Interesting article on Nature news about the effects of extreme sports and the heart:
Athletes who carry one type of a well-known ‘fitness’ gene might actually push themselves so hard that they tire out their hearts. That’s the finding from a study of individuals who competed in one of the most gruelling races in the world.
Exercise experts have long assumed that heart muscles, unlike those of our legs and arms, don’t tire. But a few studies of athletes have hinted that the heart also gets worn down by many hours of extreme exercise…
Link.