The three extracapsular ligaments of the hip joint.

http://en.wikipedia.org/wiki/Pelvis

The three extracapsular ligaments of the hip joint — the iliofemoral, ischiofemoral, and pubofemoral ligaments — form a twisting mechanism encircling the neck of the femur. When sitting, with the hip joint flexed, these ligaments become lax permitting a high degree of mobility in the joint.