The following story is excerpted from TIME’s special edition, The Science of Exercise, which is available at Amazon. Dr. Dena Oaklander, a psychiatry resident—who also happens to be my sister—is the last person you’d ever expect to become a bodybuilder. She’s naturally scrawny and a little bit shy, not the type of person to beast out at the gym—or so I once thought. In medical school, she’d counse