The abdominal head of the pectoralis major muscle is the inferior portion of the pectoralis major muscle. It produces adduction and extension of the humerus. The abdominal head varies greatly from person to person; it may be large, small, or completely absent.
The abdominal head of the pectoralis major muscle arises from a tendon sheath inserted into the superior edge of the external oblique muscle. From its insertion, it ascends the chest as a thin, flat muscular belly inferior and lateral to the sternocostal head of the pectoralis major. The abdominal head crosses the anterior shoulder joint and joins with the clavicular and sternocostal heads to insert on a long string on the anterior humerus just distal to the shoulder joint.
Contraction of the abdominal head of the pectoralis major moves the arm in two different directions depending on the starting location of the arm. When the arm is in the anatomical position (hanging down along the side of the body with palm facing forward), the abdominal head works with the other heads of the pectoralis major muscle to pull the humerus anteriorly and medially toward the sternum and medial abdomen. From a flexed position with the arm pointing anteriorly from the trunk, the abdominal head of the pectoralis major muscle extends and adducts the arm by pulling it inferiorly and medially toward the abdomen.
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