The sacroiliac joint is a joint between the sacrum and the ilium, or pelvic bone. The two sides of the sacroiliac joint normally work together. If 1 side becomes stiff, they will not move together and this causes pain or muscle stiffness in the area. Pain is often made worse with walking and bending activities. It is also possible that one side may become too loose (lax) as well, resulting in SIJ dysfunction. This may occur during the menstrual cycle or pregnancy due to hormonal changes that cause the ligaments to become more lax. SIJ dysfunction can occur with injury, such as when a person falls and lands on 1 side of the body and alters the position of the joint, or when an athlete overtrains. Muscle imbalances and hip problems, such as hypermobility or dysplasia, may also lead to SIJ dysfunction. Sacroiliac pain is also related to some types of arthritis, such as ankylosingspondylitis, an inflammatory process most often affecting the lower back, which may cause the vertebrae to fuse.