I've had afib for 20+ years. From my perspective, endurance exercise is a double edged sword. If you haven't been exercising, then getting up off the couch can be beneficial for afib. However, if you do endurance exercise chronically in high volumes with intensity, it can lead to afib and was my path to afib. This is a
thread on this topic.
In my case, I had my first afib episode as a delayed
vagal trigger after a training run on a 14,000' peak - training for a 13+ mile race that gained 7800' elevation, topping out over 14,000'. This was for the 9th or 10th time I'd done the race. Within two months of that first episode, I started an episode that lasted 2.5 months and was converted with a loading dose of oral flecainide.
I'd already figured out that chronic fitness was my path to afib. When, during the 2.5 month episode, the EP proposed I stay out of rhythm, I proposed a "plan B" that included training very differently, electrolyte supplementation and a script for
on-demand flecainide to be used to convert if I went out of rhythm. He accepted my proposal and I've been basically following this plan since, though I have and do tweak it. It has worked pretty well as many years my afib burden is <0.05%.
What I've learned is that, for me, the exercise trigger is the product of intensity times duration. I learned to limit long duration endurance activity to an intensity I could do with nasal breathing only (AKA Zone 2). Short duration, high intensity workouts (like Tabatas on a fan bike) are fine (and I now do even HIIT workouts nasal breathing, having trained a high CO2 tolerance). I follow these rules and maintain excellent fitness at age 69. For example, Friday I went climbing with a 37 year old partner. I had a 60# pack and went up the long, steep approach trail at 6,000' elevation with only nasal breathing. My climbing partner was out of breath and had to stop frequently on the way up (and was carrying a 5# pack with only harness, helmet, shoes & water). I ski frequently at elevations to 13,000' and on off piste', steep runs. I strength train with both isometrics (with some 90 second holds) and calisthenics to failure. I also train with a setup that allows me to breathe 7.5% O2 (like being at 27,500' elevation) or 80% O2 through a mask and I use this doing simultaneous intensity and hypoxia/hyperoxia intervals on a fan bike, rower and skier.