The biceps brachii is a prominent muscle located in the anterior compartment of the upper arm, situated between the shoulder and the elbow. Its name, which translates to “two-headed muscle of the arm,” accurately describes its structure, which consists of two distinct heads of origin. The overall muscle belly is spindle-shaped, and despite its size, the muscle does not directly attach to the humerus, the long bone of the upper arm.
The Origin Point of the Short Head
The short head of the biceps brachii muscle originates at the coracoid process of the scapula. This point of attachment is located on the medial side, closer to the body’s midline, compared to the long head. The coracoid process is a small, hook-like bony projection extending from the top, front part of the scapula.
This specific origin is shared with the coracobrachialis, and their combined proximal ends are sometimes referred to as the conjoined tendon. However, the short head’s attachment is often described as muscle fibers attaching directly to the bone, covered by a thin aponeurosis, rather than a distinct, true tendon. From this point, the short head descends, passing in front of the humerus, forming a broader and more robust muscle belly than the long head.
The short head’s attachment to the coracoid process gives it a minor influence on movement at the shoulder joint. By crossing the joint, it can contribute weakly to moving the arm forward and across the body. This proximal attachment also plays a role in stabilizing the shoulder joint, especially when the arm is extended and carrying a heavy load.
The Long Head and Muscle Path
In contrast to the short head, the long head of the biceps brachii originates at the supraglenoid tubercle of the scapula. This is a small bump located just above the glenoid cavity, the socket of the shoulder joint. The tendon for the long head is notably more complex, as it remains tendinous and passes directly through the shoulder joint capsule.
The long head’s tendon then travels down the arm through the bicipital groove of the humerus, which is a deep furrow between two bony prominences. Both the short and long heads merge together to form the large, common muscle belly in the middle of the upper arm. This unified muscle then tapers down to a single, strong tendon that crosses the elbow joint.
The final insertion point for the combined muscle is the radial tuberosity, a bony projection on the radius, one of the two forearm bones. A second, fan-shaped extension of the tendon, called the bicipital aponeurosis, also extends from the distal biceps tendon to blend into the fascia of the forearm. This dual attachment is crucial for transmitting force across the elbow joint.
Key Functions of the Biceps Brachii
The primary actions of the biceps brachii muscle occur at the elbow and forearm. The muscle is particularly powerful in supination of the forearm, the movement that turns the palm upward. This turning action is effective when the elbow is already bent, due to the muscle’s angle of attachment to the radial tuberosity.
The second major function is the flexion of the forearm, or bending the elbow. Although the biceps is often considered the main muscle for this action, the brachialis is actually the stronger elbow flexor. The biceps works alongside the brachialis and the brachioradialis to complete the bending motion. Because both heads originate on the scapula, the muscle also assists weakly in shoulder flexion, which involves lifting the arm forward and upward.