How the lower back is built

The lower back carries more of the body's weight than any other part of the spine, and it bends, lifts and straightens many times a day. Knowing how it is built makes the words people use about low back pain easier to follow.

The lumbar vertebrae

The lumbar spine is the section between the bottom of the rib cage and the pelvis. Its vertebrae are the largest in the spine, with thick, sturdy bodies at the front to bear weight. The American Academy of Orthopaedic Surgeons' OrthoInfo page on low back pain describes the spine as small bones, called vertebrae, stacked on top of one another, with muscles, ligaments, nerves and intervertebral discs as its other parts. In the lower back the stack curves gently inward, forming the hollow in the small of the back.

The discs

Between each pair of lumbar vertebrae sits a disc, with a tough fibrous outer ring and a softer centre. OrthoInfo explains that the discs provide cushioning so the spine can tolerate the movements and stresses placed on it. The lumbar discs are thicker than those in the neck and carry heavy loads when you bend and lift, which is part of why disc problems are most often talked about in the lower back. The discs and sciatica page explains what "herniated" and "bulging" mean.

Facet joints

At the back of each vertebra, a pair of small facet joints links it to its neighbours. These joints guide how the lower back moves: their angle in the lumbar spine allows a good deal of bending forwards and backwards but relatively little twisting. Like any joint, they have cartilage surfaces and a capsule of tissue around them, and they change with age. When people talk about arthritis in the lower back, the facet joints are often part of what they mean.

The sacrum, pelvis and sacroiliac joints

Below the lowest lumbar vertebra sits the sacrum, a triangular bone formed from vertebrae that fuse together during development. It wedges between the two halves of the pelvis, and the joints on either side, where the sacrum meets the pelvis, are the sacroiliac joints. They move only slightly but transfer the weight of the upper body into the hips and legs. Pain felt low down, to one side of the spine near the dimples above the buttocks, is sometimes linked with this area.

Muscles and ligaments

Strong ligaments bind the lumbar vertebrae to one another and to the pelvis. Layers of muscle surround them: small deep muscles between neighbouring vertebrae, long muscles running up either side of the spine, the deep and outer muscles of the abdomen that wrap around the trunk, and the large muscles of the hips and buttocks. OrthoInfo notes that the muscles and ligaments help stabilise the bones of the spine. When people talk about the "core", they usually mean this whole group working together to hold the trunk steady while the arms and legs move.

Because these tissues work so hard, they are also the most common source of everyday lower back soreness. OrthoInfo describes muscle soreness from unaccustomed activity, with muscle and ligament fibres overstretched or injured, as one of the more common causes of low back pain.

Nerves of the lower back

The spinal cord itself usually ends around the top of the lumbar spine. Below that point, a bundle of nerve roots continues down the spinal canal, and pairs of roots leave between each set of vertebrae, heading for the buttocks and legs. Some of them join to form the sciatic nerve, which runs down the back of each leg. The National Institute of Arthritis and Musculoskeletal and Skin Diseases, in its overview of back pain, notes that pain can sometimes radiate away from the back to the buttocks, legs or abdomen. This close neighbourhood of discs, joints and nerve roots is why a problem in the lower back is sometimes felt in the leg, and why leg pain, numbness or weakness is worth mentioning to a health care provider.

Putting it together

In everyday life, all of these parts share the work. Bending to tie a shoelace flexes the lumbar discs and facet joints, stretches the muscles and ligaments at the back, and calls on the hips to do much of the movement. Lifting a box adds load to all of them at once. The lifting and carrying page looks at how people spread that load, and the low back pain page explains how pain in this area is usually described.