Energy Expenditure and Metabolism
The total energy expenditure of your body is relatively constant from day to day and consists of three major components: resting metabolic rate RMR; the energy cost of food absorption, metabolism, and storage F; and the energy cost of exercise and other movements EX. RMR accounts for the largest portion of daily energy expenditure -, and is affected by many intrinsic and extrinsic factors, including age, fat-free muscle mass, prolonged exercise, clothing, and ambient temperature. This final factor is relevant to the present chapter because it may or may not alter RMR, depending on the severity of the environment. Figureillustrates this fact; it was published by Eric Poehlman and colleagues at the University of Vermont. This figure shows that there is a zone in which ambient temperature changes do not alter RMR appreciably at the lowest point in the curve. When air temperature is considerably lower than core body temperature, RMR increases, via involuntary muscular activity shivering and hormonal effects, to replace the heat that was lost to the environment see figure pageFigurealso allows you to mentally compare the energy expended during shivering at rest to typical energy expenditures during exercise kcal/mi of running or walking.
The nutrients that are used by cells for fuel metabolic substrates during rest or light daily activities also change in cold environments. In mild conditions, lipids are the primary substrates, accounting for of all energy expenditure. When resting for in cold air C, F, carbohydrates become the preferred fuel for shivering thermogenesis, derived from both blood glucose and intramuscular glycogen stores. As you will see in the next section of this chapter, these responses are different during exercise in cold air. Interestingly, the onset of low blood sugar hypoglycemia has been shown to inhibit shivering thermogenesis in humans; this further emphasizes the importance of carbohydrates as a substrate for shivering.
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Cold Air and Physical Performance
Numerous difficulties arise from exercise or work in cold air; all these are stressors involving either insulative clothing or body cooling. The first involves the awkwardness and extra weight of protective clothing. As the weight of clothing increases, caloric expenditure increases at a rate that is greater than can be accounted for by the weight of the clothing alone. This extra caloric output has been attributed to both a ”hobbling effect interference with joint movements caused by bulky clothing and frictional drag caused by layers of material sliding over each other. The second difficulty involves shivering, or an increase in muscle tone before shivering begins. Both conditions increase the metabolic cost VO of exercise at a given speed; this means that exercise is less efficient during shivering. Also, because shivering occurs in both agonists and antagonists, normal movement patterns and muscular coordination can be impaired. Third, intense skin cooling effectively anesthetizes sensory receptors in the hand. This markedly impairs manipulative motor skills requiring finger dexterity, such as marksmanship, catching, and throwing. One published experiment reported that manual dexterity declined when skin temperature fell belowF. Although not directly related to exercise, two other difficulties arise from prolonged cold-air exposure: a tissue injury to skin from frostbite see below, and b the negative psychological and emotional effects impaired cognition and clinical depression that develop during prolonged labor and daily activities in a cold climate.
Controlled laboratory studies have evaluated differences between physiological responses to exercise in cold air and those in thermoneutral conditions. T able summarizes the findings of these studies with regard to specific thermoregulatory, cardiopulmonary, and metabolic measurements. However, it is important that you realize that this table presents a generalized statement about each variable, and that a cluster of physiological responses, such as those in table cannot describe all exercise responses to all cold conditions
Increases with increased workload Increases with increased workload a Conditions of light-moderate workloads, mild ambient temperature, and steady-state exercise. b Relative to that of the thermoneutral condition.