Seven Japanese male adults aged 23-28 were subjected to a cold environmental stress. Three of them were born and grew in Hokkaido district which is a northerland in Japan (HK group), and the other four were born, grew up in Honshu or Kyushu district in Japan and immigrated into Hokkaido (Non-H K group).
Measurements of heart rate (HR) and bipolar surface EMG of 11 muscles were performed before and during exposure to the cold environment which was consisted of the air temperature of about 10°C, the radiant temperature of about 10°C, the relative humidity of about 50% and the air movement 10-20 cm/sec.
In consideration of a large number of other previous papers on H R during cold exposure, it could be supposed that H R might be suppressed by cold stimuli and facilitated by shivering. It seemed to be quite right to consider that increase in H R would be caused in the intense cold environment for individuals as a whole and decrease in the slight cold. Under the above assumption, H R changes of H K group and Non-H K group were discussed. It could be considered that H K group was superior in the cold tolelance to Non-H K group within the limits of the present experiment.
From the EMG results in HK group shiverng occurred mainly in trunk region, and two of Non-H K group had the same result but in the other two of Non-H K group shivering occurred not only in trunk region but also in inferior extremity region. It might be pointed out that in the cold environment when shivering extended to extremity region especially great inferior extremity muscles from trunk region HR increased compared with that in the optimal thermal environment.