How does the shoulder work?
Your shoulder is the most mobile joint in your body. This is because your main shoulder joint (the glenohumeral joint) is a ball-and-socket joint. Ball-and-socket joints allow for the most amount of movement of all the different types of joints in your body.
Ball-and-socket joints get this name because the ball-shaped upper arm bone (the humerus) fits into the cup-like hollow of the top part of your shoulder blade bone (the scapula).
Your shoulder joint is surrounded by a tough fibrous sleeve, called the capsule, which helps to hold the joint together. The inner layer of the capsule (the synovium) produces a fluid which nourishes the cartilage and lubricates the joint so it moves more smoothly. A group of four muscles and their tendons make up the rotator cuff, which controls movement and also helps to hold the joint together.
There’s also a smaller joint (the acromioclavicular joint) where the top of your shoulder blade meets your collarbone, above the main shoulder joint.