The metabolic and thermal responses to muscular exertion in a cold environment were studied in outdoor and indoor workers. The metabolic rate in all subjects during exercise was higher in the cold environment than in the warm environment at low levels of work; at higher workloads it was the same. In the cold environment there was a tendency towards lower oxygen uptake of the outdoor workers than of the indoor workers at low levels of work; at high levels it was essentially the same. Differences in skin temperature and onset of rewarming indicate an adaptability to cold of the vasomotor control mechanism of the peripheral circulation. It is suggested that habituation to cold leads to a lower set point of the thermosensitive cells of the thermoregulatory center, so that vasodilation impulses are discharged at a lower temperature.
During 7 hours of comfortably warm sleep, average metabolic rates of 11 Indian and 7 white men were alike. Since the Indians were 15% lighter their metabolic rates (MR's) referred to weight were greater. During a night at 0° with insufficient covering, MR's rose to 129% and 132% in the two groups. Cold caused equal myographic records of shivering (15% and 13% of records) and gross muscular movement appeared in 6.5% of the records for each group. Encephalograms showed that Indians slept more (51%) than whites (31%). Shivering was recorded in Indians and white men during encephalographic indications of sleep. Rectal temperatures of Indians declined about 0.5°. During cold nights skin on the bodies of all subjects cooled 3°–5° and about 15° on the feet. All subjects were disagreeably cold, but their cold sensations stimulated metabolic heat production only half as much as would be necessary to maintain fairly comfortable warmth.
Physical fitness in terms of aerobic working capacity was measured in nomadic Lapps living in the northern part of the Scandinavian peninsula. Forty-nine men between 10 and 55 years of age and 21 girls were studied. Aerobic capacity was determined by measuring oxygen consumption during exercise on a bicycle ergometer. Two or three submaximal loads were used. The maximal work lasted three to four minutes, during which time the subjects worked as hard as they could. Blood lactate taken after this heavy run showed that the oxygen requirement exceeded oxygen intake, thus indicating that maximal values for oxygen intake were achieved during this type of exercise.
The values for maximal oxygen intake of nomadic Lapps increased steadily from the age of 10 up to 18 years, from an average of 1. 4 liters/minute to about 3. 5 liters/minute . The latter value remained essentially unchanged up to the age of 30 in men. Maximum oxygen consumption then decreased to about 2. 5 liters/minute at. 50 years of age. No sex differences in maximum oxygen consumption were noted in subjects below 15 years of age.
Adaptations which equip a mammal to cope with the cold stresses of the Arctic environment must at the same time be accompanied by responses which enable it to dissipate large quantities of heat produced during exercise. Some aspects of the heat producing and heat dissipating mechanisms were investigated in the reindeer, an example of a large well adapted Arctic mammal. The oxygen consumption of a reindeer while standing quietly was 606 ml/minute; while pulling a heavily loaded sled, 2390 ml/minute. The evaporative heat loss from the respiratory tract of a standing reindeer was 12 kcal/hour, or seven percent of the heat production; of a vigorously exercising reindeer, 130 kcal/hour, or 20 percent of the heat production. The temperature of the air expired at the nostril was as low as 14° C when the reindeer was standing in a wind at -16° C, and about 30° C at ambient temperatures near 0° C. After the animal exercised, the nostril temperature was 35° to 37° C. The heat production of the rumen ingesta was found to be as high as 0.09 kcal/hour per kilogram of body weight soon after feeding, or 5% to 10% of the basal heat production. The average surface temperature of the thinly furred parts of the reindeer was 5° to 11° C above ambient temperature, the hoof temperature 5° to 9° above ambient and the thickly furred parts only 2° to 4° C above ambient when the reindeer was conserving heat during rest. On the other hand, during vigorous exercise the thinly furred surface was 18° to 22° C above ambient temperature, the hoof was 21° to 28° C above ambient, and the thickly furred surfaces 12° to 15° C above ambient.