Upper extremity abduction and scapular movement
Abduction of the upper limb to a horizontal position is performed by the deltoid muscles. Since these muscles are attached to the pectoral girdle and humerus, the pectoral girdle (scapula) itself does not move when the upper limb is abducted to a horizontal position. If one tries to abduct more upward in this position, the humerus and acromion collide. Therefore the humerus cannot be moved by the shoulder joint.
It is the rotation of the scapula that allows the upper limb to abduct beyond the horizontal. The upper fibers of the trapezius muscle attached to the upper limb girdle rotate the scapula, allowing the upper limb to abduct to a vertical position.