The Parasympathetic Nervous System acts as a Brake for heart rate. I never realized how much until the afib started. Metoprolol does a great job controlling rate, but I had an altercation with another driver recently, OMG, what was I thinking. Although the rate probably didn't go above 100, it hasn't been near that at rest in awhile, more like the 70s at rest, 90s after climbing the stairs. It took hours for my heart rate to return to normal. Adrenaline causes a higher rate, the parasympathetic nervous system brings the rate down, but something that upsets it lets the rate go, or remain, high.

Reading a medical article recently, the doctor said when they see heart rates above X amount, it means Parasympathetic Response has been withdrawn entirely.

So, in those approved to exercise, and in those cases exercising commensurate with their doctor's oversight and in ways approved by the doctor, increased conditioning brings increased parasympathetic tone and a naturally lower heart rate.