What axis does the sacrum move around during cranial flexion and extension?

• A. Transverse axis (S2)
• B. Longitudinal axis
• C. Anteroposterior axis
• D. Vertical axis

Answer: (A)

During cranial flexion and extension, the sacrum rotates about a horizontal, transverse axis that passes through the second sacral segment (S2). This axis aligns with the craniosacral rhythm, linking the motion of the cranium to sacral movement so that the sacrum’s rotation complements the cranial cycle. The other axes describe rotations in planes that don’t match how the sacrum moves in this context: a longitudinal axis runs head-to-toe, an anteroposterior axis would rotate around a front-to-back line, and a vertical axis would rotate around a up-down line. The key point is that the sacrum moves around the transverse axis at S2 during the cranial rhythmic motion.