Control of  these factors falls into various realms: Temperature, relative humidity, radiant heat and wind are greatly influenced by the race organizer's choice of date, time and course route. The athlete's fitness level and acclimatization will be determined by chronic training techniques and pre-race environmental factors. The only element which the athlete can control during the race is his metabolic rate. If he expects to win with a 2:13:00 marathon, he will need to exert at 76.2cc/kg/min. If he wants to rest, he can slow up to whatever rate he wishes, or even stop and sit down, at which point his oxygen uptake will rapidly drop to 3.5cc/kg/min. This concept is vitally important in understanding the effects of environment on athletic performance.

Why? Core temperature. Resting core temperature for an adult is about 99°F. As metabolic rate increases, heat must be dissipated or core temperature will rise. Rising core temperature is a major component of perceived fatigue. At a core temperature level of between 101.5°F and 104°F, most athletes will collapse. Some elite long distance runners can sustain a core temperature of 104.5 to 105 degrees. No one can sustain a core temperature of 108 degrees, which will rapidly lead to severe heat illness and risk of injury or death.

Recognition of this delicate balance between temperature and performance has led to the practical development of scheduling most marathons in the spring or fall, when climatologic conditions are likely to be ideal. The Boston Marathon, the oldest of all modern long-distance races, is traditionally run on Patriot's Day, April 19th. Here is the heat index chart for that day:

Starting at 12 noon, the average Boston Marathon will be run at a heat index of 51°F. This is probably the ideal temperature for endurance exercise, as determined by the work of Galloway and Maughan. This idea is supported by Trapasso and Cooper's analysis of record performances over 30 years at the Boston Marathon: Record breaking performances are characterized by a wet bulb temperature of less than 46°F (which, it should be noted is equal to a dry bulb temperature of 51°F and a relative humidity of 70%!), 100% cloud cover and (sometimes) a light drizzle.

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For a nice picture from the Boston Marathon, click here.

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