The lumbar region is located between the thoracic (chest) region of the spine and the sacrum, a large bone at the base of the spine. The lumbar spine usually curves slightly inward, called lordosis.

the L5-S1 disc is a spinal joint found at the bottom of the lumbar spine (L5) and top of the sacrum (S1). This disc is critical in “supporting the lower back and facilitating movement.” ...