Is my sacroiliac joint supposed to move? Ever since I got injured in a car accident, I keep feeling a clunk in my low back when I lift my leg up.

The sacroiliac joint does not usually move very much. The joint moves a different amount depending on whether you're male or female and how old you are. Movement in the joint decreases slowly from birth to puberty. For males, the sacroiliac joint keeps losing movement after that. In females, however, movement in the joint increases after puberty, peaking around age 25. Overall, the movement in the sacroiliac joint is greater in women than men. 

People who've injured the ligaments that connect around the sacroiliac joint may end up with too much movement in the joint. This can be a source of pain and occasional popping or "clunking" in the joint. Try to avoid movements and activities that cause the pain or clunk. If your symptoms keep bothering you, consider seeing a physical therapist. A therapist can help you learn safe ways to lift and move and can train you in exercises to stabilize the joint.