Need another reason to push yourself harder?

Researchers at the University of Southern California found that women who did high-intensity sessions, such as swimming or running, for at least five hours per week had a 20 to 30 percent lower risk of breast cancer than women who did more moderate activities (like walking) for less than 30 minutes a week. "Strenuous exercise can lower production of estrogen and progesterone," says lead researcher Leslie Bernstein, Ph.D. "Increased levels of these hormones have been associated with a higher risk of breast cancer." To make sure you're hitting the mark, spend half of your weekly cardio workouts at RPE 6 or 7 (see below for RPE chart).