The biceps brachii is located on the front of the upper arm, spanning the distance between the shoulder and the elbow joint. Its name, meaning “two-headed muscle of the arm,” refers to its structure, composed of two distinct bundles: the long head and the short head. The biceps is situated in the anterior compartment of the upper arm, alongside the brachialis and the coracobrachialis. Both heads originate high on the shoulder blade (scapula) and merge to form a single muscle belly that attaches to the forearm, allowing for movement at both the shoulder and the elbow.
Precise Origin and Insertion Points
The location of the short head of the biceps is defined by its origin point on the scapula, which is the coracoid process. This coracoid process is a small, hook-like bony projection extending anteriorly and laterally from the upper part of the scapula. The short head’s tendon arises from the tip of this projection, often alongside the coracobrachialis muscle, sometimes referred to as a conjoined tendon. Anatomical studies suggest that the short head may attach directly to the coracoid process via muscle fibers and an aponeurosis, rather than a distinct tendon, which is a structural difference from the long head.
The two muscle heads converge in the middle of the upper arm to form a shared muscle belly. This single, thick muscle then extends downward to cross the elbow joint, where it terminates at its insertion point. The primary distal attachment site for the entire biceps muscle is the radial tuberosity, which is a rough, bony prominence located just beneath the neck of the radius bone in the forearm.
An additional, broad sheet of connective tissue, called the bicipital aponeurosis, extends from the biceps tendon near the elbow. This aponeurosis radiates over the inner part of the forearm, attaching to the fascia that surrounds the deeper muscles. This dual attachment to both the radial tuberosity and the forearm fascia ensures that the muscle can effectively transmit force for its primary actions at the elbow and forearm.
Structural Differences Between the Short and Long Heads
The short head of the biceps is positioned medially, or toward the midline of the body, relative to the long head. This positioning gives the short head a straighter, more direct path down the arm, and it runs adjacent to the coracobrachialis muscle. The short head has a simple attachment, originating directly from the tip of the coracoid process.
The long head, by contrast, has a complex path, originating from the supraglenoid tubercle, a small bump located just above the socket of the shoulder joint. The tendon of the long head must travel through the shoulder joint capsule and then descend through a vertical channel in the humerus bone called the intertubercular groove. This trajectory means the long head crosses two joints, the shoulder and the elbow, giving it a more diverse range of actions.
Although the two heads arise from distinct locations, they eventually merge to form a single muscle mass in the central third of the upper arm. The short head is often associated with the width and thickness of the biceps, while the long head contributes more to the height of the muscle’s peak.
Functional Contribution to Arm Movement
The primary roles of the short head are integrated into the overall function of the biceps muscle, which is a powerful supinator and a strong flexor of the forearm. Forearm supination is the rotational movement that turns the palm upward, an action where the biceps is most effective when the elbow is already partially bent. This is facilitated by the muscle’s insertion onto the radius, which allows it to rotate the bone when it contracts.
The short head also contributes significantly to elbow flexion, which is the action of bending the arm. While the deeper brachialis muscle is considered the strongest elbow flexor, the biceps assists this motion, particularly when the forearm is supinated. The short head’s medial position means it may be preferentially engaged in certain movements.
A unique contribution of the short head is its function at the shoulder joint, which is the site of its origin. Due to its attachment on the coracoid process, the short head assists in stabilizing the shoulder joint. Furthermore, the short head can assist in horizontal adduction, which is the movement of bringing the arm across the front of the body.