Sacroiliac (SI) joint anatomy and function
The sacroiliac joint is located in the pelvis, linking the iliac bone (pelvis) to the sacrum (lowest part of the spine above the tailbone). This joint transfers weight and forces between your upper body and legs. It is an essential component for energy transfer between the legs and the torso.
The sacroiliac joint is stabilized by a network of ligaments and muscles, which also limit motion. The normal sacroiliac joint has a small amount of normal motion of approximately 2-4 mm of movement in any direction. The sacroiliac ligaments in women are less stiff than in men, allowing the mobility necessary for childbirth.