The shoulder muscle is composed of four main muscles: the delt (also known as the DELTOID), the ROTATOR cuff, the teres, and the DELTOID. The delt is a triangular muscle located at the top of the shoulder and is responsible for shoulder abduction and flexion. The ROTATOR cuff is a group of four muscles that provide stability to the shoulder joint and allow for rotation of the arm. The DELTOID is a large, triangular muscle located on the outer side of the shoulder and is responsible for shoulder abduction and flexion. The teres is a small, round muscle located on the back of the shoulder and is responsible for shoulder extension and adduction. All four of these muscles work together to provide stability and movement to the shoulder joint.