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NFL veterans face a much higher risk of a type of irregular heartbeat that can lead to stroke, a new study suggests.

Ex-players were nearly six times more likely to have atrial fibrillation, also called AFib, than men of a similar age who did not play professional football. The athletes' risk was higher even though they had fewer risk factors for cardiovascular disease, including Type 2 diabetes and high blood pressure, and had lower resting heart rates.

This is the first study to connect AFib with an elite sport that requires muscle strength, said the authors, whose work appeared Wednesday in the .

"Most former NFL athletes with AFib were unaware of any symptoms and yet should have been taking blood thinners to prevent stroke," lead author Dr. Dermot Phelan said in a news release. Phelan, director of the Sports Cardiology Center at the Cleveland Clinic in Ohio, said the findings highlight "the importance of being vigilant and intermittently checking for AFib in this group."

AFib occurs when the electrical impulses that initiate each heartbeat fire erratically, causing the atria – the top chambers of the heart – to quiver. This can cause blood to pool in the atria and form a clot. The clot can travel to a blood vessel leading to the brain and cause a stroke. Pacemakers help treat the problems of electrical conduction in the heart.

Previous studies linked long-term participation in endurance sports, such as marathon running, with an increased risk of AFib. Phelan said the prolonged strength training involved in football might increase heart chamber size and wall thickness, which can alter heart rhythms and electrical signals in the heart.

The researchers compared 460 former NFL players to 925 non-players. Both groups were middle-aged, and about half in each group were African American. Among former player, 5% had AFib compared to 0.5% of men in the control group.

AFib risk was higher among men who were older, white and weighed more, all known risk factors for AFib.

The authors caution, however, the new data should be placed in the context of indicating former NFL players are at lower risk of death overall and from heart-related issues compared to the general population.

"For the majority of people, the benefits of both aerobic exercise, such as walking, and strength training, such as working with weights, is strongly linked to a healthier heart, and this study should not discourage people from being physically active," said Phelan. "Mild to moderate exercise reduces the risk of atrial fibrillation for most people."

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