The axis goes from the back to the front, like the direction in which a person normally walks.

The anteroposterior axis runs from the back (posterior) to the front (anterior), matching the direction a person normally walks. The sagittal axis runs from side to side, the frontal axis runs front to back in some conventions but is not the named back-to-front axis here, and the vertical axis runs up and down. Therefore, the correct choice is anteroposterior.

AI-written and checked against the verified answer.