The iliofemoral ligament is one of several ligaments that restrain the movement of joints in the pelvic region. A ligament is a type of connective tissue that exerts a passive elastic force when elongated by certain movements; this force ensures that joints are limited to a particular range of motion. This prevents overextension, which may cause injury to the joint.
Extension occurs when points of attachment in a joint are pulled further apart, making the joint angle increase. Flexion describes the opposite motion, which occurs when the joint bends and the angle decreases. Ligaments can control these movements, as well as others such as rotation (in which the joint is twisted around the axis formed at its point of contact).
Hip extension is limited by the iliofemoral ligament, which passes over the front of the hip joint and connects the ilium, or hip bone, to the femur, or thigh bone. This ligament elongates when the pelvis is tilted backwards, restricting the distance the joint can be extended. The iliofemoral ligament also restrains external rotation of the hip joint when flexed, and it restrains both internal and external rotation when the joint is extended.