Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology
ReviewExertion-induced fatigue and thermoregulation in the cold☆
Introduction
Humans rely primarily on behavioral thermoregulation to protect themselves against the cold. That is, they wear clothing, remain in shelters and use various heat generating devices. However, many participants in outdoor sports and winter recreational activities choose to disregard such behavioral strategies, or the strategies are inadequate to defend body temperature homeostasis during cold exposure. When behavior provides inadequate protection from the cold, physiological responses are elicited. The physiological responses elicited by cold exposure may influence, or be influenced by, the physiological responses to exercise. This paper will review the normal human physiological responses elicited by cold exposure, and then consider the potential interactive effects of exercise and cold.
Section snippets
Normal heat balance in the cold
Body temperature reflects the balance between body heat production and loss as illustrated by the heat balance equation:
M represents metabolic heat production and Wk represents energy gained or lost due to external work (positive for concentric and negative for eccentric work). Heat exchange between the body and environment occurs via evaporation (E), radiation (R), convection (C) and conduction (K). The sum of these, heat storage (S), represents heat gain if positive, or
Exercise in the cold
In the cold, oxygen uptake during exercise can be higher than, the same as, or lower than in warm conditions, depending on the exercise intensity (Young, 1990). Fig. 3 schematically depicts the effect of cold exposure on oxygen uptake during exercise over a range of submaximal intensities. At low exercise intensities in the cold, metabolic heat production is not high enough to prevent shivering. Thus, oxygen uptake is higher, with the increase representing the added oxygen requirement for
Exercise-induced fatigue and thermoregulation in the cold
Exercise can lead to exertional fatigue. An anecdotal association between exertional fatigue and susceptibility to hypothermia has been reported (Pugh, 1964, Pugh, 1966). Two recently reported experimental studies (Thompson and Hayward, 1996, Weller et al., 1997) attempted to demonstrate the effect of prolonged fatiguing exercise on maintenance of thermal balance in the cold. Those reports both showed that as fatigue develops during prolonged exercise in the cold, the intensity of exercise that
Summary
Humans exhibit vasomotor and thermogenic responses to cold. Peripheral vasoconstriction limits body heat loss, but allows the periphery of the body to cool. Shivering, physical activity, or both increase metabolic heat production. Heat balance in the cold, thus the requirement for shivering, is dependent on the severity of environmental stress, the effectiveness of the vasoconstriction for conserving heat, as well as intensity and mode of activity or exercise. Body size and composition are
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This paper was presented at the International Conference on Physiological and Cognitive Performance in Extreme Environments, Canberra, Australia, March 2000.
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