If you’re unmotivated to go on your daily run or attend your weekly spin class, you should change things up, says Jordan Metzl, a sports medicine physician at the Hospital for Special Surgery in New York. “Trying something new that shakes up your routine can really give you a fresh perspective and get you excited,” Dr. Metzl says.

Make sure your routine includes a mix of exercises that assures good muscular balance, he says. Doing the same exercises over and over can lead to the overdevelopment of certain muscles and the...