The sacroiliac (SI) joint connects the hip bones to the sanctum, which is the triangular bone between the lumbar spine and the tailbone. This joint is important for absorbing shock between the upper body and the pelvis/legs.
Problems with this joint can lead to back and leg pain. It can be hard to tell if the pain is coming from the SI joint or a herniated disc however, since they can feel very similar.
SI joint dysfunction can come from overuse, or too much movement of the joint. Problems can also arise from underuse, or too little movement. Sacroiliac joint dysfunction may cause inflammation. It is more common in young and middle-aged women.