Low back pain (LBP), also known as lower back pain or lumbago, is a common disorder involving the muscles and bones of the back. It affects about 40% of people at some point in their lives. Low back pain may be classified by duration as acute (pain lasting less than 6 weeks), sub-chronic (6 to 12 weeks), or chronic (more than 12 weeks). The condition may be further classified by the underlying cause as either mechanical, non-mechanical, or referred pain.