Teres major muscle
The teres major muscle controls the adduction of the arm - the movement of bringing the arm in toward to the body. It helps medially rotate the arm, or being able to draw elbow in and away from the side of the body. It also extends the arm at the shoulder allowing the arm to reach forward.
The teres major is innervated by the posterior cord of the lower subscapular nerve (C5 and C6).