For those of us who like to run, a few studies over the years have caused reconsideration about excessive exercise. A few years back there was a study that showed consistent training at the level of marathon and ultramarathon may increase the frequency of atrial fibrillation. A new study (linked below) suggests that in people who have survived MI, moderate exercise is good but excessive exercise may not be. Excessiveness was defined as 7.1 km/day of running or 10.7 km/day of brisk walking. The take home message at this point seems to be that moderate exercise is good but make sure you talk with your doctor if you want to consistently train at levels more than 5-7 miles/day (for both overall afib risk and post mi complication risk).