Because exercise causes the body to generate so much extra heat, optimal temperatures for intense physical activity are lower than those for daily life. Athletes can raise their core temperatures six degrees just by working out. Add an environment that makes heat dispersal more difficult — not to mention possible dehydration from sweat losses that sometimes exceed six liters (for marathoners) or two liters per hour (team game players) — and performance can take a nosedive. (Might the misery of exercise in torrid weather explain why the South boasts higher obesity rates than cool and crisp Colorado?)
Endurance can also diminish in the heat as the heart works ever harder to power the same feats. For example, researchers in Darwin, Australia, observing a long-distance runner taking a 30-minute jog through the humid air, noted that his body temperature increased from 98.96 degrees to 105.8 degrees. When he'd gone on a similar jaunt under cooler conditions, his temperature had risen by just two degrees. Such a spike spells trouble for maintaining an optimal heart rate: The man's rate soared to 200 beats per minute during the last 15 minutes of his run, where, previously, it was a more sustainable 154 beats per minute.
There are some benefits to exercising in the heat, however. It may enhance later athletic performance in more temperate weather.
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