The shoulder-girdle muscles are the upper trapezius, levator scapulae and rhomboid muscles. These muscles run in pairs, one on each side of the spinal column. The massive trapezius of ancient Chamorro male or female may pull on the tendons attached to the lower edge of the skull such that cranial turbercles or bone projections may result at the base of the skull during the lifespan of the individual.

The trapezius is a flat sheet of fibers located on the upper-middle back directly below the skin. It originates on the base of the skull, a ligament of the neck and the row of spinous processes of the vertebrae from the seventh cervical to the 12th thoracic. The uppermost fibers are ver thin and curve sideward and forward to insert on the clavicle (collarbone). The fibers that are slightly lower are thicker and stronger and insert on the acromion of the scapulae. They run downward and sideware from the spine to the scapulae. The middle and lower portions of the trapezius insert on the upper border of the spine of the spine of the scapulae.

The rhomboid muscle lies beneath the middle portion of the trapezius (in the middle of the back). It originates on the row of spinous processes of the vertebrae from the seventh cervical to the fifth thoracic and insers on the medial border of the scapulae from the spine to the lower border. The fibers run at an upward angle from the inner edge of the scapula to the vertebrae of the spine.

The levator scapulae is a small muscle on the back and sides of the neck and lies beneath the upper trapezius. Its origin is on the transverse processes of the upper four or five cervical vertebrae and its insertion is on the medial border of the scapula, from the spine to the superior angle. Although small, it is an important muscle in lifting the shoulder blades.