Final answer:
In cranial flexion, the sacrum moves slightly forward and downward at its base, while the coccyx moves slightly upward and forward due to the articulation at the sacroiliac joint.
Step-by-step explanation:
The sacrum is part of the vertebral column and interacts with other components of the spine through movements like flexion and extension. Flexion movement in the vertebral column is an anterior forward bending, which occurs within the sagittal plane.
During cranial flexion specifically, as it relates to the sacrum, the top (base) of the sacrum moves slightly forward and down in relation to the ilium of the pelvis, while the tailbone (coccyx) moves in the opposite direction, slightly upward and forward. This is due to the articulation between the sacrum and the ilium forming a joint called the sacroiliac joint, which has limited movement but still allows for the slight anterior and posterior movement that comprises flexion and extension.
This movement is minimal and is summed with the movements of other vertebral sections to contribute to the overall flexibility and motion of the spinal column. The ligaments and intervertebral discs play critical roles in the movement and stability of the spine, including the sacral region. As part of the sacrococcygeal curve, the sacrum also retains a primary curvature which is concave anteriorly.