The biceps brachii is a prominent muscle located on the front of the upper arm. The name itself, meaning “two-headed muscle of the arm,” describes its structure, which begins near the shoulder and extends down toward the elbow. This muscle’s primary function is to move the forearm, connecting the shoulder girdle to the forearm bones. Its attachments allow it to work across both the shoulder and elbow joints, giving it a unique role in upper limb mobility.
The Anatomy of the Biceps Brachii Origin
The biceps brachii begins high up near the shoulder joint, a structure known as the scapula. The muscle is characterized by its two separate points of attachment, referred to as heads. The long head originates from the supraglenoid tubercle, a bony bump situated above the socket of the shoulder joint.
The short head arises from the coracoid process, a hook-like projection on the scapula. These two separate tendons run down the upper arm before their muscle bellies merge into a single mass. This mass then tapers down to form a single, thick tendon as it approaches the elbow joint.
Pinpointing the Insertion Site
The biceps brachii tendon attaches to the forearm bones at a two-part insertion site. The main insertion is on the radial tuberosity, a bony prominence on the radius, the forearm bone on the thumb side. This point is situated just below the neck of the radius bone, deep within the elbow joint.
The tendon wraps slightly around the radius before attaching, a detail that influences the muscle’s mechanical action. This primary attachment allows the biceps to pull directly on the radius bone during contraction.
The second, broader insertion is the bicipital aponeurosis (lacertus fibrosus), a flat, sheet-like extension of the main tendon. This aponeurosis extends obliquely and blends into the deep fascia of the forearm on the medial side. It plays a role in distributing force across the forearm muscles and acts as a protective roof over the median nerve and brachial artery.
The dual insertion points ensure that the force generated by the contracting muscle is widely anchored across both the bone and the surrounding soft tissue.
How the Insertion Drives Movement
The biceps insertion on the radial tuberosity dictates the muscle’s primary functions. When the muscle contracts, the main tendon pulls on the radius, resulting in two distinct movements at the elbow joint. The first action is forearm flexion, which is the bending of the elbow.
The second and often more powerful action is supination, the rotation of the forearm so the palm faces upward or forward. The biceps is a powerful supinator because its tendon attaches to the radius opposite the direction of pronation. When the muscle shortens, the pulling action on the radial tuberosity causes the radius to spin around the ulna, turning the hand over.
This mechanical advantage for supination is most pronounced when the elbow is partially bent. The insertion site converts the muscle’s contraction into a powerful twisting force. This specialization allows the biceps brachii to perform tasks requiring both lifting and turning, such as using a screwdriver.