The humerus is the large bone in the upper arm. It is categorized as a long bone and its major features include the humeral head, greater and lesser tubercles, deltoid tuberosity, medial and lateral epicondyles, trochlea, and capitulum.
Biomechanically, the humerus enables a variety of different activities, including throwing, lifting, and writing. Some of these activities involve gross movement and require strength, while others involve fine movement and require precision and delicacy. The humerus is the first major component in the kinematic chain of the arm, followed by the forearm and the hand.
The humerus is part of two major body joints, the shoulder and the elbow. The head of the humerus lies at the proximal end and attaches to the glenoid cavity of the scapula (shoulder blade). This forms the shoulder or glenohumeral joint. Surrounding the joint are ligaments that keep the bones from separating during activity. The shoulder is a very mobile joint and must be dynamically stabilized, primarily by the deltoid, latissimus dorsi, teres major, and pectoralis major muscles, as well as by the rotator cuff group.
On the distal end of the humerus are two articular surfaces, the capitulum and trochlea. The trochlea is a groove on the medial side of the humerus that interlocks with the trochlear notch of the ulna to form half the elbow joint. This joint acts as a simple hinge allowing flexion and extension of the lower arm. The second joint in the elbow is between the capitulum of the humerus and the radius. The capitulum is a rounded surface on the lateral side of the humerus that engages with a cup-shaped feature on the head of the proximal radius bone in the forearm. It acts as a limited ball-and-socket joint enabling the forearm to pronate—turn the palm up—and supinate—turn the palm down—as the forearm rotates about its long axis.
