What is the primary function of the SI joint?

• A. Stability
• B. Mobility
• C. Weight bearing
• D. Shock absorption

Answer: (A)

The sacroiliac joint is built to provide a solid base for transferring forces from the spine to the pelvis and legs. Its surfaces are large and the joint is held together by strong ligaments, which limits motion. That emphasis on rigidity means it acts mainly to stabilize the pelvis and trunk, especially under load during standing and walking. Tiny movements do occur to accommodate small changes, but these are secondary to maintaining stability. So, while it does bear weight and contribute to load transmission, the defining function is stability.