The teres minor muscle rotates the arm laterally and assists in bringing it toward the body. As it draws the upper arm bone (humerus) up, it strengthens the shoulder joint. There are two teres muscles. The other one is the teres major muscle, a thick, flattened muscle that brings the arm toward the body and assists in extending it when the arm is in a flexed position. The teres major muscle also aids in rotating the arm but its function is just the opposite of the teres minor and other muscles in the rotator cuff.