The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow. Anatomically, it connects the scapula and the lower arm (consisting of the radius and ulna), and consists of three sections. The upper extremity consists of a rounded head, a narrow neck, and two short processes. Its body is cylindrical in its upper portion, and more prismatic below. The lower extremity consists of two epicondyles, two processes, and three fossae.
The muscles of the humerus that act on the forearm (Figure 1) are primarily involved in flexion. This is the movement of the forearm back towards the shoulder joint or the decreasing of the angle between the bones of the limb at the elbow joint and extension (Figure 2). Flexion is produced by the action of the following muscles:
- Biceps brachii is a two-headed muscle that lies on the upper arm between the shoulder and the elbow. Both heads arise on the scapula and join to form a single muscle belly which is attached to the upper forearm. While the biceps crosses both the shoulder and elbow joints, its main function is at the latter where it flexes the elbow and supinates the forearm. Both heads join on the middle of the humerus, usually near the insertion of the deltoid, to form a common muscle belly.
- Brachialis is a muscle in the upper arm that flexes the elbow joint. It lies deeper than the biceps brachii, and is a synergist that assists the biceps brachii in flexing at the elbow. The brachialis originates from the lower half of the front of the humerus, near the insertion of the deltoid muscle, which it embraces by two angular processes.
- Brachioradialis is a muscle of the forearm that acts to flex the forearm at the elbow. It is also capable of both pronation and supination, depending on the position of the forearm. It is attached to the distal styloid process of the radius by way of the brachioradialis tendon, and to the lateral supracondylar ridge of the humerus.
- Medial epicondyle muscles of the humerus provide a tiny contribution.
When the forearm is extended and supinated, the axis of the arm and forearm are not in the same line; the arm forms an obtuse angle with the forearm (the carrying angle). During flexion, however, the forearm and the hand tend to approach the middle line of the body, and thus enable the hand to be easily carried to the face.
Extension is produced by the following muscles:
- Triceps brachii muscle is the large muscle on the back of the upper limb of many vertebrates. The medial head arises distally from the groove of the radial nerve; from the dorsal (back) surface of the humerus; from the medial intermuscular septum; and its distal part also arises from the lateral intermuscular septum.
- Anconeus muscle is a small muscle on the posterior aspect of the elbow joint. Anconeus muscle originates on the posterior surface of the lateral epicondyle of the humerus and is inserted on the ulna lateral to the olecranon from where it extends down on the dorsal side of the bone. It assists in extension of the elbow, where the triceps brachii is the principal agonist, and supports the elbow in full extension.
- The muscles arising from the lateral epicondyle of the humerus, such as the extensor digitorum communis, also contribute to extension.