The human backbone is a column of 33 total vertebrae, of which 24 are movable and free (the remainder are fused). The movable vertebrae are divided into three regions: cervical, thoracic, and lumbar. There are five lumbar vertebrae, although occasionally some people have six. The first lumbar spinal vertebra (L1) is the first of this series.

The lumbar vertebrae are the largest movable bones of the backbone. They are wider from side to side than from front to back. They are thicker in the front than the back. The large size and bone strength is necessary because these vertebrae support more weight than the upper two segments of the backbone.

The L1 vertebra is level with the ninth rib and the connection between the stomach and the small intestine. Flexion and extension in the L1 vertebrae averages about 10 to 12 degrees. Flexion and extension are both angled movements; flexion refers to bending movements around a joint, while extension refers to movements that straighten or extend away from the joint.

There are five pairs of lumbar nerves that emerge from the lumbar vertebrae of the backbone. The nerves emerging from the L1 vertebrae provide nervous system services to the abdomen and then as the nerves branch, to the thighs.