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Exertion-induced fatigue and thermoregulation in the cold

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Abstract

Cold exposure facilitates body heat loss which can reduce body temperature, unless mitigated by enhanced heat conservation or increased heat production. When behavioral strategies inadequately defend body temperature, vasomotor and thermogenic responses are elicited, both of which are modulated if not mediated by sympathetic nervous activation. Both exercise and shivering increase metabolic heat production which helps offset body heat losses in the cold. However, exercise also increases peripheral blood flow, in turn facilitating heat loss, an effect that can persist for some time after exercise ceases. Whether exercise alleviates or exacerbates heat debt during cold exposure depends on the heat transfer coefficient of the environment, mode of activity and exercise intensity. Prolonged exhaustive exercise leading to energy substrate depletion could compromise maintenance of thermal balance in the cold simply by precluding continuation of further exercise and the associated thermogenesis. Hypoglycemia impairs shivering, but this appears to be centrally mediated, rather than a limitation to peripheral energy metabolism. Research is equivocal regarding the importance of muscle glycogen depletion in explaining shivering impairments. Recent research suggests that when acute exercise leads to fatigue without depleting energy stores, vasoconstrictor responses to cold are impaired, thus body heat conservation becomes degraded. Fatigue that was induced by chronic overexertion sustained over many weeks, appeared to delay the onset of shivering until body temperature fell lower than when subjects were rested, as well as impair vasoconstrictor responses. When heavy physical activity is coupled with underfeeding for prolonged periods, the resulting negative energy balance leads to loss of body mass, and the corresponding reduction in tissue insulation, in turn, compromises thermal balance by facilitating conductive transfer of body heat from core to shell. The possibility that impairments in thermoregulatory responses to cold associated with exertional fatigue are mediated by blunted sympathetic nervous responsiveness to cold is suggested by some experimental observations and merits further study.

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:S=M−±Wk±E±R±C±KW/m2

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.

    DISCLAIMER: The views, opinions and/or findings contained in this report are those of the authors and should not be construed as an official Department of the Army position, policy, or decisions, unless so designated by other official documentation. Approved for public release; distribution is unlimited.

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