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).
The purpose of the present study was to examine the discrepancies in rectal temperature (T (re)) at various depths. Nineteen young males performed two bouts of bicycle exercise and recovery. T (re) was simultaneously measured at depth of 4, 6, 8, 10, 13, 16, and 19 cm, alongside the measurement of skin temperatures. We found small but statistically significant differences by depth in the absolute T (re), the magnitude of rise in T (re) and the lag of response in T (re). During the stabilization stage before exercise, T (re) at 4 cm-depth was 0.5 degrees C lower than T (re) at 16 cm-depth (p < 0.05). As the depth measured in the rectum was shallower, the rise in T (re) during exercise was greater. However the rise in T (re) at 10, 13, 16 and 19 cm showed no systemic difference. Among seven depths, T (re) at 16 cm-depth had the most stable feature with the longest latent period (3.1 +/- 1.3 min) and the smallest rise (0.8 +/- 0.3 degrees C), while T (re) at 4 cm-depth was the most responsive to the change of exercise and rest with the shortest latent period (1.0 +/- 0.6 min) and the greatest rise (1.2 +/- 0.5 degrees C). The differences observed in the depths from 4 to 19 cm were offset by exercise to some extent. In summary, T (re) appeared in different manners according to the seven depths during the repetition of exercise and rest, but T (re) deeper than 10 cm-depth seemed to have no systematic differences.
BackgroundHeat acclimatization studies have reported that tropical natives have better physiological function to tolerate heat exposure compared to those from temperate natives, in which may result in a better ability to show a better resistance to performance losses during heat stress. In this study, we investigate whether the degree of heat acclimatization affects cognitive abilities during heat exposure by comparing heat acclimatization level of subjects from Southeast Asia and temperate natives from Japan.MethodsEleven tropical males from Southeast Asia and ten temperate males from Japan participated in this study and performed two types of cognitive task: short-term memory test and mental arithmetic test, under control and passive heat exposure conditions. Passive heat condition was stimulated through leg immersion protocol by immersing subjects’ lower legs into a hot water maintained at 42 °C in a chamber controlled at 28 °C air temperature and 50% relative humidity.ResultsThe results show that the subjects in tropical group, who had smaller increase of rectal temperature, did not show any performance losses in both cognitive tests during heat exposure, while for Japanese group, there was performance decrement in mental arithmetic test during heat exposure (P < 0.05). We also found that the subjects in both tropical and Japanese groups tried to maintain their performance by increasing oxyhemoglobin in their prefrontal cortex area during performing the tasks during heat exposure. In addition, the subjects in the Japanese group showed higher increase of oxyhemoglobin when they performed the tasks during heat exposure than those when they performed the tasks in control condition (P < 0.05), while the subjects in tropical group did not show any differences in oxyhemoglobin during task performance between control and heating conditions.ConclusionsIn addition to a better ability to maintain their homeostasis during heat exposure, tropical natives from Southeast Asia showed better resistance to performance loss during heat exposure in comparison with temperate natives from Japan. The tropical natives also showed smaller increase of oxyhemoglobin indicating less cognitive effort to maintain performance.
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