Hi, @soph and all. The article recommended by @kanaazpereira is a good thumbnail sketch of how exercise and A-fib co-exist in my life. My A-fib is essentially asymptomatic — I rarely feel it or get any warnings from it — but it always appears on an EKG, and it speeds up my heart rate a bit (from a natural 50bpm to sometimes 70bpm). These "symptoms" are influenced by my medication — a beta blocker twice a day and (for hypertension) Lisinopril and a diuretic. I also take an anticoagulant (Warfarin) to discourage clot formation in my heart due to impeded blood flow caused by irregular heart beats.
As to exercise, regular and moderate are in effect with me. A year ago, I thought exercise might have caused a stroke during a brisk walk around the community. But at Urgent Care, echocardiogram and MRI produced a different diagnosis: A "small stroke" (ischemic rather than hemorrhagic) from a small blood clot, attributable not to the exercise from the walk but to a sharp rise in my blood's ability to coagulate. This, in turn, came from a wide swing in the effectiveness of my anticoagulant medication, probably attributable to a couple of glasses of wine with dinner the night before.
Sorry to bother you with these extremely complex factors, but it seems important to make the point that unusual factors raise vexing questions that cry out for expert advice from the best physicians you can find — doctors who not only know their specialty, but have experience with similar cases in the past; doctors who will take the time to listen to your detailed description of how your symptoms behave, discuss them with you thoroughly, and answer your questions completely right away or as soon as diagnostic evidence is available.