A All fibers:Adduct the shoulder (glenohumeral joint)Medially rotate the shoulder (G/H joint)Assist to elevate the thorax during forced inhalation (with the armfixed)Upper fibers:Flex the shoulder (G/H joint)Horizontally adduct the shoulder (G/H joint)Lower fibers:Extend the shoulder (G/H joint)O Medial half of clavicle, sternum, and cartilage of first through sixthribsI Crest of greater tubercle of humerus The main function of this chest muscle as a whole is the adduction and internal rotation of the arm in the shoulder joint. Pectoralis Major