From a very young age, Dr. Jordan Metzl knew that playing sports energized and helped him focus. It wasn't until years into his career as a sports medicine doctor that he began prescribing exercise to others. "It's the most ubiquitous, effective, totally no-side-effects drug in the world," Metzl, who recently published "Push: Unlock the Science of Fitness Motivation to Embrace Health and Longevity," told Business Insider. Exercise is also something Metzl feels is sometimes overlooked in the