The biceps tendon is one of two tendons connecting the upper and lower ends of the biceps brachii muscle to the shoulder blade and the forearm, respectively. Each a band of dense fibrous tissue, these tendons are known as the proximal and distal biceps tendons, proximal referring to the one nearest the trunk of the body and distal referring to the one farthest away. Because each tendon traverses a major joint — the shoulder joint and the elbow joint — it helps the biceps muscle to produce movement at the joint it crosses.
Originating at two points on the top of the scapula via the proximal biceps tendon, this muscle has two separate sections known as heads. The long and short head of the biceps run parallel to each other along the front of the upper arm and converge to form the distal biceps tendon just above the elbow. This tendon then crosses the elbow joint and inserts in the forearm at the top of the radius bone just below the elbow.
Located on the anterior surface of the upper arm, the biceps brachii flexes the elbow joint and rotates the forearm so the palm of the hand turns face-up, an action known as supination. It also weakly assists in flexing the arm forward at the glenohumeral or shoulder joint. This minimal involvement at the shoulder, where other large muscles like the pectoralis major and anterior deltoid produce most of the forward-flexing movement, means that the force created by the biceps and transferred across the glenohumeral joint by the proximal biceps tendon is relatively weak. The proximal tendon is mostly responsible for attaching the upper portion of the biceps muscle to the shoulder blade.
As most of the action of the biceps brachii is performed at the elbow joint, the distal biceps tendon transfers the majority of the force created by the muscle. This tendon actually crosses two joints: the humeroulnar or elbow joint, and the proximal radioulnar joint, the upper of the two junctions between the parallel radius and ulna bones in the forearm. It is at the latter of the two where the biceps does the most work.
While the biceps brachii is a flexor of the elbow and therefore is involved in curling the elbow joint, it is primarily a supinator of the forearm, meaning that it rotates the palm of the hand face-up. When the biceps contracts, it pulls on the radius bone via the distal biceps tendon. This pulls the proximal end of the radius medially, or inward, which in turn rotates the top of the forearm.