Humid heat acclimation does not elicit a preferential sweat redistribution toward the limbs

We tested the hypothesis that local sweat rates would not display a systematic postadaptation redistribution toward the limbs after humid heat acclimation. Eleven nonadapted males were acclimated over 3 wk (16 exposures), cycling 90 min/day, 6 days/wk (40degreesC, 60% relative humidity), using the controlled-hyperthermia acclimation technique, in which work rate was modified to achieve and maintain a target core temperature (38.5degreesC). Local sudomotor adaptation (forehead, chest, scapula, forearm, thigh) and onset thresholds were studied during constant work intensity heat stress tests (39.8degreesC, 59.2% relative humidity) conducted on days 1, 8, and 22 of acclimation. The mean body temperature ((T) over bar (b)) at which sweating commenced (threshold) was reduced on days 8 and 22 (P less than or equal to 0.05), and these displacements paralleled the resting thermoneutral (T) over bar (b) shift, such that the (T) over bar (b) change to elicit sweating remained constant from days 1 to 22. Whole body sweat rate increased significantly from 0.87 +/- 0.06 l/h on day 1 to 1.09 +/- 0.08 and 1.16 +/- 0.11 l/ h on days 8 and 22, respectively. However, not all skin regions exhibited equivalent relative sweat rate elevations from day 1 to day 22. The relative increase in forearm sweat rate (117 +/- 31%) exceeded that at the forehead (47 +/- 18%; P less than or equal to 0.05) and thigh (42 +/- 16%; P < 0.05), while the chest sweat rate elevation (106 ± 29%) also exceeded the thigh (P < 0.05). Two unique postacclimation observations arose from this project. First, reduced sweat thresholds appeared to be primarily related to a lower resting (T) over bar (b), and more dependent on (T) over bar (b) change. Second, our data did not support the hypothesis of a generalized and preferential trunk-to-limb sweat redistribution after heat acclimation.