MECHANISMS FOR HEAT LOSS DURING EXERCISE
As exercise begins, the blood flow to the muscles increases. Not only does the heart pump more blood, but blood is preferentially diverted away from nonessential organs and toward the working muscles and skin. As blood passes through the muscles, it is heated, and it distributes the added heat throughout the body, particularly to the skin. In this manner, as well as by direct transfer from muscles lying close to the skin, heat is conducted to the skin surface. Here, circulating air currents convect this heat away, and any nearby objects whose surface temperatures are lower than the skin temperature attract this heat, which travels by electromagnetic waves in a form of energy transfer known as radiation.
In another method of heat loss, surface heat evaporates the sweat produced by the sweat glands in the skin. Sweating itself does not lose heat; heat is lost only when that sweat actually evaporates. The efficiencies of all these mechanisms depend on a variety of factors, most of which are open to modification by the athlete.
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The Exercise Intensity
As the intensity of exercise increases, the body must decide whether to pump more blood to the muscles to maintain their increased energy requirements or to
Assist heat dissipation by increasing the skin blood flow. Faced with conflicting demands, the body always favors an increased blood flow to the muscles. The result is that while body heat production is increased, the ability to lose that heat is decreased.
It appears that athletes running at world-record speeds, at least in races up to 16 km, develop marked limitations to skin blood, flow and therefore have limited abilities to lose heat. They thus run in microenvironments in which their abilities to maintain heat equilibrium depend entirely on the prevailing environmental conditions (Pugh, 1972). If these conditions are unfavorable, the athletes will continually accumulate heat until their body temperatures reach the critical level at which heatstroke occurs.
Two famous athletes running at world-record pace who developed heatstroke when forced to race in unfavorable environmental conditions were Jim Peters in the 1954 Empire Games Marathon (see post 8) and Alberto Salazar in the 1980 Falmouth 12-km Road Race.