Your sartorius, also called the tailor muscle, performs all three of the motions necessary to assume a cross-legged sitting position: Flexing and externally rotating the hip, abducting the leg away from the body and flexing the knee. Because the sartorius performs so many motions at multiple joints, isolating it for a stretch is next to impossible. But you can use stretches that target similar muscles -- those that perform the same motions -- to help you target the sartorius.
Complications
At least one other muscle group performs every action the sartorius helps with. Your hip flexors also flex your leg at the hip; your hamstrings flex your leg at the knee, and your glutes both externally rotate your hip and abduct your leg. So when you stretch your sartorius, you will unavoidably target at least one of these other muscle groups at the same time. You'll feel the stretch most in your tightest muscles, so if your sartorius is shortened relative to your other muscles you can still, theoretically, isolate it.
Standing Quad Stretch
The standing quadriceps stretch targets your hip flexors, quads and sartorius. Stand on one leg, grasp your other foot, and bring that heel as close as possible to your buttocks. Then press your hip gently forward on the side of the bent leg until you feel a stretch. Maintain upright posture throughout the stretch, and hold onto a sturdy wall or railing for balance if need be.
Standing Hip Flexor Stretch
If you still don't feel a stretch in your sartorius, which runs like a ribbon from the top, outside of your thigh across the front of your thigh to attach just below and inside the knee, try a standing hip flexor stretch. Stand with your feet square, then take a large step forward with your right leg. Drop your weight slowly, bending your right knee but keeping your left leg straight. The toes of your left leg should point forward; don't allow them to rotate out. Stay upright and think of pressing both hips forward slightly to keep from arching your back. You should feel a stretch in front of your hips and down your thighs.
Kneeling Hip Flexor Stretch
You can perform a similar hip flexor stretch while kneeling. Kneel, then raise one leg and plant that foot on the floor in front of you. Lean forward, gently pressing your hips straight forward until you feel a stretch in the hip flexors and thigh of the knee still pointing down. Hold this and every other stretch at the point of tension, not pain, breathing normally, for 15 to 30 seconds.
