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The amount of skin a human possesses is determined by his height and weight and is referred to as Body Surface Area. Because of the geometry of the human form, the relationship of height, weight and body surface area does not vary in a linear way. That is, some combinations of height and weight produce relatively more skin surface area than others. Many studies have examined risk factors for exertional heat illness in large populations of athletes. Their results have been remarkably consistent in one finding: For any given height, increasing weight above the ideal level is a substantial risk factor for heat illness, and the greater the surplus weight , the greater the risk. This is exactly what we would expect if the skin acted as an air-conditioner with sweat as its coolant. As an individual gains weight, more body mass must be moved from Point A to Point B. The metabolic heat cost of moving this weight progresses in a proportional way. However, the amount of skin available to produce sweat does not change much with weight gain, and the relative ability to cope with heat stress will decrease. (Of course, since fat does not contribute to movement, the ratio of working muscle to total mass also falls, another disadvantage. Surprisingly, having too much muscle is also a hazard. The ideal long distance runner has very little fat and no more muscle than is needed to move himself.) Unexpectedly, some studies have also found increasing height to be an independent risk factor for exertional heat illness. This seemingly paradoxical effect can be explained by examining the relationship of increasing height to body surface area. Because of human geometry, increasing height confers a small but definite worsening of surface to mass ratio, within the range of typical height-weight relationships. The air conditioner becomes relatively smaller as the individual gets taller and heavier. Is shorter and lighter really better for athletes participating in endurance events in hot weather? Up to a point, it would seem that this is true. Marathon runners have the lowest per cent body fat of any athletes. Female long distance runners on average weigh 15 pounds less than their teammates of identical height who are sprinters. Although there have been a few long distance runners who are very tall and very successful, many of them are surprisingly short. For example, Daniel Kamen of Kenya, the current world record holder at 3,000 m (7:20:67) and 2 miles (7:58:61) is 67 inches tall. Haile Gebrselassie of Ethiopia, the current world record holder at 5,000 meters (12:39:36) and 10,000 meters (26:22:75) is 64 inches tall. Professional tennis players routinely play matches lasting from 1 to 5 hours, often in conditions of high heat and humidity. It is safe to say that some of these players have become known for competing best in severe heat stress. Let's examine anthropometric features of several top players:
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