When you’re sitting around, you heart is pumping about five quarts of blood a minute, and if you run up the stairs or hard or push yourself physically, it can go up 35 or 40 quarts a minute. If you go and run for 26 miles, or do a full-distance triathalon, it completely overtaxes the heart. The heart is pumping 25 quarts a minute for hours and hours, and that starts to cause muscle fibers to tear, which leads to a bump in troponin and other enzymes associated with inflammation, and it causes the death of some muscle cells in the heart.
Dr. James O’Keefe
Time Healthland