Anatomically, the sacrum and coccyx are part of the spine. Unlike the vertebra in the cervical, thoracic, and lumbar parts of the spine, however, the sacrum and coccyx are fused with the right and left innominate (pelvis) bones of the pelvic girdle.
The fifth lumbar vertebrae (L5) of the lumbar spine connects to the sacrum (S1) at the lumbosacral joint to form a base to support the spine. As the only connection to the upper portion of the body, this joint experiences a great amount of force, especially when engaged in dynamic activities. The sacrum provides an attachment point for several muscles including the gluteus maximus, iliacus, and piriformis muscles.
The coccyx is attached to the bottom of the sacrum. Biomechanically, it helps support body weight during sitting. Load on the coccyx increases when sitting in a reclined positon. This can lead to pain if the coccyx is inflamed or damaged.
