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Last modified: Saturday, July 1, 2000

Exercise physiologist offers tips for exercising in hot weather

BLOOMINGTON, Ind. -- Humans have a huge capacity to produce heat, and our environment determines how we get rid of it. That's why hot weather and high humidity can be causes for concern.

Exercise physiologist Joel Stager, research director of IU's Human Performance Laboratory, said common sense is the best advice for those engaged in such summer activities as bicycling, running, hiking, tennis and softball.

"We focus on heat being external, but it's internal heat that we're trying to unload," Stager explained.

Adults can generate heat at about the same rate as a 100-watt light bulb when at rest, and this can increase to 1,000-1,500 watts in heavy exercise, he said. People should exercise in the mornings or evenings in hot weather, because frequently it's too hot at other times of the day.

The temperature of the asphalt used by runners can easily exceed 100 degrees in the summer, and with buildings also radiating heat, it's best to find a park, especially if it is shaded.

"When we exercise, our temperature can easily rise from the 98-99 degree normal range to 101-102, and that doesn't leave much margin of error before it can become life-threatening at 106 degrees," Stager said.

The major threats are heat exhaustion, heat stroke and dehydration. Dizziness, lightheadedness or feeling lethargic are symptoms of heat exhaustion. Even more dangerous is heat stroke, where early symptoms are cool skin, pale color and a somewhat clammy feeling.

During exercise, we can lose one to two liters of water per hour in sweat, so it's important to use common sense and stay hydrated.

"You don't have to drink a lot of water before exercise, as over-hydration doesn't help much, but it is very important to not start a summer exercise program feeling thirsty," Stager said. "We are relatively poor regulators of water balance, so by the time we feel thirsty, it's too late to adequately replenish our supply."

He cautioned that even swimmers need to be aware of dehydration during a strenuous workout, because this loss of water through sweat isn't noticeable to them.

Humidity also can be a problem, he said, because sweat must be able to evaporate, and high levels of humidity prevent this.

A final common-sense tip is to wear light-colored, loose-fitting clothes that allow sweat to evaporate.

Stager can be reached at 812-855-1637 or

(Prepared by Richard Doty)