As the skin temperature rises, it causes blood to pool in the veins of the arms and legs. This is because the elevated skin temperature “paralyzes” the veins, which dilate and soon fill with a large volume of blood.
This blood is effectively lost from the circulation and can only be returned to the circulation if the skin temperature is again lowered. This can be achieved by sponging (literally wetting the skin with a sponge).
A recent study confirmed that wetting the skin did indeed lower the skin temperature during exercise but did not aid heat loss (Bassett et al, 1987). Thus the benefits of sponging during exercise probably relate to its effect on the central circulation.
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Factors Explaining Impaired Running Performance in the Heat
Anyone who has run a marathon or longer race in the heat knows that such races are much more difficult than are races of the same distance run in cool conditions. The most likely explanation for this comes from recent studies showing that precooling of the body or of the active muscles either prior to or during exercise prolongs endurance time to exhaustion in both dogs and humans (Kozlowski et al, 1985; Kruk et al, 1985; Olschewski & Briick, 1988; Schmidt & Briick, 1981). This cooling keeps the muscle temperature lower during subsequent exercise and alters the metabolic response by decreasing the rates of muscle glycogen utilization, muscle lactate accumulation, and the fall in muscle high-energy phosphate content (Kozlowski et al, 1985), thereby allowing the cooler muscles to exercise for longer. By inference we may conclude that the sustained elevation of muscle temperature that occurs during prolonged exercise may be one of the most important factors limiting endurance performance.
Calculating Sweat Rate During Exercise
To calculate your sweat rate or to Exercises how much your current drinking pattern during races falls short of replacing your sweat losses, you could try the following experiment.
Weigh yourself (naked) on a scale reading in kilograms, immediately before (WB) and immediately after (WA) a run in conditions and at a pace to which you are accustomed. Measure carefully the total amount of fluid (F) in liters that you ingest while running. You can then calculate your sweat rate fairly accurately.
Your fluid replacement will have been adequate if, after races longer than 30 km, you have lost less than 2 to 3 kg and are not dehydrated by more than 3 %, calculated by this equation:
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