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How Big Is Big?

An enlarged left atrium was 40 millimeters or bigger. Only 2% of the athletes had a left atrium that was 45 millimeters or bigger. The upper range was 46 millimeters in women and 50 millimeters in men.

That might be the "outer limit" of exercise's effect on heart size, and enlargements beyond that might stem from health problems, write the researchers.

Chalk It Up to Training?

The heart's left atrium probably got bigger in response to intense athletic conditioning, note the researchers.

"Left atrial remodeling in competitive athletes may be regarded as a physiological adaptation to exercise conditioning, largely without adverse clinical consequences," they write.

Other Heart Chambers Enlarged

WebMD contacted professor Edward Coyle, PhD, about the study.

Coyle directs the Human Performance Laboratory at the University of Texas at Austin. He studies the physiological factors that limit human exercise performance. His subjects have included seven-time Tour de France winner and cancer survivor Lance Armstrong.

In an email, Coyle tells WebMD that the study showed that endurance athletes have larger left atria, as well as the thicker chamber below it (the left ventricle).

"Although this is well known for the ventricles, [which] perform most of the [heart's] work in pumping, it is clear this is the case for the atria," writes Coyle.

"It would be surprising if both chambers were not enlarged, as they work in tandem, and the volume and size of one should generally match the other," he continues. "More importantly, this study documented that enlargement of the atrial chamber is a healthy adaptation and it is not associated with increased irregular heartbeats or disruption of heart rhythm."

Coyle writes that he has not attempted to measure the size of the left atria but has seen "a healthy enlargement" of the left ventricle in endurance athletes. "This has been well documented by other investigators," writes Coyle.

More Study of Athletes

In a news release from the American College of Cardiology, heart expert Norbert van Hemel, MD, PhD, calls for more study of aging athletes.

"One must question whether this 'adaptation' is sound, and whether it might become the source of physical problems in the years after the intensive sport training is stopped," says Hemel. He is the Pasman Chair of Cardiac Stimulation at Utrecht University in the Netherlands.

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