The Piriformis arises from the anterior surface of segments two through four of the sacrum. It exits the pelvis via the greater sciatic foramen and attaches to the greater trochanter. It is the primary external rotator of the thigh. It also participates in abduction of the thigh.
The only sacroiliac muscle, connects the anterior surface of the sacrum to the greater trochanter of the femur and participates in sacral torsion and hip tightness. Spasm of the piriformis results in lateral rotation of the thigh at the hip joint, and potential compression of the sciatic nerve.