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

Abstract

The hypothesis tested in the present study is a seasonal difference in the core interthreshold zone (CIZ), as we suggested in an earlier study that individual awareness of heat may change the CIZ due to thermoregulatory behavior. A series of human experiments were carried out in a climatic chamber in January and August of 2009 and January of 2010. The air temperature in the chamber was controlled at 20–24°C. Subjects wore a water-perfused suit that was perfused with 25°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 until their sweating rate increased and then remained seated without exercise until oxygen uptake increased. Subjects' rectal temperature and skin temperatures at four sites were monitored by thermistors. The sweating rate was measured at the forehead with a sweat rate monitor (SKD 4000, Skinos Co.). Oxygen uptake was monitored with a gas analyzer (Respiromonitor RM-300i, Minato Med. Science Co.). In the 2009 winter experiment, 5 male subjects were exposed to lighting of 36 cd/m²/1,050 lx, and in the 2009 summer and 2010 winter experiments, 10 male subjects were exposed to lighting of 18 cd/m2/510 lx. The results showed that the CIZ of 0.69±0.29°C (n=22, data from 2005–2007 experiments) at 36 cd/m² and that of 0.37±0.17°C (n=10) at 18 cd/m² in summer were greater than the CIZ of 0.37±0.13°C (n=5) at 36 cd/m² and that of 0.18±0.17°C (n=10) at 18 cd/m² in winter, and thus demonstrated a seasonal difference in the CIZ as well as an effect of lighting conditions on the CIZ.