The lumbar spine is composed of the lower 5 vertebrae, which have been numbered L1–L5. The lowest vertebra of the lumbar spine (L5) is connected to the top of the sacrum, which is a triangular bone present at the base of the spine fitting into the two pelvic bones. In some cases, an extra or sixth lumbar vertebra may be present.
The lumbar vertebral bodies are seen to be taller and bulkier than the rest of the spine, as the lower back has to withstand higher pressure due to body weight and other movements such as lifting, pulling, and twisting. In addition, large and powerful muscles are found to be attached on or near the lumbar spine to provide extra strength to the lumbar vertebral bodies.