Before you take off on your run, you need to make sure your muscles are properly warmed up and ready to go. There are a number of good, dynamic stretches ideally suited to runners, and they almost all focus on various muscle groups in the legs. Runner's World has a number of helpful stretching guides, including this one from physical therapist Nikki Kimball. These stretches are designed to get your heart rate going in addition to loosening up your muscles. More passive (or "static") stretches are best for cooling down after your run [source: Static Flexibility Stretches for Runners].