Brachialis

Origin, Insertion, Action, Innervation

Origin: Distal half of the anterior surface of the humerus, medial and lateral intermuscular septa
Insertion: Coronoid process and ulnar tuberosity of the ulna
Action:
Primary: Flexion of the elbow (the only action — pure elbow flexor)
Active in all forearm positions (supinated, neutral, pronated) because it attaches to the ulna, which does not rotate
Innervation: Musculocutaneous nerve (C5–C6); lateral portion may receive a small branch from the radial nerve (C7)

Palpation Guide

Client position: Supine or seated with the elbow flexed to approximately 90 degrees and the forearm in neutral or pronated position (pronation reduces biceps activity, making brachialis more prominent).
Landmark sequence:

Locate the lateral border of the biceps brachii in the distal third of the arm. Press laterally and posteriorly past the biceps edge — you are now on the brachialis.
The brachialis is palpable as a broad, flat muscle deep to the lateral edge of biceps, filling the space between the biceps and the lateral intermuscular septum.
It can also be accessed medially by pressing deep to the medial edge of the biceps, though neurovascular structures (brachial artery, median nerve) make this approach more cautious.
Trace distally — the brachialis inserts on the coronoid process of the ulna, palpable just distal to the elbow crease on the anterior ulna.

Tissue feel: Feels flat and broad compared to the rounded biceps belly. When the client flexes the elbow with the forearm pronated, brachialis firms up beneath and lateral to the biceps. It has a "dense, meaty" quality distinct from the biceps.
Confirmation test: Ask the client to flex the elbow against resistance with the forearm fully pronated. Pronation minimizes biceps contribution (biceps is a supinator), so brachialis becomes the dominant flexor. You will feel it contract deep to the lateral biceps edge.
Common errors:
Palpating biceps brachii instead — students often remain on the superficial biceps belly. You must deliberately press past the lateral or medial border of biceps to access brachialis.
Pressing too aggressively on the medial approach — the brachial artery and median nerve lie between the biceps and brachialis on the medial side.
Confusing brachialis with brachioradialis — brachioradialis originates from the lateral supracondylar ridge and crosses the elbow anterolaterally; brachialis lies more posterior and inserts on the ulna (not the radius).

Trigger Point Referral

Common TrP locations: The primary TrP is located in the mid-belly of the muscle, accessible from the lateral edge of the biceps in the distal half of the arm.
Referral pattern: Refers to the base of the thumb (dorsal and palmar surfaces of the thenar eminence) and sometimes into the anterior elbow crease.
Clinical significance: The referral to the thumb base mimics de Quervain's tenosynovitis or CMC joint arthritis — if thumb pain does not correlate with positive Finkelstein's or CMC grind tests, check brachialis TrPs.

Trigger point referral diagram — coming soon

Image coming soon. For visual reference, see [Brachialis at TriggerPoints.net](http://www.triggerpoints.net/muscle/brachialis).

Clinical Notes

Innervation significance:

Brachialis shares the musculocutaneous nerve (C5–C6) with biceps and coracobrachialis. A lateral portion may receive radial nerve input, which explains why some elbow flexion is preserved even with complete musculocutaneous nerve injury.

Common conditions:

Chronic elbow flexion contracture — brachialis is the primary contributor to fixed flexion deformity at the elbow. Because it crosses only one joint and attaches to the non-rotating ulna, it shortens efficiently with prolonged flexion postures (desk work, carrying, sleeping with flexed elbows).
Relevant to conditions/lateral-epicondylitis rehabilitation — brachialis weakness or TrP involvement can alter elbow mechanics and loading patterns at the lateral epicondyle.
Myositis ossificans — brachialis is the most common site of myositis ossificans in the upper extremity, typically following direct trauma to the anterior arm (e.g., a blow to the distal arm). If post-traumatic anterior arm swelling does not resolve normally or becomes progressively harder on palpation, refer for imaging.

What you'll typically find:

In clients with chronic elbow flexion (desk workers, manual laborers who carry loads), brachialis is almost always shortened and tender. Because it hides beneath biceps, students rarely assess it directly. Palpation from the lateral approach typically reveals dense, fibrotic tissue with poor pliability.
Clients are often surprised by the intensity of tenderness when brachialis is accessed — they did not know the muscle was there, let alone hypertonic.

Treatment effects:

Responds well to sustained compression and cross-fiber techniques accessed from the lateral biceps border. Work slowly — the muscle is deep and requires gradual pressure to reach effectively.
Post-treatment, elbow extension ROM often improves immediately. If a client has been unable to fully straighten the elbow, releasing brachialis (not just biceps) is often the key to restoring the last 10–15 degrees of extension.

Cautions:

The brachial artery and median nerve run between biceps and brachialis on the medial side of the arm. When accessing brachialis from the medial approach, use light pressure and monitor for arterial pulsation under your fingers.
Post-traumatic anterior arm swelling should be treated with caution — aggressive massage to a brachialis with early myositis ossificans can worsen the condition. If the anterior arm is hard and swollen after trauma, do not apply deep pressure until imaging rules out heterotopic bone formation.

Clinical pearl:

When a client cannot achieve full elbow extension despite biceps stretching and treatment, brachialis is almost always the missing piece. Biceps crosses two joints and can be stretched effectively with shoulder extension. Brachialis crosses only the elbow — it requires specific elbow extension stretching and direct manual release from the lateral approach to restore full extension.

Assessment

Manual muscle testing:

Elbow flexion in pronation: Client seated with forearm fully pronated. Apply resistance to the distal forearm into extension. Pronation minimizes biceps contribution, making this a more specific test for brachialis.

Stretch test:

Full elbow extension: Client supine or seated. Passively extend the elbow fully. Loss of terminal extension (the last 5–15 degrees) with a firm end-feel suggests brachialis shortening. Compare bilaterally.

Related special orthopedic tests:

Biceps reflex (C5–C6) — while primarily testing biceps, also assesses the musculocutaneous nerve that innervates brachialis

Muscle Groups

Elbow flexors (functional):

anatomy/muscles/biceps-brachii
Brachialis (this article)
anatomy/muscles/brachioradialis
anatomy/muscles/pronator-teres (weak flexor)

Musculocutaneous nerve group (innervation):

Related Muscles

Synergists for elbow flexion:

anatomy/muscles/biceps-brachii — strongest when forearm is supinated; also supinates
anatomy/muscles/brachioradialis — most active from neutral forearm position

Antagonist:

anatomy/muscles/triceps-brachii — sole elbow extensor

Same innervation (musculocutaneous nerve):

anatomy/muscles/biceps-brachii — superficial two-joint flexor-supinator
anatomy/muscles/coracobrachialis — shoulder flexor and adductor; shares the coracoid process attachment area

Key Takeaways

Brachialis is the strongest elbow flexor and the primary cause of elbow flexion contracture — always assess it directly when terminal elbow extension is restricted.
Access from the lateral border of biceps with the forearm pronated — pronation quiets biceps and exposes brachialis for palpation and treatment.
The TrP referral to the thumb base mimics de Quervain's or CMC joint pathology — check brachialis when thumb pain tests are negative.