@tallbackhip I'm not clear on your question. Are you asking if a person wishes to do aerobic or HIIT exercise while in obvious, measured, AF, what should the rate be, or what upper limit would be appropriate? I can't answer that as the individual's symptoms would govern the limits. I was at the end of a 10 km run when I had my first bout of AF. I was at the time retired, and had a history of training for running races. My resting HR was 45 BPM, up from 38 when I was in my 30's. My typical training when I was diagnosed comprised 10 runs up to 20 km, sometimes running quickly, sometimes just plodding. My HR would rise to 170 BPM if running up a hill, of which there were plenty locally, or back off to 140 if just running at a 'maintenance' pace, say 8'/mile. It turns out that my running was not the initiator of my AF. It was late-diagnosed 'severe obstructive sleep apnea.'

When I was experiencing AF, seated and watching TV, my rate would exceed 138 BPM typically, sometimes rising to 180. That is not a dangerous state, especially for a healthy and fit heart used to sustained effort. But it can't be maintained indefinitely as it can lead to cardiomyopathy and eventually to the 'remodelling' that we are warned to avoid. So, my instructions, and what we routinely counsel newcomers to afibbers.org forum, is that AF is not a dangerous disorder unless it is sustained beyond 24 hrs at a rate above 100 BPM, is accompanied with RVR (rapid ventricular response), and is highly symptomatic (shortness of breath, fainting, excessive anxiety, etc).