Technical developments have changed the situations in which humans are exposed to cold. The number of people working outdoors in cold conditions is presently declining, while that of people participating in recreational activities, e.g. winter sports is evidently increasing. Physical fitness is important for the ther-moregulatory responses to cold. Fit people have a higher metabolic response and a higher skin temperature at the onset of shivering. On the other hand, body fat provides protection against cooling .
Cold can be defined as conditions which activate heat conservation responses, and may be experienced in air or water, or in contact with solid materials. Unclothed or clothed parts of the body can be locally exposed to cold (hands, face and legs) or the whole body may be cooled. The duration of the cold exposure may last for seconds to several weeks, and can be recurrent. The effects of cold exposure will therefore depend on these factors.
Many of the cold-induced physiologic responses attenuate physical performance. Low temperature of muscles causes poor efficiency and coordination, and risk of muscle and tendon tears. Shivering muscles make use of energy stores, and shivering may also cause clumsiness. Physiologic mechanisms activated by cold are presented in the box below. Cold-induced skin vasoconstriction leads to increased blood pressure, plasma extravasation (leakage of fluid from the plasma to the interstitium) and diuresis. Increased sympathetic activation and hemoconcentration reduce maximal physical performance. Finally bron-choconstriction in winter athletes is common and may lead to exercise-induced asthma (see Chapter 4.5). A recent study showed that 23% of the Olympic winter sport athletes in the US had exercise-induced bron-choconstriction .
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