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 ...

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).

A forceful injury can tear or rupture the bicep or tricep tendons. Injuries to the upper arm muscles and tendons are most common in men 30-50, but women also may experience them.