Which sport has the most heart—or, better said, builds you the biggest heart? Rowing. To make sure astronauts are fit for space (and to ease their reacclimation to earth), the National Space Biomedical Research Institute trains them like elite-level rowers before launching them toward the final frontier. “Because of the unique cardiovascular stresses, rowers have the largest hearts of any competitive athlete,” says Benjamin Levine, M.D. So swap the treadmill for a rowing machine.