Biceps Brachii Muscle: Anatomy, Function, Imaging

Topic

Biceps brachii muscle

The biceps brachii is a prominent, two-headed muscle of the anterior compartment of the arm. It spans from the scapula to the radius, crossing both the shoulder and elbow joints, and functions primarily as a forearm supinator and elbow flexor. The muscle also assists in shoulder stabilization and flexion.

It consists of two distinct heads — long head and short head — which unite midway down the arm to form a common belly and tendon that inserts into the radial tuberosity. A fibrous expansion, the bicipital aponeurosis, blends medially with the deep fascia of the forearm.

Synonyms

Biceps muscle of arm
Two-headed arm flexor
Biceps brachii longus et brevis

Origin, Course, and Insertion

Origin:
- Long head: Supraglenoid tubercle of the scapula, tendon passing through the shoulder joint capsule and bicipital (intertubercular) groove.
- Short head: Apex of the coracoid process of the scapula, along with the coracobrachialis.
Course:
- Both heads descend anteriorly in the arm, merging into a common belly in the mid-arm.
- The tendon passes anterior to the elbow joint and inserts on the radius.
Insertion:
- Radial tuberosity of the radius via a strong rounded tendon.
- Bicipital aponeurosis fans medially to merge with the deep fascia of the forearm.

Tendon Attachments

Proximal tendon: Long head runs intra-articularly, stabilized by the transverse humeral ligament in the intertubercular groove.
Distal tendon: Inserts into the radial tuberosity and gives off the bicipital aponeurosis.
Provides mechanical leverage for supination and elbow flexion.

Relations

Anteriorly: Skin, superficial fascia, cephalic vein.
Posteriorly: Brachialis muscle and anterior surface of the humerus.
Medially: Median nerve and brachial artery.
Laterally: Deltoid insertion and lateral cutaneous nerve of forearm.
Superiorly: Pectoralis major and deltoid muscles.
Inferiorly: Cubital fossa structures (brachial artery bifurcation, median nerve, radial nerve branches).

Nerve Supply

Musculocutaneous nerve (C5–C6), a branch of the lateral cord of the brachial plexus.

Arterial Supply

Brachial artery (primary supply).
Collateral contributions from the anterior circumflex humeral and profunda brachii arteries.

Venous Drainage

Drains via brachial veins and cephalic vein, which empty into the axillary vein.

Function

Elbow flexion: Primary flexor when forearm is supinated.
Forearm supination: Most powerful supinator, especially with elbow flexed.
Shoulder stabilization: Long head contributes to glenohumeral stability.
Shoulder flexion: Assists with anterior arm elevation.

Clinical Significance

Tendon rupture:
- Proximal long head tear: Common in elderly and athletes, presenting with “Popeye” deformity.
- Distal tendon rupture: Less common; causes weakness in supination and flexion.
Tendinitis/Tendinosis: Chronic inflammation or degeneration at the long head tendon within the bicipital groove.
Tenosynovitis: Fluid and synovial thickening around long head tendon sheath.
Bicipital aponeurosis injury: May cause forearm pain or compression neuropathy.
Imaging importance: MRI best visualizes tendon tears, muscle edema, and partial detachments. CT evaluates calcification or avulsion fractures.

MRI Appearance

T1-weighted images:
- Muscle belly: Intermediate signal intensity with clear fascicular pattern.
- Tendons: Low signal (dark) along both long and short head tendinous portions.
- Marrow of humerus: Bright fatty signal.
- Pathology: Ruptures show discontinuity or retraction of the tendon; hematoma appears intermediate to bright.
T2-weighted images:
- Normal muscle: Intermediate-to-low signal, slightly darker than on T1.
- Tendons: Low signal unless fluid surrounds or infiltrates (bright hyperintense rim).
- Inflammation, edema, or strain: Bright hyperintense signal in muscle belly or tendon sheath.
- Tendinitis: Increased signal within the long head tendon at the bicipital groove.
STIR:
- Normal muscle: Intermediate-to-dark signal.
- Pathology: Bright hyperintense regions indicate edema, strain, or partial tear.
- Highlights fluid in the bicipital groove and peritendinous inflammation.
Proton Density Fat-Saturated (PD FS):
- Normal: Intermediate-to-dark signal muscle with sharp margins.
- Pathologic: Bright hyperintensity within tendon or musculotendinous junction in tendinitis or partial tear.
- Excellent for detecting subtle longitudinal tendon splits or synovial fluid tracking.
T1 Fat-Sat Post-Contrast:
- Normal muscle: Mild homogeneous enhancement.
- Tendinitis or synovitis: Focal or diffuse tendon sheath enhancement.
- Tear or hematoma: Peripheral enhancement with central low signal intensity.

CT Appearance

Non-Contrast CT:

Muscle: Soft-tissue density with smooth contours.
Tendon: Linear soft-tissue structure inserting on radial tuberosity.
Bicipital groove: Clearly visualized; may show cortical irregularity in tendinitis.
Pathology: Detects calcific tendinitis, avulsion fractures, and retracted tendon stump in rupture.

Post-Contrast CT (standard):

Muscle: Homogeneous enhancement.
Inflamed tendon sheath or aponeurosis: Enhanced outlining or thickening.
Utility: Assesses soft-tissue masses, vascular lesions, and neoplastic or inflammatory infiltration around the biceps.

MRI image