Mobility of the sacro-iliac joint
- The surfaces of the sacrum and the hip bone have articular cartilage, and an articular cavity exists between them. Thus, although it is a joint in the narrow sense (synovial joint), the joint capsule of this joint is so tightly reinforced by the anterior sacro-iliac ligament, interosseous sacro-iliac ligament, and posterior sacro-iliac ligament that it can hardly be moved.
- In cows and some other animals, progesteron act on these ligaments to make them more flexible, facilitating childbirth. This may also be the case in humans.