You might think you're doing everything right to stay fit, but some "healthy" habits can be counterproductive. As you age, your body becomes more rigid and less flexible, and if you exercise incorrectly, you can suffer more serious injuries than sore muscles. On the other hand, if you are intentional about your exercising, you can benefit in more ways than one — for instance, a recent study found that even small amounts of exercise can increase women's thinking speed reserve. "As time goes by, muscles and joints become less responsive and adaptable compared to the peak young adult years," says physical therapist Rob Cowell. "Sports and exercises that involve sudden changes in direction and quick, powerful movements can easily lead to strains or more serious injuries, especially for the untrained." Here are some exercises to approach with caution, including those intense Peloton sweat sessions.