The extracapsular ligaments are the iliofemoral, ischiofemoral, and pubofemoral ligaments attached to the bones of the pelvis (the ilium, ischium, and pubis respectively). All three strengthen the capsule and prevent an excessive range of movement in the joint. Of these, the Y-shaped and twisted iliofemoral ligament is the strongest ligament in the human body.
In the upright position, it prevents the trunk from falling backward without the need for muscular activity. In the sitting position, it becomes relaxed, thus permitting the pelvis to tilt backward into its sitting position. The iliofemoral ligament prevents excessive adduction and internal rotation of the hip. The ischiofemoral ligament prevents medial (internal) rotation while the pubofemoral ligament restricts abduction and internal rotation of the hip joint.
The zona orbicularis, which lies like a collar around the most narrow part of the femoral neck, is covered by the other ligaments which partly radiate into it. The zona orbicularis acts like a buttonhole on the femoral head and assists in maintaining the contact in the joint.
All three ligaments become taut when the joint is extended – this stabilises the joint, and reduces the energy demand of muscles when standing