The Teres major muscle is located at the back of the armpit. It originates on the posterior aspect of the inferior angle of the scapula, and inserts and inserts on the medial lip of the intertubercular sulcus of the humerus. It’s main function is to assist adduction, internal rotation, and extension of the arm. This muscle only becomes active when there is resistance added to these movements. Trigger point pain from this muscle will cause pain to be felt deep into the posterior shoulder and down the back of the arm.