The sternocostal head of the pectoralis major is the largest part of the pectoralis major muscle. It works with the clavicular head to adduct the arm and works alone to extend the arm at the shoulder. The name “sternocostal” refers to the origins of this head on the sternum and ribs, or costal region.
The sternocostal head is one of two major heads of origin of the pectoralis major muscle. It arises from a wide band of origins along the manubrium, body of the sternum, costal cartilages, and the sixth rib. Tendons arise from these origins and extend a short distance before forming muscular slips that run laterally toward the axillary region. As they approach the axial region, the muscular slips converge to give the sternocostal head a fan-like shape. The muscular slips pass inferior to the shoulder joint to reach the arm and form a tendon that inserts along a narrow strip on the crest of the greater tubercle of the humerus.
Contraction of the sternocostal head of the pectoralis major muscle draws the insertion on the humerus toward the origins along the anterior chest. When both the clavicular and sternocostal heads contract together, they pull the arm anteriorly and medially, resulting in the adduction and medial rotation of the humerus at the shoulder joint. The sternocostal head can also work independently of the clavicular head when the arm is raised, pulling the arm inferiorly towards its origins and extending the arm at the shoulder.