The humerus is the bone of the upper arm. It is part of the elbow and shoulder joints. At the elbow joint, the humerus connects to the ulna, a lower arm bone. At the shoulder joint, the humerus fits into the shoulder socket.
The humerus is one of the longest bones in the human body. Due to this, it is an insertion and attachment point for many muscles. These muscles are the reason you can move your shoulder and other parts of your arm. The major deltoid muscle, which is responsible for flexion, extension, and abduction of the shoulder, attaches at this bone. Other muscles that attach at the humerus include the triceps brachii and the 4 rotator cuff muscles.
Due to its vast movement and the fact that it is one of the longer bones in the body, the humerus is prone to fractures or breaks. The most common spot for a break to occur is at the surgical neck. The surgical neck is below the head of the humerus, located near the shoulder joint.