The doctor told me that my shoulder is “unstable” after a pitching injury. What makes a joint “stable?”

There are at least four items that hold the shoulder joint steady and keep it from dislocating. The first is the bulk of the surrounding muscles, which provides some passive tension to hold the joint. The second is muscle contraction. When the muscle contracts, it presses the joint together. Bringing the joint surfaces closer increases stability. Third is joint motion. When the shoulder moves, some of the ligaments tighten up. This is called passive constraint. One other factor is the barrier provided by the muscles. This keeps the head of the humerus from coming out of the socket. Injury or damage to any of these structures can cause the joint to become unstable.