When the base of the sacrum moves posteriorly, which motion unit is primarily affected?

• A. Lumbosacral
• B. Sacroiliac joints
• C. Lumbar
• D. Pelvis

Answer: (A)

When the sacrum’s base moves posteriorly, the direct interface that bears the change is the junction between the last lumbar vertebra and the sacrum. That L5–S1 area forms the lumbosacral motion unit, so a posterior motion of the sacral base primarily alters how this specific joint moves and carries load. The sacroiliac joints can be affected secondarily by changes in sacral tilt, but they are not the primary unit involved. The broader lumbar region or the pelvis isn’t the main focus here because the key movement at stake is the relative motion between L5 and S1.