The finding came as a bit of a surprise, because until recently, the conventional wisdom was that the heart never slowed down, according to lead investigator Dr. Euan A. Ashley, an assistant professor of cardiology at Stanford University.

"Your heart is going to beat two or three billion times in your lifetime," Ashley said. "It was believed that in the absence of disease, it would not slow down. What we showed was that if you exercise for 19 or 20 hours at a time, your heart will tire a bit, about 10 percent."

And the slowdown is greater in people who carry what's been called the "fitness gene," Ashley's team reported in the August issue of the Journal of the American Cardiology. The gene is called "ACE" because it is linked to the angiotensin-converting enzyme, the target of ACE inhibitor heart medications.

For the study, Ashley and his colleagues set up shop at the finishing line of an ultra-endurance race called the "Adrenalin Rush," held in the Scottish Highlands. The annual event is grueling even by "iron man" standards, with one or two competitors usually requiring hospitalization after every race.

As athletes crossed the line after 90 hours of biking, climbing, swimming, paddling and rope work, the researchers tested their hearts.

The athletes' average heartbeat had slowed from what was measured before the race, by about 8 percent for athletes who did not carry the ACE fitness gene and 13 percent for those who did carry it.

The ACE gene has been associated with improved athletic performance, and Ashley said the association could explain the difference. "It could be that people with the fitness gene pushed themselves harder," he said. "They were the ones pulling the others along."

Other studies have suggested the heart might tire with intense effort, said study senior author Dr. Pamela Douglas, chief of cardiovascular medicine at Duke University in Durham, N.C.

But it's the relationship of the fitness gene with heart performance that interested Douglas the most. Ashley's explanation of the relationship is reasonable but remains unproven, she said.

The study results might have some application to the ordinary world of cardiology, Douglas said. For one thing, "people with heart disease or borderline heart disease should not be running marathons," she said. "There are data to suggest much more subtle changes occur in marathon running."

The information gathered in the study might help shed more light on heart failure, in which the heart cannot pump enough blood, Ashley said. "There may be a similar mechanism involved," he said.