American magazine "Best Life" ran a feature on Dick Dreissigacker, the inventor of the Concept2 Indoor Rower recently. You can find the article on the Best Life website, an extract of which is below:

"My doctor calls it an athlete’s heart, and it’s common among experienced rowers. My left ventricular chamber is so much larger, and its walls so many millimeters thicker than an average heart, that an inexperienced cardiologist could easily mistake it for an abnormality. Cyclists, as would be expected, have strong hearts too, but experts say that rowers’ hearts are stronger, although it’s not completely understood why."