…though a figurative phrase is a good description of what hard core endurance athletes might be doing every week. Of course the novelty of an Iron Man Triathlon (swimming 2.5 miles, …though a figurative phrase, is a good description of what hard core endurance athletes might be doing every week. Of course the novelty of an Iron Man Triathlon (swimming 2.5 miles, cycling 112, and running the 26 of a marathon) has worn off, but it is still an activity requiring a lot of dedication and effort. But it might age your heart muscle and its electrical system at an accelerated rate. All of that “combustion” — especially if influenced by a genetic inability to detoxify the by products at a normal rate — looks like it causes a lot of microscopic scar tissues, aka, “myofibrosis”, to form that itself will change and interrupt the electrical impulses that make your heart beat normally. Now, it is generally recognized that for many, “more fit = greater risk” for atrial fibrillation. The Mayo Clinic looked at this comprehensively in 2016 and concluded that moderation becomes more and more important as we age.