Your shoulder is a ball and socket joint capable of moving and twisting in all directions. The ball part is formed by the upper part of your arm called the humeral head. The socket part is formed by a golf-tee like dish on the side of your shoulder blade. The ball is dynamically head in the socket by four muscles working together called your rotator cuff. Your arm, shoulder blade and collar bone together form your shoulder girdle which is attached to your trunk where your collar bone meets the center of your chest. You also have a number of large muscles that attach from your shoulder to your neck, chest and back that have a strong influence on your posture. See the video for more detail.