The pectoralis major or “pecs” is commonly thought of as the chest muscle. It originates on the clavicle, sternum, costal cartilage , and the external oblique aponeurosis. It inserts on the intertubercular groove of the humerus. Its main actions are to adduct the shoulder and to internally rotate the humerus. This muscle gets chronically shortened by a rounded shoulder forward posture such as from prolonged sitting. Tightness in this muscle can cause strain in the rhomboids and traps. When trigger points form in this muscle they refer pain into the anterior shoulder, as well as the anterior chest and medial aspect of the arm. Trigger points in the pecs can also cause nipple hypersensitivity. Trigger points in the left pec muscle can mimic heart pain.