The biceps brachii muscle is one of the most recognizable muscles in the human body, prominently situated on the front of the upper arm. Despite its common name, this muscle is a two-headed structure, reflected in its Latin name meaning “two-headed muscle of the arm.” These two distinct bundles of muscle are known as the long head and the short head. While both heads work together to perform the main actions of the arm, the short head has unique anatomical features that give it specialized functions. This dual-headed design allows the muscle to exert force and control across both the shoulder and elbow joints.
Anatomical Placement of the Short Head
The short head of the biceps brachii, also called the caput breve, has a distinct starting point on the shoulder blade, or scapula. It originates from the apex of the coracoid process, which is a small, hook-like bony projection on the front of the shoulder blade. This origin point places the short head on the medial side, or closer to the body’s midline, compared to its counterpart.
In contrast, the long head originates from a different point on the scapula, traveling through the shoulder joint itself. Both the short and long heads then merge to form the single, large muscle belly visible in the upper arm. A common tendon forms from this combined muscle and inserts primarily onto the radial tuberosity, a bony bump on the radius bone of the forearm.
Primary Functions of the Biceps Brachii
The biceps muscle complex as a whole is primarily responsible for two powerful movements of the forearm. Its most commonly known action is elbow flexion, which involves bending the arm to bring the forearm closer to the upper arm. This movement is also significantly assisted by the deeper brachialis muscle, which is considered the forearm’s most powerful flexor.
The second major function is forearm supination, the action of rotating the forearm so the palm faces upward. The biceps is a particularly effective supinator, especially when the elbow is already bent. This turning action is made powerful by the muscle’s attachment point on the radius, allowing it to efficiently rotate the bone. Both the short and long heads contribute to these two main forearm movements.
Unique Contribution of the Short Head
The short head’s attachment to the coracoid process dictates its unique mechanical advantages and contributions to shoulder movement. Because it crosses the shoulder joint, it assists with arm flexion, which is the action of lifting the entire arm forward. It also aids in arm adduction, the movement of pulling the arm toward the center of the body.
The short head acts as an important stabilizer for the shoulder joint. It helps to keep the head of the humerus, or the ball of the shoulder joint, secure in the socket, especially when lifting or carrying heavy objects with the arm extended. Furthermore, the short head’s medial location provides a protective layer. It shields the underlying neurovascular structures, such as the median nerve and the brachial artery, as they pass through the upper arm.