Brachialis Muscle: Anatomy, Function, Relations, and Imaging

Topic

Brachialis muscle

The brachialis muscle is a strong, flat muscle located in the anterior compartment of the arm, deep to the biceps brachii. It forms the main flexor of the elbow joint and provides essential strength and stability during forearm flexion, especially when the forearm is pronated. It bridges the humerus and ulna, playing a crucial role in both static and dynamic elbow movements.

Anatomically, the brachialis serves as a transition muscle between the arm and forearm, forming the floor of the cubital fossa and lying directly over the brachial artery and median nerve. It is a vital landmark in upper limb surgery and imaging.

Synonyms

Deep flexor of the arm
Humeroulnar flexor muscle

Origin, Course, and Insertion

Origin: Distal half of the anterior surface of the humerus and adjacent intermuscular septa.
Course: Muscle fibers converge into a thick tendon that descends anterior to the elbow joint, deep to the biceps brachii tendon.
Insertion:
- Main insertion: Coronoid process and tuberosity of the ulna.
- Accessory slips: Occasionally to the joint capsule of the elbow, reinforcing it anteriorly.

Relations

Anteriorly: Biceps brachii and overlying skin and fascia.
Posteriorly: Brachial artery, median nerve, and elbow joint capsule.
Laterally: Brachioradialis and extensor carpi radialis longus.
Medially: Pronator teres and neurovascular bundle of the arm.

Nerve Supply

Primary: Musculocutaneous nerve (C5–C6).
Accessory (lateral part): Radial nerve (C7) — supplies a small lateral portion of the muscle.

Arterial Supply

Brachial artery — muscular branches supplying the main belly.
Radial recurrent artery — contributes to the distal portion near the elbow.
Superior ulnar collateral artery — occasional minor contribution.

Venous Drainage

Brachial veins — primary drainage accompanying the brachial artery.
Basilic vein — superficial drainage through communicating veins.
Radial and ulnar recurrent veins — assist drainage around the elbow.

Function

Primary elbow flexor: Flexes the forearm at the elbow joint in all positions (pronation/supination).
Stability: Maintains anterior stability of the elbow during motion.
Static support: Acts as an antigravity stabilizer during load-bearing in semiflexed positions.
Synergy: Works with biceps brachii for smooth flexion and deceleration of forearm movements.

Clinical Significance

Tendinopathy or strain: Overuse in lifting or repetitive flexion can cause anterior elbow pain.
Tear or rupture: May mimic distal biceps tendon injury on imaging or physical exam.
Entrapment: Close proximity to brachial artery and median nerve — important during surgical approaches.
Myositis ossificans: Can develop after trauma to the anterior arm.
Imaging relevance: Distinguishing brachialis from biceps pathology is crucial in evaluating elbow pain and trauma.

MRI Appearance

T1-weighted images:
- Muscle belly: Intermediate signal intensity with clear fascicular pattern.
- Tendon: Low signal (dark linear band) at ulnar insertion.
- Fat planes: Bright, delineating muscle borders.
- Pathology: Muscle tears show focal intermediate-to-bright signal with contour disruption.
T2-weighted images:
- Normal muscle: Intermediate-to-dark signal, slightly darker than on T1.
- Tendon: Low signal throughout.
- Pathology: Strain, edema, or partial tears show localized bright hyperintensity in the muscle belly or myotendinous junction.
- Hematoma: Hyperintense with surrounding low-signal rim if subacute.
STIR:
- Normal muscle: Intermediate-to-dark signal.
- Injury or inflammation: Bright hyperintense regions within muscle fibers.
- Excellent for detecting early edema or myositis.
Proton Density Fat-Saturated (PD FS):
- Normal muscle: Intermediate-to-dark homogeneous signal.
- Pathologic: Bright foci indicating fluid, strain, or interstitial injury.
- Sensitive for subtle edema and small partial tears.
T1 Fat-Sat Post-Contrast:
- Normal muscle: Uniform mild enhancement.
- Inflamed or healing tissue: Moderate enhancement at the myotendinous junction.
- Chronic fibrosis: Low-signal areas with peripheral enhancement.

CT Appearance

Non-Contrast CT:

Muscle: Uniform soft-tissue density with smooth margins anterior to the humerus.
Tendon: Linear dense structure inserting into the coronoid process.
Pathology: Detects calcification, myositis ossificans, or post-traumatic changes.
Hematoma: Appears as localized soft-tissue density or fluid collection within the muscle.

Post-Contrast CT (standard):

Normal muscle: Homogeneous enhancement following contrast.
Inflammation or strain: Focal increased enhancement or thickening near insertion.
Chronic fibrosis or ossification: May show heterogeneous density or peripheral enhancement.

MRI images