The pectoralis minor muscle is located in the deep surface of the pectoralis major muscle, i.e. the deep layer of the anterior and superior part of the thorax, starting from the 3rd to 5th ribs, with the muscle bundle facing outward and upward, and ending at the rostral eminence of the scapula. The pectoralis minor is innervated by the medial thoracic nerve. The function of the pectoralis minor is to pull the scapula forward and downward, and to lift the ribs to help inhale when fixing the scapula. Shortening of the pectoralis minor leads to a rounded shoulder posture; contracture causes the trapezius muscle to be passively pulled, resulting in pain, or causes the inner edge of the scapula to be turned out; damage to the pectoralis minor affects expiration, resulting in numbness of the arm due to the involvement of the blood vessels around the pectoralis minor. If you have difficulty in breathing out, arm numbness, shoulder pain and other symptoms, you should pay attention to check whether the pectoralis minor muscle is damaged, and choose the appropriate treatment under the guidance of a professional doctor.