when you strengthen your muscles, they generate more nuclei, or “little protein factories,” that contain DNA necessary for increasing muscle volume, says Kristian Gundersen, professor of physiology at the University of Oslo in Norway. A study led by Gundersen in 2010 confirms that even after you quit exercising, these nuclei stick around, meaning a runner is one step ahead when he decides to get back into it.
“When you do an activity, the brain sends messages to your muscles in the form of electrical charges through pathways in the central nervous system, and the muscles send messages back,” says Matt Silvis, M.D., a primary-care sports-medicine physician at Penn State Milton S. Hershey Medical Center. It’s because of this constant feedback loop that the right muscles are activated, and at the right force, in order to perform a particular task. Do this task enough, and these nervous-system pathways become well-trodden, which is why you never forget how to ride a bike-or how to run.