Practically no studies exist in which the effects of cold acclimatization on physical performance have been studied. We know from some studies that the unpleasantness of cold sensations becomes reduced or habituated after 1-4 daily cold exposures, and that increased sympathetic activity and shivering is attenuated within a week [12-14]. True cold acclimatization is difficult to induce in humans. Three types of adaptation to cold are described: (i) metabolic, where a greater metabolic response to cold stress is developed [15,16]; (ii) hypothermic, where core temperature falls (e.g ); and (iii) insulative  with a lowering of the skin to environment gradient and heat loss, and with little change in core temperature and metabolic rate during cold exposure.
Immersion in cold water 5 days per week over 5 weeks has been found to induce the type of adaptation described as insulative : a lowering of resting rectal temperature, a slower rise in metabolic rate (indicating a delay in onset of shivering) and a lower skin temperature. It appears that a repeated fall in core temperature is necessary to induce the sympathetic activation, while a cold skin alone is enough to stimulate the increased vasoconstrictor response obtained after 5 weeks' daily cold water immersions . However, when healthy men were exposed to cold air for 11 days, a hypothermic type of acclimatization was observed instead (i.e. reduced cold sensations, decreased core and increased skin temperature in some places, reduced norepinephrine response, and no changes in metabolic rate or heat debt responses) . It is evident that acclimatization in cold water is different from that in cold air. Moreover, cold water acclimatization increases norepinephrine response and peripheral resistance, and decreases cardiac output [18,19], all of which are not beneficial for physical performance.
Therefore, cold air and not cold water acclimatization is perhaps the type of acclimatization that should be used if performance in cold air is to be improved.
It is evident that unacclimatized subjects perform less well than acclimatized, and therefore some kind of cold acclimatization should be obtained before winter sports competitions. It is in the author's knowledge that subjects who are about to participate in the demanding polar expeditions acclimatize by sleeping overnight outdoors for several weeks before their expeditions (see Case story 2.3.1).
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