Individual Variability in the Peripheral and Core Interthreshold Zones

Abstract

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.

The Effect of Season and Light Intensity on the Core Interthreshold Zone

“…The mean CIZ of the five subjects was 0.37Ϯ0.13°C. This value was about half as wide as that (ϭ0.69°C, nϭ22) observed from the past experiments carried out in summer in the same lighting condition (Kakitsuba et al, 2007;Kakitsuba et al, 2009). There was a significant effect of season (pϭ0.05), and the post-hoc analyses showed that the CIZ in summer was significantly ( pϽ0.01) wider than that in winter at a brightness level of 36 cd/m 2 .…”

“…1 Diagram of the cooling system. This diagram appeared in the previous publications by Kakitsuba et al (2007Kakitsuba et al ( , 2009. For the purpose of providing details of the system, it is shown again here.…”

“…Adiposity of 31.2Ϯ5.53% (meanϮSD) for the subject group who participated in the 2009 summer and 2010 winter experiments 164 Seasonal Difference in the Core Threshold Zone was significantly (pϽ0.01) higher than for the other two groups. However, according to the study by Kakitsuba et al (2009), who analyzed the relationship between the CIZ and factors reflecting physical constitution, no physical factors appeared to be correlated with the CIZ. In addition, their analyses demonstrated that onset of shivering of the subjects who answered "susceptible to heat" becomes lower as compared with that of the subjects who answered "not susceptible to heat."…”

“…The details of the experimental protocol were described in 162 Seasonal Difference in the Core Threshold Zone our previous works (Kakitsuba et al, 2007;Kakitsuba et al, 2009). Briefly, we maintained the mean skin temperature (T sk ) at 28°C by perfusing a water-perfused suit with water at 25°C and controlling ambient temperature at 25°C/60%.…”

“…Their result may be strongly associated with the seasonal change in the CIZ. In our recent study (Kakitsuba et al, 2009), we analyzed individual variability in the CIZ as related to body physique, somatotype, and physical constitution and showed a good correlation between susceptibility to heat and the CIZ. Considering that individual awareness of heat may change the CIZ due to thermoregulatory behavior, a difference in the CIZ between summer and winter is expected.…”