APPLIED RADIOLOGICAL ANATOMY

Detailed explanation-1: -The piriformis muscle is an external (or lateral) rotator of the hip along with the superior and inferior gemellus, quadratus femoris, and obturator internus, and externus. The piriformis muscle rotates the femur during the hip extension and abducts the femur during flexion of the hip.

Detailed explanation-2: -With the hip flexed, the piriformis pulls the femur in medial rotation instead of lateral. Once you are past 60 degrees of hip flexion, the piriformis becomes a medial rotator and a prominent hip abductor.

Detailed explanation-3: -In this position, the line of pull has shifted anterior to the longitudinal axis of rotation, turning the piriformis muscle into an internal rotator when the hip is flexed past 90 degrees. Therefore, to properly stretch this muscle, the hip should be placed in a position of flexion and external rotation.