The teres major is a thick, rounded muscle of the shoulder joint. It is a thick but somewhat flattened muscle. Supraspinatus , Infraspinatus , Subscapularis , and Teres Minor together This Four Muscle work as rotatory to the Humerus. So, also known as Rotator Cuff. teres major ORIGIN: teres major Originates from the posterior surface of the scapula at the inferior angle. teres major INSERTION: teres major inserted at the intertubercular groove of the humerus, tendon fused with the tendon of latissimus dorsi. teres major ACTION: teres major posteromedially extends, medially rotates, & adducts arm. teres major synergist of latissimus dorsi . teres major NERVE SUPPLY: The lower scapular nerve.