They also protect the main shoulder joint, the glenohumeral. About eight shoulder muscles attach to the shoulder blade (scapula), upper arm (humerus), and collar bone (clavicle).

Your deltoid muscles start near your clavicle and scapula. The end of the muscles, or insertion point, attaches near your humerus. There are other muscles and joints that support the deltoids, too.

The head of your upper arm bone, also called the humerus, fits into the socket of your shoulder blade, or scapula. When you extend your arm out away from your body, the rotator cuff muscles keep ...