The teres major is a short and thick, somewhat flattened muscle that extends from the scapula (shoulder blade) to the humerus (upper arm bone). This muscle creates part of the posterior wall of the armpit and helps adduct the arm. It also rotates the arm medially behind the back. The teres major tenses when the arm lies next to the body. When the arm is extended the teres major is stretched, becoming elongated and thinner.
