The lower back is made up of the lumbar spine (the five vertebrae, or bones, of the lower back); the spinal discs (which rest between the vertebrae); the muscles, tendons, and ligaments; and the spinal cord and nerves. The purpose of the lower back is to provide support for the weight of the upper body. It is also involved in bending, extending, and rotating. The lumbar spine (the bones of the lower back) has a particularly important job: it is tasked with protecting the spinal cord and the soft tissues of the nervous system. Therefore, any trauma or disorder in the ligaments, tendons, bones, muscles, discs, and/or vertebrae can cause serious pain and disability.
Very generally, lower back pain may be caused by a number of factors, including strain in the lower back (injury to the muscles, tendons, and ligaments). Some patients suffer from pain related to nerve irritation caused by injuries to the spinal discs. This type of nerve irritation is known as radiculopathy. When spinal disc material compresses or irritates the nerves of the lower back, the result may be the symptoms of sciatica. Other major causes of low back pain include spinal stenosis (a narrowing of the spinal canal) as well as arthritis, osteoporosis, compression fractures of the lumbar vertebrae, scoliosis, and (in rare cases) tumors.
Problems in the lower back not only compromise the functions of the lower back itself, but also the workings of the lower extremities. The sciatic nerves exit from the lower back and move down the legs to the feet. When the sciatic nerves are subject to compression or irritation (as described above), messages between the legs and the brain are disrupted; this leads to the leg pain, abnormal sensation (numbness and tingling), and muscle weakness of sciatica.