Generally considered to be the lumbar region, and the site of much of the world’s pain, possibly because of man’s unnatural adoption of the upright posture. In the early development of the body the spine has a single C-curve, later it develops concave curves at neck and lumbar regions (lordosis) and convex or persistence of the C-curve in the upper back (dorsal kyphosis).
The vertebral column has five lumbar vertebrae, the bodies and processes becoming thicker and heavier in the progression of L1 to L5. They are separated by intervertebral discs, the essential strength in binding the vertebrae together; the thickest of these discs binds L5 to the upper sacrum.
The spinal cord in the early fetus has the same length as the spinal canal, formed by the circular spaces in the vertebrae behind the body and in front of the posterior element of laminae; as the fetus grows, the rates of lengthening of cord and body do not match, so by full growth the cord ends at about the junction of thoracic and lumbar vertebrae, and the nerve roots, grouped together as the “horse’s tail” (cauda equina) continue on down to their points of emergence between their respective vertebrae.
The flexor muscles of the lumbar spine are located within the abdominal cavity, posterior to the peritoneum. The psoas muscle arises from the bodies and transverse processes of all five lumbar vertebrae and passes down the lower back to leave the abdomen beneath the inguinal ligament and insert into the femur. Both psoas muscles contracting together will flex the spine forward, one working alone will laterally flex the spine; if the spine is held still, the psoas flexes the hip, and is the muscle (with Iliacus) that will flex the hip in the seated position.
Quadratus lumborum is as its name implies, a rectangular muscle that runs between pelvic crest and 12th rib; as in psoas, one contacting will laterally flex the spine, both contracting will forward flex the spine.
At the back of the lumbar spine there is a complicated set of muscles, collectively known as erector spinae. They are enclosed in a dense fascial envelope. The muscles arise from the posterior aspects of the lumbar vertebrae, laminae, spinous and transverse processes, and insert at varying points in the spine above that, some reaching as far as the upper cervical vertebrae.