The shoulder is the most flexible of all our joints, made up of three joints in total.
The first joint in the shoulder is called the glenohumeral joint. That's where the humerus (the upper arm bone) connects neatly into the scapula (the shoulder blade).
Together, they form a ball and socket which allows for a large amount of motion. Layered between the 'socket' of the scapula and the humerus's 'cup' is the labrum - cartilage that ensures the two can fit together, whilst maintaining flexibility.
The second joint lies with the scapula and the clavicle (the collarbone). This is called the acromioclavicular joint. Finally, there's the sternoclavicular joint, where the clavicle and the sternum meet just below the neck, anchoring the clavicle and allowing it to move.
Holding this all of this together is a series of muscles and ligaments. The supraspinatus and the infraspinatus, are the upper and lower muscles of the shoulder blade. Below these, you'll find the teres minor and the subscapularis muscles. These four muscles make up the shoulder's rotator cuff.
Sitting above these is the powerful deltoid muscle. There are three parts to this particular muscle, which each part creating movement in specific directions. The front section deals specifically with flexion: raising your arm in front of you. The rear section performs extension: stretching your arm out behind you. The middle part works in tandem with the supraspinatus for abduction of the humerus.
Shoulder related AnatomyStuff resources