The sacrum is at the bottom of the spine and lies between the fifth lumbar segment and the coccyx (tailbone). The sacrum is a triangular-shaped bone and consists of five segments (S1 - S5) that are fused together. The sacrum contains a series of four openings on each side through which the sacral nerves run. Back pain can typically arise where the lumbar spine and sacrum connect because this section of the spine is subjected to a large amount of stress during certain activities, such as sports and sitting for long periods of time. Low back pain can also arise from a dysfunction in motion at the sacroiliac joint connecting the sacrum to the pelvis.