The biceps brachii is the most recognized muscle of the upper arm, located on the front side of the arm. It performs important actions at both the shoulder and elbow joints. Understanding its anatomy requires knowledge of where the muscle anchors to the skeleton at its beginning (origin) and its end (insertion). This article focuses on the distal attachment point, or the insertion, of the biceps brachii muscle.
Anatomy of the Biceps Brachii
The name “biceps brachii” means “two-headed muscle of the arm,” referring to its structure composed of a long head and a short head. Both heads originate high on the shoulder blade before merging into a single muscle belly in the middle of the upper arm. This muscle is the most superficial of the three muscles situated in the anterior compartment of the arm.
The long head and the short head travel downward until they converge into a flattened, common tendon just above the elbow joint. The muscle belly runs along the humerus but has no direct connection to the bone itself. The common tendon then crosses the elbow joint to connect the muscle to the forearm bones.
The Distal Attachment: Specifics of the Insertion
The terminal attachment point of the biceps brachii splits to anchor the muscle at two distinct locations on the forearm. The primary and strongest anchor point is the radial tuberosity, a roughened prominence found on the radius bone just below the elbow joint. This direct tendon attachment is the main transmission point for the force generated by the muscle’s contraction.
The tendon’s footprint on the radial tuberosity is ribbon-shaped and located on the posterior margin of this bony landmark. A small bursa, a fluid-filled sac, is situated between the tendon and the bone to reduce friction, especially during the rotational movements of the forearm. The orientation of this insertion is mechanically advantageous for the powerful twisting motion of the forearm.
The secondary insertion is a broad, thin sheet of connective tissue called the bicipital aponeurosis, also known as the lacertus fibrosus. This aponeurosis arises from the medial side of the distal biceps tendon before it dives toward the radial tuberosity. It extends obliquely across the front of the elbow, or cubital fossa, blending into the deep fascia that covers the flexor muscles of the forearm.
This secondary attachment reinforces the muscle’s anchoring and distributes the tensile forces generated during elbow flexion and forearm rotation across a wider area. Furthermore, the bicipital aponeurosis acts as a protective shield for sensitive neurovascular structures that pass underneath it. It covers the median nerve and the brachial artery, helping to offload some of the strain that would otherwise be concentrated solely on the radial tuberosity.
Contextualizing the Proximal Attachments (The Origins)
Before reaching its distal insertion point, the biceps brachii muscle begins with its two proximal attachments, or origins, on the scapula (shoulder blade). The long head originates from the supraglenoid tubercle, a bony projection located just above the socket of the shoulder joint. The tendon of the long head passes through the shoulder joint capsule before descending down the upper arm.
The short head has a simpler, more medial origin, attaching to the coracoid process of the scapula. This process is a hook-like structure that points forward and laterally, and the short head tendon runs alongside the coracobrachialis muscle. The distinct origins of the two heads allow the biceps to influence both the elbow and the shoulder joints, making it a bi-articular muscle.
Functional Role of the Biceps Brachii
The placement of the biceps brachii’s insertion point on the radius dictates its powerful primary functions at the elbow joint. The first action is elbow flexion, the motion of bending the arm, which the biceps performs alongside other muscles like the brachialis and brachioradialis. It plays a greater role in this action when the forearm is already in a supinated position (palm facing up).
The second function is forearm supination, the rotational movement that turns the palm of the hand upward. Because the biceps tendon inserts on the radius, the muscle pulls on the tuberosity to effectively twist the forearm. This supination action is strongest when the elbow is slightly bent. The biceps also contributes a minor role at the shoulder joint, assisting in the flexion of the arm forward and providing dynamic stability.