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Sweating and fluid supply

During physical exertion the body’s core temperature increases. The cooling effects of wind and heat loss by radiation are insufficient to prevent it rising dangerously; they are supplemented by the production and evaporation of sweat. Sweating begins after 1½–3 minutes of work and increases steadily before leveling out after 10–15 minutes, or longer in a humid environment.70% of the human body is composed of water, most of which is contained inside the cells. When sweating, fluid is drawn mainly from this intracellular supply with a resulting adverse effect on cell metabolism. Fluid loss therefore causes impaired performance, which becomes apparent when 1–2% of the body weight has been lost as sweat. When the fluid loss amounts to 4–5% of the body weight the capacity for hard physical work is reduced by nearly 50%. Further fluid losses are likely to lead to collapse, and this does occasionally happen during sporting competitions, especially long events conducted in warm weather.


Muscles need energy in order to work. At rest, energy supplied by the metabolism of substances within the body is used primarily to maintain a temperature of 37°C (98.6°F) and to fuel the vital functions of the internal organs. During exercise energy production in the musculature is 50–100 times greater than during rest. Fatty tissue, of which an average man has 8–10 kg (22 33 lb) distributed in various parts of the body, is the largest of our fuel stores and provides 36 kJ/g (9 kcal/g) of energy.
The carbohydrates that we eat (for example, bread, rice and potatoes) are stored as glycogen in the muscles and liver. Normally muscle contains 10–15 g/kg (¼–½ oz/lb) of glycogen, and combined muscle and liver glycogen totals 400–500 g (14–17 oz). Glycogen provides 17 kJ/g (4.1 kcal/g) of energy, or 6,600– 8,300 kJ (1,500–2,000 kcal) in total. This is approximately the amount of energy used by a top-level skier or cyclist during 1½ hours of hard competition. Thus glycogen is a limited energy reserve.

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