Name the functional regions of the deltoid and one action for each.

• A. Anterior fibers flex and medially rotate; Middle fibers abduct; Posterior fibers extend and laterally rotate
• B. Anterior fibers extend and laterally rotate; Middle fibers abduct; Posterior fibers flex and medially rotate
• C. Anterior fibers flex and medially rotate; Middle fibers abduct; Posterior fibers extend and laterally rotate
• D. All deltoid fibers perform the same action

Answer: (C)

Deltoid has three functional regions whose fiber directions give distinct actions at the shoulder. The anterior fibers pull the humerus forward and rotate it inward, so they flex the shoulder and medially rotate the arm. The middle fibers pull straight up to abduct the arm. The posterior fibers pull the humerus backward and rotate it outward, so they extend the shoulder and laterally rotate the arm. This arrangement explains why abduction is mainly by the middle portion, with the anterior and posterior fibers contributing as the arm moves. Initial abduction is also guided by the supraspinatus, but as the motion continues, the middle fibers provide the bulk of the lift. The combination described—anterior flexion and medial rotation, middle abduction, posterior extension and lateral rotation—best matches how the deltoid functions.