Deep, aching pain in the lower back that worsens when you sit for long periods, climb stairs, or roll over in bed is a familiar complaint for millions of people. What is far less familiar is the joint most likely responsible for it. The sacroiliac joint, which connects the base of the spine to the pelvis on each side, is one of the most commonly overlooked sources of low back pain in adults. It is frequently mistaken for a disc problem, a hip issue, or general lumbar strain, and as a result, many patients spend months or years pursuing treatments that target the wrong structure entirely.