The quadratus femoris is a muscle of the posterior hip. Situated immediately behind and slightly below the hip joint, its fibers are laterally oriented between the lower pelvis and upper femur, the long bone of the thigh. This muscle is one of several responsible for turning the thigh outward in the hip socket. It also assists the muscles of the inner thigh in adduction of the leg, or drawing the leg inward toward the midline of the body. The quadratus femoris also plays a role in holding the head of the femur in place in the acetabulum, or hip socket, during these leg movements.
A broad band-shaped muscle on the back of the hip, the quadratus femoris is almost as wide as it is long, giving it a square-shaped appearance. It arises from the ischium of the pelvis, the lower outer pelvic bone, along a club-shaped protrusion known formally as the ischial tuberosity and informally as the sit or sitz bone. From the outer edge of the ischial tuberosity, the quadratus femoris runs horizontally across the back of the hip joint toward the top of the posterior femur bone. It then inserts along a vertical line found at the base of the neck of the femur called the quadrate tubercle.
Bordered by the inferior gemellus muscle above and the adductor magnus muscle below, the quadratus femoris is the bottommost of the external rotators in the hip and the topmost of the adductor group in the thigh. It is a deep muscle, situated immediately beneath the large gluteus maximus, the muscle that extends and abducts, or lifts sideways, the leg in the hip joint. Though it is technically counted among the hip muscles along with the glutes, the superior and inferior gemelli, the obturator internus and externus, and the piriformis muscles, the quadratus femoris’s horizontal fibers are practically flush with those of the adductor magnus. They are separated only by the presence of the medial femoral circumflex blood vessels. The fact that the quadratus directly crosses the hip joint and the adductor does not also differentiates the two.
This muscle’s lateral orientation means that when it contracts, it shortens in a sideways direction and therefore must either pull the thigh inward or rotate it back. During adduction, it assists the more powerful adductor muscles of the inner thigh in drawing the leg in sideways, as in jumping together during a jumping jack. To perform lateral or external rotation, it works with the muscles on the back of the hip to pull on the posterior femur, turning the head of the femur within the hip socket and pointing the toe outward like that of a ballet dancer.