An evaluation of the role of skin temperature during heat adaptation.

This project sought to evaluate the importance of skin temperature during heat acclimation, using an isothermal-strain model. Two groups of seven matched males, participated (1 h per day, 10 days) in one of two conditions: (i) temperate physical training (TEMP: 22.4 +/- 0.7 degrees C, relative humidity (r.h.) 41.0 +/- 0.9%); or (ii) combined physical training and heat acclimation (HEAT: 38.2 +/- 0.7 degrees C, r.h. 39.7 +/- 1.3%). Isothermal strain was induced in both groups by rapidly elevating rectal temperature by 1 degree C (cycling), then holding it constant by manipulating external work. Subjects completed two three-phase heat stress tests (39.8 +/- 0.1 degrees C, r.h. 38.6 +/- 1.2), consisting of 20 min rest, then 20 min cycling at each of 30% and 45% of peak power, before and after each regimen. While there was a difference of 4.2 degrees C in mean skin temperature between treatments, both regimens elicited a similar peripheral sudomotor increase, indicating a core temperature dependent adaptation. However, based on significant pre- vs. post-acclimation decreases in average auditory canal temperature (0.4 +/- 0.1 degree C), average forehead skin blood flow (26%), average perceived exertion (11%), and a 5% increase in average forehead sweat rate (0.1 +/- 0.04 mg cm-2 min-1), the HEAT regimen elicited a more complete acclimation. While elevation in core temperature is critical to acclimation, it also appears necessary to expose subjects to an external thermal stress. This observation has not been previously demonstrated under conditions of isothermal strain, and verifies the importance of skin temperature elevation in the acclimation process.