A new study shows runners do not have increased incidence of atrial fibrillation

 Past studies have reported that walking reduces the risk of atrial fibrillation (AF) by 48%, whereby running  increased AF risk by 53%. A resent study (linked below) found "the risk for cardiac arrhythmias was similar in walkers and runners who expended comparable METhr/d during structured exercise. We found no significant risk increase for self-reported cardiac arrhythmias associated with running distance, exercise intensity, or marathon participation. Rhythm abnormalities were based on self-report, precluding definitive categorization of the nature of the rhythm disturbance. However, even if the runners’ arrhythmias include sinus bradycardia due to running itself, there was no increase in arrhythmias with greater running distance."

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0065302