The sartorius is a small, ribbon-like muscle that runs from the forward outside portion of your pelvis down to the inside of your leg, just below the knee. It helps flex both the hip and the knee. The sartorius also aids in performing external hip rotation as the hip and knee flex (think of turning your toes out into a duck-walk position--that's external hip rotation) and abducts the hip (in other words, it moves your ankles further apart from each other if you keep your legs straight).
Kneel with one knee on the ground, the other bent at a 90-degree angle in front of you, with that foot flat on the floor. Support yourself against a wall or sturdy piece of furniture, if necessary, to keep your balance.
Keep your spine upright and think of your pelvis as a bucket full of water. Keeping your pelvis in neutral position--that is, neither tilting forward nor back--will keep the imaginary water from spilling out.
Lean forward with your spine still completely upright. Think of pushing the bucket of water (your pelvis) forward while still keeping it level. Clenching your buttock muscles while you do this may help you get a feel for the right motion.
Hold the stretch for between 10 and 30 seconds, breathing normally as you do so, then slowly release and repeat on the other side. Repeat the stretch between two and five times on each leg, so that you hold the stretch for one minute total on each side.
Stand on your right leg. Support yourself against a piece of solid exercise equipment, a wall, or a sturdy piece of furniture if necessary to keep your balance.
Bring your left heel as close to your buttocks as possible. Grasp the left foot in both hands--if possible--or in the left hand to help keep it close to the body.
Think of pushing the hips forward without arching your back. You should feel the stretch in the front of your hip and possibly down the inside of your thigh as well.
Breathe normally. Hold the stretch for 10 to 30 seconds, then slowly release and repeat on the other side. Repeat the stretch until you've held it for one full minute on each leg.
Warm up with about 10 minutes of light cardiovascular activity before stretching. Your muscles are more responsive to stretching when they are warm.
Stretching should never cause intense pain. Hold the stretches at the point where you feel a gentle stretch. If you experience pain, despite backing off the intensity of the stretch, consult a trainer, physical therapist or healthcare professional.