If you run, bike, are desk-bound all day, or have been sitting in a car or plane traveling, your hamstrings could use some extra love and length. It not only feels good to stretch this commonly tight area, but hamstring flexibility is also important for the health of your back, hips, and knees. Here are six easy and essential stretches that target the backs of your legs. To avoid injury, it's best to do them at the end of a workout, when the muscles are warm.