I knew a professor of physiology who claimed to have spoken with another professor (somewhere) who had actually done some measurements on Miguel Indurain. IIRC

I knew a professor of physiology who claimed to have spoken with another professor (somewhere) who had actually done some measurements on Miguel Indurain. IIRC, his resting heart rate was 25. The average human heart at rest pumps ~5 L/min, so his was pushing out ~200 mL/beat. According to the story, his wife wouldn't sleep in the same bed as him because his heart shook the bed so much every time it beat. Since the stroke volume of the heart is more or less fixed in the short term, the fact that he could meet his needs at such a low heart rate indicated that his maximum cardiac output was in the neighborhood of 35 L/min (max heart rate 175 x 200 mL/beat, he could probably get his rate higher but the heart would not fill completely and so stroke volume would be a bit less). I have no way to know if this was true, but he wasn't known for exaggerating.