The piriformis is one of the six muscles of the deep lateral rotator group. They are (from superior to inferior):
- Superior gemellus
- Obturator internus
- Inferior gemellus
- Obturator externus (deep to quadratus femoris)
- Quadratus femoris
- The piriformis attaches from the anterior surface of the sacrum to the superior aspect of the greater trochanter of the femur.
- Laterally rotates the thigh at the hip joint (from anatomic position)
- Abducts the thigh at the hip joint (from anatomic position) (note: this action is relatively weak)
- Medially rotates the thigh at the hip joint (if the thigh is first flexed approximately 60 degrees or more)
- Horizontally abducts the thigh at the hip joint (if the thigh is flexed to 90 degrees)
- Contralaterally rotates the pelvis at the hip joint (reverse closed-chain joint action – from anatomic position)
- The sciatic nerve can be compressed by a tight piriformis, causing symptoms of sciatica. When this occurs, it is called piriformis syndrome.