A very common muscle to become overloaded and damaged, and thus develop trigger points, is the pectoralis major muscle. This is the muscle that makes up the chest. It’s main functions are adduction and internal rotation at the shoulder. Poor rounded shoulder posture is a common cause of pec tightness and trigger points. trigger points in this muscle will produce pain felt into the front of the shoulder, the chest, and down the medial arm. If these symptoms occur in the left pec it can mimick heart pain. In women, these points can be a cause of breast pain and nipple hypersensitivity.