The shoulder joint – where the upper arm and shoulder blade meet – is the most flexible joint in the body. This mobility, combined with long arms and grasping hands, enables humans to perform a huge range of arm movements.
The bony framework of the shoulder joint is formed by three bones – the shoulder blade (scapula), the collarbone (clavicle), and the top of the upper arm bone (humerus). Deep muscles make the joint stable, while an outer layer of muscles pulls on the bones to move the joint.
The shoulder has about eight muscles that attach to the scapula, humerus, and clavicle. These muscles form the outer shape of the shoulder and underarm. The muscles in the shoulder aid in a wide range of movement and help protect and maintain the main shoulder joint, known as the glenohumeral joint.
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