The spine is composed of 33 interlocking bones called vertebrae. The lumbosacral region of the spine (or lumbar spine) consists of 5 lumbar vertebrae and 5 sacral vertebrae. The sacral vertebrae, which are fused together, are called the sacrum.

• The lumbar vertebrae of the spine extend from the bottom of the rib cage to the sacrum.
• The fifth lumbar vertebra is connected to the sacrum. The sacrum is connected to the pelvic bones and the tailbone (coccyx).
• Because this part of the spine bears the weight and stress of the trunk and upper body, the lumbar vertebrae are larger and thicker than those in any other part of the spine.

A disc is located between each pair of lumbar vertebrae. The discs are capsules of connective tissue with a soft, jellylike center. The discs absorb shock and make the spine flexible. The bones of the sacrum are joined to one another and have no discs between them.

The vertebrae are supported by ropelike tissues called ligaments and groups of muscles:

• Abdominal muscles support the spine from the front and sides of the body.
• Iliopsoas muscles support the spine and flex the hips.
• Erector spinae muscles support the back of the trunk and spine and are the major stabilizers of the spine during lifting.

The spinal cord runs through the spinal canal, a tunnel that is formed by the holes in the center of each of the vertebrae. Nerves branch from the spinal cord, pass through openings (foraminae) between the vertebrae, and branch to the lower body.