The sacroiliac joint is located in the pelvis, on both sides of the sacrum, and is composed of iliac bones. It is surrounded by ligaments; there are no muscles at this particular joint. It permits pelvis movements, both forward and backward, that are useful during childbirth, even if they are not used on a daily basis. A localized injury can trigger sacroiliac and lumbar pains. Ankylosing spondylitis, an arthritic disorder, may also cause sacroiliac pain.