The effect of low-intensity exercise in the heat on thermoregulation and certain biochemical changes in temperate and tropical subjects under poorly and well-hydrated states was examined. Two VO 2max matched groups of subjects consisting of 8 Japanese (JS) and 8 Malaysians (MS) participated in this study under two conditions: poorlyhydrated (no water was given) and well-hydrated (3 mL · Kg Ϫ1 body weight of water was provided at onset of exercise, and the 15th, 35th and 55th min of exercise). The experimental room in both countries was adjusted to a constant level (Ta: 31.6Ϯ0.03°C, rh: 72.3Ϯ0.13%). Subjects spent an initial 10 min rest, 60 min of cycling at 40% VO 2max and then 40 min recovery in the experimental room. Rectal temperatures (T re ) skin temperatures (T sk ), heart rate (HR), heat-activated sweat glands density (HASG), local sweat rate (M sw-back ) and percent dehydration were recorded during the test. Blood samples were analysed for plasma glucose and lactate levels.The extent of dehydration was significantly higher in the combined groups of JS (1.43Ϯ0.08%) compared to MS (1.15Ϯ0.05%). During exercise M sw-back was significantly higher in JS compared to MS in the well-hydrated condition. The HASG was significantly more in JS compared to MS at rest and recovery. T re was higher in MS during the test. T sk was significantly higher starting at the 5th min of exercise until the end of the recovery period in MS compared to JS.In conclusion, tropical natives have lower M sw-back associated with higher T sk and T re during the rest, exercise and recovery periods. However, temperate natives have higher M sw-back and lower T sk and T re during experiments in a hot environment. This phenomenon occurs in both poorly-hydrated and wellhydrated states with low intensity exercise. The differences in M sw-back , T sk and T re are probably due to a setting of the core temperature at a higher level and enhancement of dry heat loss, which occurred during passive heat exposure.

This study investigated the differences in heat dissipation response to intense heat stress during exercise in hot and humid environments between tropical and temperate indigenes with matched physical characteristics. Ten Japanese (JP) and ten Malaysian (MY) males participated in this study. Subjects performed exercise for 60 min at 55% peak oxygen uptake in 32°C air with 70% relative humidity, followed by 30 min recovery. The increase in rectal temperature (T(re)) was smaller in MY during exercise compared to JP. The local sweat rate and total body mass loss were similar in both groups. Both skin blood flow and mean skin temperature was lower in MY compared to JP. A significantly greater increase in hand skin temperature was observed in MY during exercise, which is attributable to heat loss due to the greater surface area to mass ratio and large number of arteriovenous anastomoses. Also, the smaller increase in T(re) in MY may be explained by the presence of a significantly greater core-skin temperature gradient in MY than JP. The thermal gradient is also a major factor in increasing the convective heat transfer from core to skin as well as skin blood flow. It is concluded that the greater core-skin temperature gradient observed in MY is responsible for the smaller increase in T(re).

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The purpose of this study was to investigate the effects of a thermal environment where air temperature closer to the ground was lower compared to that above on thermal comfort and mental performance in both sexes. Temperatures at the upper and lower parts of the body were controlled independently using a climatic box placed in a climatic chamber. Sixteen healthy subjects (8 males and 8 females) were exposed to the four conditions with various temperature differences between the upper (25 degrees C) and lower part of the body (16, 19, 22, or 25 degrees C). Skin temperature and subjective votes were measured, and two kinds of task using a computer were performed during exposure. Skin temperature on the back for females was higher than that for males during exposure, and the decrease in thigh skin temperature for females under lower air temperature conditions was significantly larger than that for males. A significant difference in thermal comfort at the beginning of the exposure was indicated between genders, especially in the 16 and 19 degrees C conditions, so females became aware of thermal discomfort before males. Although the score of mental performance based on perceptual speed for females was higher than that for males, there was no significant effect from the different vertical air temperatures. The effect of the unequal thermal environment, where air temperature closer to the ground was lower than above, on skin temperature and thermal discomfort for females was significantly higher compared to males.

The objective of this study was to investigate thermoregulatory responses to heat in tropical (Malaysian) and temperate (Japanese) natives, during 60 min of passive heating. Ten Japanese (mean ages: 20.8 ± 0.9 years) and ten Malaysian males (mean ages: 22.3 ± 1.6 years) with matched morphological characteristics and physical fitness participated in this study. Passive heating was induced through leg immersion in hot water (42°C) for 60 min under conditions of 28°C air temperature and 50% RH. Local sweat rate on the forehead and thigh were significantly lower in Malaysians during leg immersion, but no significant differences in total sweat rate were observed between Malaysians (86.3 ± 11.8 g m(-2) h(-1)) and Japanese (83.2 ± 6.4 g m(-2) h(-1)) after leg immersion. In addition, Malaysians displayed a smaller rise in rectal temperature (0.3 ± 0.1°C) than Japanese (0.7 ± 0.1°C) during leg immersion, with a greater increase in hand skin temperature. Skin blood flow was significantly lower on the forehead and forearm in Malaysians during leg immersion. No significant different in mean skin temperature during leg immersion was observed between the two groups. These findings indicated that regional differences in body sweating distribution might exist between Malaysians and Japanese during heat exposure, with more uniform distribution of local sweat rate over the whole body among tropical Malaysians. Altogether, Malaysians appear to display enhanced efficiency of thermal sweating and thermoregulatory responses in dissipating heat loss during heat loading. Thermoregulatory differences between tropical and temperate natives in this study can be interpreted as a result of heat adaptations to physiological function.

For the coherent understanding of heat acclimatization in tropical natives, we compared ethnic differences between tropical and temperate natives during resting, passive and active heating conditions. Experimental protocols included: (1) a resting condition (an air temperature of 28°C with 50% RH), (2) a passive heating condition (28°C with 50% RH; leg immersion in a hot tub at a water temperature of 42°C), and (3) an active heating condition (32°C with 70% RH; a bicycle exercise). Morphologically and physically matched tropical natives (ten Malaysian males, MY) and temperate natives (ten Japanese males, JP) participated in all three trials. The results saw that: tropical natives had a higher resting rectal temperature and lower hand and foot temperatures at rest, smaller rise of rectal temperature and greater temperature rise in bodily extremities, and a lower sensation of thirst during passive and active heating than the matched temperate natives. It is suggested that tropical natives' homeostasis during heating is effectively controlled with the improved stability in internal body temperature and the increased capability of vascular circulation in extremities, with a lower thirst sensation. The enhanced stability of internal body temperature and the extended thermoregulatory capability of vascular circulation in the extremities of tropical natives can be interpreted as an interactive change to accomplish a thermal dynamic equilibrium in hot environments. These heat adaptive traits were explained by Wilder's law of initial value and Werner's process and controller adaptation model.

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