Final answer:
When the lumbar spine is in flexion and side bending is added, the sacrum moves in the opposite direction of the bend, known as counter-nutation, due to the biomechanics of the pelvis and the stabilizing ligaments. The sacrum's slight posterior rotation helps maintain stability and alignment in the pelvis and lower back.
Step-by-step explanation:
If the lumbar spine is in flexion, and lateral side bending is added, the sacrum will counter-nutate (posteriorly rotate). Flexion of the lumbar spine is when there is an anterior (forward) bending movement in the sagittal plane. Lateral flexion involves bending to the right or left in the coronal plane. When these two movements are combined, the sacroiliac ligaments, which are meant to support and stabilize the sacrum, limit the movement of the sacrum. Since the sacrum and lumbar spine usually move in opposite directions due to the sacroiliac joint's limited range of motion, the sacrum will typically move in the opposite direction of the lumbar spine when it side bends.
Thus, if the lumbar spine is in flexion and it bends to the side, the sacrum will rotate slightly in the opposite direction of the lumbar side bend due to the biomechanics of the pelvis and the presence of supporting ligaments like the anterior and posterior sacroiliac ligaments, the sacrospinous ligament, and the sacrotuberous ligament. This is also in part to balance the forces and stabilize the pelvis and spine during movement. The sacrum's movement in this situation aids in maintaining stability and alignment in the pelvis and lower back.