Brachialis: Origin, Insertion, Action, Nerve & The "Workhorse"

The Brachialis is the true powerhouse of elbow flexion. Often overshadowed by the more superficial Biceps Brachii, the Brachialis lies deep against the humerus. Unlike the biceps, it does not supinate; its only job is to bend the elbow, and it does so with immense force regardless of forearm position.

[Image of Brachialis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Distal half of the anterior surface of the Humerus.
Insertion (Distal)	Coronoid Process and Tuberosity of the Ulna.
Nerve Supply	Primary: Musculocutaneous Nerve (C5, C6). Secondary: Radial Nerve (C5, C7) - supplies a small lateral portion.
Blood Supply	Brachial Artery, Radial Recurrent Artery.
Primary Actions	Elbow Flexion (in all forearm positions: supinated, pronated, and neutral).

Deep Dive: The "Workhorse" of the Elbow

Physiologists often call the Brachialis the "workhorse" and the Biceps the "racehorse."

1. The Pure Flexor

Because the Brachialis inserts into the Ulna (which does not rotate during pronation/supination), its line of pull remains constant. Whether your hand is palm up, palm down, or neutral, the Brachialis pulls with equal strength.

2. Dual Innervation

While predominantly supplied by the Musculocutaneous nerve (like the Biceps), the lateral aspect of the Brachialis receives a branch from the Radial Nerve. This makes it a unique transition muscle between the anterior (flexor) and posterior (extensor) compartments.

Physio Corner: Clinical Relevance

💪 Functional Fact: When you pick up a grocery bag with your palms facing your body (pronated), you are relying almost entirely on your Brachialis (and Brachioradialis), as the Biceps is mechanically disadvantaged.

Palpation

The Brachialis lies deep to the Biceps. To feel it, have the patient flex their elbow gently. Push the Biceps muscle belly medially. Palpate in the space between the Biceps and the Triceps on the lateral side of the arm. The firm muscle belly felt there is the Brachialis.

⚠️ Clinical Pathology: Myositis Ossificans
Because the Brachialis lies directly against the humerus, severe blunt trauma (like a helmet hit in football) can cause bone tissue to form inside the muscle (Myositis Ossificans), severely restricting elbow motion.

Manual Muscle Testing (MMT)

To isolate the Brachialis, we must disadvantage the Biceps Brachii.

Testing Tip: Pronating the forearm puts the Biceps tendon in a twisted, slack position (active insufficiency), forcing the Brachialis to do the heavy lifting.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Sitting or Standing. Arm at side, forearm Pronated (Palm down). Action: Patient flexes the elbow (lifts hand toward shoulder). Resistance: Applied at the distal forearm (wrist), pushing down. Grade 3: Full range against gravity. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Sitting with arm supported on a table at 90° abduction, forearm pronated. Action: Patient slides the hand toward the shoulder. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate lateral to the biceps tendon in the cubital fossa or deep to the biceps distally. Cue: "Try to bend your elbow." Grade 1: Trace contraction felt. Grade 0: No activity.

Frequently Asked Questions

Does Brachialis participate in supination?

No. It attaches to the Ulna. The Ulna is the stationary bone around which the Radius rotates. Therefore, Brachialis has zero rotary component.

Why is it called the "Workhorse"?

Because it is the primary flexor in all positions (fast, slow, heavy, light, pronated, supinated). It is always working, unlike the Biceps which prefers supination.

What is the "Dual" nerve supply?

Most of the muscle is Musculocutaneous (C5, C6). However, a small inferolateral part is supplied by the Radial Nerve (C7), which is a remnant of embryonic development.

Test Your Knowledge: Brachialis Quiz

1. What is the insertion of the Brachialis?

2. Which position best isolates the Brachialis?

3. What is the primary nerve supply of the Brachialis?

4. Why does the Brachialis NOT supinate?

5. Which nickname is often given to the Brachialis?

6. Where does the Brachialis originate?

7. The lateral portion of the Brachialis often receives innervation from:

8. Myositis Ossificans is a risk for the Brachialis because:

9. The Brachialis acts primarily on which joint?

10. True or False: The Brachialis is deep to the Biceps Brachii.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Magee, D. J. (2014). Orthopedic Physical Assessment. 6th ed. Elsevier.