TUESDAY, Aug. 5 (HealthDay News) -- Light to moderate exercise -- just walking a few blocks or even dancing -- can help prevent the abnormal heart rhythm called atrial fibrillation in those most vulnerable to it -- older people, a new study finds.

Atrial fibrillation, in which the two upper chambers of the heart tend to twitch rather than beat steadily, is the most common heart rhythm abnormality. It is especially common after age 65. The danger is that blood can pool, causing clots that move to the heart or brain. There have been reports of an increased incidence of the abnormality in younger people who exercise vigorously.

"Prior studies have looked at atrial fibrillation in young and middle-aged and generally healthy people," said study lead author Dr. Dariush Mozaffarian, a cardiologist at Brigham and Women's Hospital in Boston. "They found that, for example, marathon runners have a higher risk of atrial fibrillation. But the vast majority of atrial fibrillation occurs later in life. After 65, about one in five people develops atrial fibrillation over 10 years."

Mozaffarian and his colleagues studied the habits of 5,446 adults, average age 73, comparing their physical activities with the risk of developing atrial fibrillation.

"No one has looked at exercise and atrial fibrillation in these older people," he said. "We found that light to moderate exercise, such as walking 10 blocks a week, was associated with a lower incidence of atrial fibrillation."

Specifically, the researchers found that the incidence of the heart abnormality was 22 percent lower in those walking five to 11 blocks a week than for those walking fewer than five blocks a week. It was 24 percent lower for those walking 12 to 23 blocks weekly, 33 percent lower for those walking 24 to 59 blocks, and 44 percent lower for those walking 60 or more blocks a week.

Overall, there was a 50 percent lower risk of developing atrial fibrillation when comparing people with the highest and lowest levels of walking distance and pace.

The findings were published in the Aug. 5 issue of the journal Circulation.

Meanwhile, a separate trial looking at the effect of exercise on atrial fibrillation from a different angle is being done by Dr. Jose A. Joglar, an associate professor of internal medicine and director of clinical cardiac physiology at the University of Texas Southwestern Medical Center at Dallas.

"We have enrolled patients who have atrial fibrillation already to see whether exercise improves their quality of life," Joglar said.

The 10 participants in the trial, all in their mid-70s and diagnosed with atrial fibrillation, are doing aerobic exercises 45 minutes a day, three or four days a week, Joglar said. "They are riding stationary bicycles or walking fast," he explained.

The study is ongoing, but "preliminary data appears to be that they feel better and function better," Joglar said.

He and Mozaffarian stressed that light exercise, whatever its effect on atrial fibrillation, has known benefits, such as helping control blood pressure and weight. Other studies have shown that the right exercise -- "not too strenuous but not too light, either" -- is helpful against angina, the chest pain caused by heart artery problems, Joglar said.

"There are additional strong reasons for the public to focus on exercise," Mozaffarian said.

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