So, for the new study, which was published in November in Frontiers in Physiology, researchers at the University of Guelph in Canada and other institutions set out to map the structure and function of elite swimmers’ and runners’ hearts.

The researchers focused on world-class performers because those athletes would have been running or swimming strenuously for years, presumably exaggerating any differential effects of their training, the researchers reasoned.

Eventually they recruited 16 national-team runners and another 16 comparable swimmers, male and female, some of them sprinters and others distance specialists.

They asked the athletes to visit the exercise lab after not exercising for 12 hours and then, when on site, to lie quietly. They checked heart rates and blood pressures and finally examined the athletes’ hearts with echocardiograms, which show both the structure and functioning of the organ.

It turned out, to no one’s surprise, that the athletes, whether runners or swimmers, enjoyed enviable heart health. Their heart rates hovered around 50 beats per minute, with the runners’ rates slightly lower than the swimmers’. But all of the athletes’ heart rates were much lower than is typical for sedentary people, signifying that their hearts were robust.

The athletes also had relatively large, efficient left ventricles, their echocardiograms showed.

But there were interesting if small differences between the swimmers and runners, the researchers found. While all of the athletes’ left ventricles filled with blood earlier than average and untwisted more quickly during each heartbeat, those desirable changes were amplified in the runners. Their ventricles filled even earlier and untwisted more emphatically than the swimmers’ hearts did.

Read The New York Times