The Anatomy of the Biceps Brachii
The biceps brachii is a prominent muscle located on the front of the upper arm, known for its distinctive two heads. The short head originates from the coracoid process of the scapula, while the long head has a more complex path. The long head originates from the supraglenoid tubercle, situated just above the glenoid cavity of the shoulder blade. From there, the long head's tendon travels through the shoulder joint and down the bicipital groove of the humerus, where it joins the short head to form the single muscle belly. Distally, the combined muscle inserts onto the radial tuberosity of the forearm. This unique anatomy, particularly the long head's path through the shoulder joint, is key to its multiple functions.
Biceps Brachii: Key Anatomical Characteristics
- Two-Headed Structure: The biceps muscle is split into a longer, lateral head and a shorter, medial head at its origin on the scapula.
- Multi-Joint Function: As a biarticular muscle, the biceps spans both the glenohumeral (shoulder) joint and the humeroulnar (elbow) joint.
- Insertion Point: Both heads converge to insert on the radius bone in the forearm, with an expansion called the bicipital aponeurosis.
Actions of the Long Head of the Biceps Brachii
While the biceps brachii works as a single unit for certain actions, the distinct starting points of its two heads give them slightly different roles, especially at the shoulder. Understanding these nuances is key to knowing what is the main action of the long head of the biceps brachii.
Primary Action: Elbow Flexion and Forearm Supination
At the elbow, the biceps is a potent flexor of the forearm, bending the arm at the elbow joint. It is also a powerful supinator, rotating the forearm to turn the palm upwards. This supination action is most effective when the elbow is already flexed. Research suggests a distinction in efficiency: the short head is relatively more efficient for elbow flexion at 90 degrees, whereas the long head becomes relatively more efficient for supination in a supinated forearm position.
Secondary Action: Shoulder Joint Contribution
Because the long head crosses the shoulder joint, it also assists in movements at that articulation. The long head plays a role in:
- Shoulder Flexion: Weakly assisting in lifting the arm forward and upward.
- Shoulder Abduction: Contributing to moving the arm out to the side, particularly when the arm is externally rotated.
Crucial Role: Humeral Head Stabilization
Perhaps one of the most critical, yet often overlooked, functions of the long head is its stabilizing effect on the shoulder joint. Its tendon runs directly through the glenohumeral joint, and its tension helps to press the head of the humerus into the glenoid cavity, preventing superior or posterior displacement. This provides essential stability, especially during powerful movements or when carrying heavy weights. In fact, unloading the long head's tendon has been shown to increase posterior translation of the humeral head, affirming its stabilizing role. This dynamic stabilization works alongside the rotator cuff to maintain proper joint mechanics.
Comparison of Long Head vs. Short Head Actions
The two heads of the biceps, while working together for many functions, have differing strengths due to their distinct origins. The following table summarizes their comparative actions.
| Function | Long Head of Biceps Brachii | Short Head of Biceps Brachii |
|---|---|---|
| Elbow Flexion | Assists, especially in supinated positions. | Primary elbow flexor, very effective at 90° flexion. |
| Forearm Supination | Most effective during supination, especially in a flexed forearm. | Assists with supination. |
| Shoulder Flexion | Assists in raising the arm forward (weak flexor). | Assists in raising the arm forward (weak flexor). |
| Shoulder Abduction | Assists in moving the arm away from the trunk. | Minimal contribution. |
| Shoulder Adduction | Moves the arm away from the trunk (abduction). | Pulls the arm back toward the trunk (adduction). |
| Shoulder Stabilization | Plays a crucial role in stabilizing the humeral head within the glenoid cavity. | Assists with stabilization, especially when carrying weights. |
Conclusion
The most prominent action of the long head of the biceps brachii is its powerful contribution to elbow flexion and, most notably, forearm supination, especially when the forearm is already bent. However, to label its function solely by its visible role in arm curls is to miss its crucial secondary, yet highly important, function: acting as a dynamic stabilizer for the shoulder joint. Its unique path through the shoulder joint capsule allows it to contribute to humeral head stability, preventing unwanted movement and working in synergy with the rotator cuff. This dual role underscores its importance not just for strength, but for overall upper limb function and stability. This comprehensive understanding is essential for physical therapists, athletes, and anyone interested in maximizing their arm and shoulder health.
An extensive review of the anatomy and function can be found through resources like the Kenhub Biceps brachii muscle overview.
