The researchers also looked at the scans of each man’s heart. The degree of plaque accumulation can generally be assessed using a coronary artery calcium score. Someone with a score higher than 100 is considered to have worrisome plaque buildup.

Comparing the groups, the researchers determined that the men in the highest-exercise group were prone to developing plaques. They were, in fact, about 11 percent more likely to have a calcium score higher than 100, compared to men who moved less. Some of the extreme exercisers had scores above 800.

Finally, the researchers checked death records for a decade or so after each man’s latest exam, to see if any had died. And some had, particularly from heart attacks among men with calcium scores higher than 100.

But few of those men came from the group that exercised the most. The extreme exercisers turned out to have less risk of dying prematurely than men with the same — or higher — calcium scores who rarely worked out.

In essence, these results suggest that large amounts of exercise can up someone’s risk of developing plaques, while also lessening the likelihood that he will die from a heart attack precipitated by those plaques, says Dr. Laura DeFina, the chief science officer for the Cooper Institute, who led the study.

This curious outcome probably occurs because extreme workouts create a unique type of plaque, she says. “There is some evidence that the plaques” in highly active people “are denser and more stable” than those in sedentary people, she says, making them less likely to break free and cause a heart attack.

But that idea is speculative, she says, and requires more study. Scientists also aren’t sure how, at a molecular level, strenuous exercise might prompt the buildup of plaques, and why some people’s arteries remain unaffected, no matter how much they exercise.