NBME Gross Anatomy High Yield Practice Test

Full arm abduction requires upward rotation of the scapula so the glenoid fossa can face the humerus as the arm lifts overhead. This upward rotation is produced mainly by the combined actions of the trapezius and serratus anterior. Serratus anterior keeps the medial border tight to the rib cage, protracts the scapula, and helps rotate the scapula upward by pulling the inferior angle laterally. The trapezius, especially its upper and lower fibers, contributes to this upward rotation by elevating and tilting the scapula in a coordinated fashion. Together, these two muscles drive the scapulohumeral rhythm that allows full abduction beyond about 90 degrees. Other muscles either retract or downwardly rotate the scapula and would oppose this motion, so they aren’t the primary drivers of full overhead abduction.