The researchers also examined the sweeps of each man's heart. The degree of plaque accumulation can usually be assessed with the aid of a calcium score in the coronary arteries. A person with a score greater than 100 is considered to have a disturbing plaque buildup.

In comparing groups, researchers determined that men in the highest exercise group were prone to developing plaques. In fact, they were about 11% more likely to have a calcium index greater than 100, compared to men who moved less. Some of the extreme users had scores above 800.

Finally, the researchers checked the death records about a decade after the last examination of each man, to determine if they were dead. And some had, especially heart attacks in men with calcium scores greater than 100.

But few of these men came from the group that exercised the most. People with extreme physical activity were less likely to die prematurely than men with equal or higher calcium scores who rarely worked.

These findings suggest in substance that significant physical activity can increase the risk of plaque development, while reducing the risk of death from heart attack, says Dr. Laura DeFina, scientific lead of the Cooper Institute, who led the study.

Curious result. This is probably due to the fact that extreme workouts create a unique type of plaque, she says. "There is evidence that plaques" in very active people "are denser and more stable" than in sedentary people, making them less likely to break free and cause a heart attack.

The idea is speculative, she says, and requires more study. Scientists also do not know how, at the molecular level, intense exercise can cause plaque buildup and why some people's arteries are unaffected, regardless of their exercise level.