Older men who exercise vigorously several times a week greatly increase their bodies' ability to dissolve blood clots that can trigger heart attacks and strokes, a new study says.

Thirteen men ranging in age from 60 to 82 who exercised four to five times a week for six months showed a 39 percent increase in their ability to dissolve blood clots, according to the study, published in the May issue of the American Heart Association journal Circulation.The same kind of exercise had little effect on the clot-dissolving abilities of 10 men ages 24 to 30 who were also studied.

"My guess is that these young men were already healthy and possibly more active at the beginning of the study, so our exercise training program affected their blood-clotting systems less than it did in the older men," Dr. Wayne L. Chandler, a University of Washington researcher who co-wrote the report, said Wednesday.

The exercise program consisted of walking, jogging and cycling for 45 minutes four or five times a week.

An Oklahoma researcher who has studied the cardiovascular systems of athletes said the study may be the first to show that older men can reduce their chances of forming blood clots through regular exercise.

"This is new," said Dr. Edward Eichner, professor of medicine at the University of Oklahoma's Health Sciences Center in Oklahoma City. "The fact that the health benefits show up the next morning after exercise is new, it suggests round-the-clock benefits."

Previous studies have shown intense exercise causes an immediate, brief increase in natural clot-dissolving power. The new study looked for prolonged effects.