Mechanically, the head teeters on the atlanto-occipital joints, shaped like cupped palms tipped slightly medially. Because the line of gravity falls anterior to these articulations, a force must be constantly provided in the upright posture by the posterior neck muscles to hold the head erect. Added to this gravitational stress is the action of the anterior muscles of the neck, essentially the masticatory, suprahyoid, and infrahyoid groups, which as a chain join the anterior cranium to the shoulder girdle.