The Heart of a Swimmer vs. the Heart of a Runner
This chamber of the heart receives oxygen-rich blood from the lungs and pumps it out to the rest of the body, using a rather strenuous twisting and unspooling motion, as if the ventricle were a sponge being wrung out before springing back into shape. Exercise, especially aerobic exercise, requires that considerable oxygen be delivered to working muscles, placing high demands on the left ventricle. A 2015 study found, for instance, that competitive rowers, whose sport combines endurance and power, had greater muscle mass in their left ventricles than runners, making their hearts strong but potentially less nimble during the twisting that pumps blood to muscles.
Source: www.nytimes.com