max were predominantly mediated by an increase in the number of active sweat glands, not the output per gland.