Those who had done intensive exercise for more than five hours a week when they were younger were 19 percent more likely to have developed a heart rhythm disorder called atrial fibrillation by age 60 than those who exercised for less than an hour a week.
That risk increased to 49 percent among those who did more than five hours of exercise at age 30 but did less than an hour a week by the time they were 60. Participants who cycled or walked briskly for an hour or more a day at age 60 were 13 percent less likely to develop atrial fibrillation.
The studies were published online May 14 in the journal Heart.
Another expert said the findings shouldn't alter standard recommendations.
"It is not standard practice to recommend strenuous activity to individuals with coronary heart disease," said Dr. Nieca Goldberg, director of the Tisch Center for Women's Health at NYU Langone Medical Center, New York City. "This study, although interesting, does not change current recommendations for moderate physical activity in coronary patients."
For her part, George said it's clear that a moderate exercise program can provide real benefit for everyone.
"A large body of scientific research has consistently shown that a sedentary lifestyle is one of the risk factors for cardiovascular disease, which includes heart disease, and that becoming more physically active can decrease your risk by as much as 50 percent," she said.
Current American Heart Association guidelines advise 30 minutes of moderate-intensity aerobic exercise most days a week or 20 minutes of vigorous activity three days a week, George added.
And in a journal editorial, a team led by Dr. Lluis Mont of the Hospital Clinic of Barcelona, Spain, agreed with the two U.S. experts.
"The benefits of exercise are definitely not to be questioned; on the contrary, they should be reinforced," the team wrote. But studies like the two published in Heart are fine-tuning recommendations for exercise, to "maximize benefits obtained by regular exercise while preventing undesirable effects -- just like all other drugs and therapies," the editorialists said.