This creates a pulley effect multiplying the piriformis' force - much like what occurs with the iliopsoas as it curves over the front of the pelvis. The sciatic nerve runs behind the piriformis and can be irritated by tightness or inflammation of this muscle, a phenomenon known as "piriformis syndrome." The piriformis acts in open and closed chain fashions. When its origin (the sacrum) is fixed, contraction produces external rotation and abduction of the femur. When the femur is fixed, contraction tilts the pelvis backward. Tightness in the piriformis limits internal rotation of the thigh in certain seated twists and in twisted standing poses.
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