Lie on your back on an exercise mat. Bend your right knee about 90 degrees and place your hands behind your right knee. Using your hands for support, slowly extend your right knee, straightening your leg to 90 degrees, if possible. Be sure to keep your lower back flat on the mat. You'll feel a stretch down the back of your right hamstring and calf. Relax and return to the starting position. Repeat on the other side.