Individual variability in the peripheral and core interthreshold zones.

The purpose of the study was to investigate the degree of subject variability in the peripheral and core temperature thresholds of the onset of shivering and sweating. Nine healthy young male subjects participated in three trials. In the first two trials, wearing only shorts, they were exposed to air temperatures of 5 degrees C and 40 degrees C until the onset of shivering and sweating, respectively. In the second experiment, subjects wore a water perfused suit that was perfused with 25 degrees C water at a rate of 600 cc/min. They exercised on an ergometer at 50% of their maximum work rate for 10-15 min. At the onset of sweating, the exercise was terminated, and they remained seated until the onset of shivering, as reflected in oxygen uptake. In the first two trials, rectal temperature (Tre) was stable, despite displacements in skin temperature (Tsk), whereas in the third trial, Tsk (measured at four sites) was almost constant (30-32 degrees C), and the thermoregulatory responses were initiated due to changes in Tre alone. The results of the first two trials established the peripheral interthreshold zone, whereas the results of the third trial established the core interthreshold zone. The results demonstrated individual variability in the peripheral and core interthreshold zones, a proportional correlation between both zones (r=0.87), and a relatively higher contribution of adiposity in both zones as compared with those of other non-thermal factors such as height, weight, body surface area, surface area-to mass ratio, and the maximum work load.