Apart from aesthetic reasons, the rationale for wearing clothing is to trap a thin layer of air next to the body. Because air is a poor conductor of heat, this thin layer rapidly heats to body temperature and acts as an insulator preventing heat loss. Clearly, any clothing that is worn during exercise in the heat must be designed for the opposite effectto promote heat loss.
Marathon runners have learned that light, porous clothing such as fish net vests best achieve this heat loss. In contrast, T-shirts or heavy rugby jerseys, particularly when soggy with sweat, become very good insulators, preventing adequate heat loss.
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Novice runners, particularly those who might consider themselves overweight, often train in full track suits in the heat. Many neophyte athletes probably believe that the more they sweat, the harder they must be exercising and therefore the greater the weight they stand to lose. The unfortunate truth is that in running, the energy cost is related only to the distance run. Thus, to lose more weight, one needs to run a greater distance.
Excessive sweating will effect a sudden loss of weight by dehydrating the body; this is the procedure used by boxers, jockeys, and wrestlers in making the weight. By exercising in the heat for as little as half an hour, one can lose as much as 1 kg, but this is a fluid loss that will be rapidly replaced if the athlete rehydrates by drinking. In contrast, to lose a real kilogram of body weight one must expend about 37,500 kJ of energy, equivalent to running about 160 km!
The insulating qualities of different clothing is expressed as CLO units. One CLO unit is equivalent to the amount of insulation provided by ordinary business apparel that provides comfort at temperatures of 21 °C when both wind speed and humidity are low. The clothing of the Eskimo provides 10 to 12 CLO units and is essential for life in arctic conditions. However, because of the considerable
Heat production during exercise, clothing that will provide 1 CLO unit of insulation is all that is required when running in temperatures as low as 22 °C, provided there is little or no wind.
Thus, a runner who lives in a moderate climate seldom (if ever) needs to train for any period in a track suit. By doing so, the runner merely increases discomfort and promotes conditions favorable for heatstroke.