Final answer:
The middle transverse axis of the sacrum is primarily involved in the movements of flexion and extension, which occur within the sagittal plane and consist of forward and backward bending of the vertebral column.
Step-by-step explanation:
The middle transverse axis of the sacrum is involved in flexion and extension. These movements take place within the sagittal plane and involve anterior or posterior movements of the body or limbs. For example, the vertebral column engages in flexion, where there is an anterior bending forward of the neck or trunk, and in extension, where there is a posterior-directed straightening from a flexed position or bending backward. The sacrum also contributes to these movements through the symphysis joints and the plane type synovial joints between the vertebrae.