Coracobrachialis: Origin, Insertion, Nerve, Action & The "Pierced" Muscle"

The Coracobrachialis is the smallest of the three muscles that attach to the coracoid process of the scapula. Located in the upper medial part of the arm, it is often called the "forgotten muscle" or the "pierced muscle" due to its unique relationship with the musculocutaneous nerve.

[Image of Coracobrachialis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Tip of the Coracoid Process of the Scapula (shared with the Short Head of Biceps).
Insertion (Distal)	Middle third of the medial surface of the Humerus.
Nerve Supply	Musculocutaneous Nerve (C5, C6, C7).
Blood Supply	Brachial Artery.
Primary Actions	Flexion: Flexes the arm at the shoulder joint. Adduction: Adducts the arm toward the trunk.

Deep Dive: The Perforated Muscle

The Coracobrachialis is anatomically distinct because it literally gets pierced by its own nerve supply.

1. The Nerve Piercing

The Musculocutaneous Nerve passes directly through the muscle belly of the Coracobrachialis before it continues down to supply the Biceps Brachii and Brachialis. This unique feature makes the muscle an important landmark in dissections and surgeries of the axilla.

[Image of Musculocutaneous nerve piercing coracobrachialis]

2. The "Coracoid Trio"

Three muscles attach to the Coracoid process:
1. Pectoralis Minor (Medial border)
2. Short Head of Biceps (Tip)
3. Coracobrachialis (Tip - conjoint tendon with Biceps)
This area acts as an anchor point for anterior shoulder stability.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Coracobrachialis acts as a "shunt muscle" when the arm is hanging at the side. It resists the downward pull of gravity, helping to prevent inferior dislocation of the shoulder joint.

Palpation

Abduct the arm to 90 degrees. Locate the Short Head of the Biceps in the anterior axilla. Move slightly medially and deeply towards the humerus. You can feel the Coracobrachialis as a distinct band, especially if you ask the patient to adduct horizontally against resistance. Be careful—the brachial artery and median nerve are very close!

⚠️ Clinical Pathology: Musculocutaneous Entrapment
Hypertrophy or spasm of the Coracobrachialis can compress the musculocutaneous nerve as it passes through the muscle. This can lead to weakness in elbow flexion (biceps) and sensory loss in the lateral forearm.

Manual Muscle Testing (MMT)

Testing this muscle requires isolating it from the Biceps and Anterior Deltoid.

Testing Tip: Flex the elbow fully to put the Biceps Brachii in active insufficiency (too short to work well), forcing the Coracobrachialis to work harder.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Sitting. Arm at side, elbow fully flexed. Action: Patient flexes the shoulder forward and slightly across the body (adduction). Resistance: Applied at the distal humerus (above elbow), pushing down and out (into extension/abduction). Grade 3: Full range against gravity. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Side-lying on the non-test side. Arm supported. Action: Patient moves arm into flexion and adduction. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate high in the medial arm, medial to the biceps tendon. Cue: "Try to lift your arm." Grade 1: Trace contraction felt. Grade 0: No activity.

Frequently Asked Questions

Why is it called "Coracobrachialis"?

The name tells you its attachments: It runs from the Coracoid process to the Brachialis (arm/humerus).

Does it cross the elbow joint?

No. It inserts on the middle of the humerus. Therefore, it has absolutely no action on the elbow joint.

Does it assist in rotation?

It acts as a weak internal rotator of the humerus because it pulls from the medial side, but this is a secondary function.

Test Your Knowledge: Coracobrachialis Quiz

1. Which nerve pierces the Coracobrachialis?

2. What is the primary insertion of the Coracobrachialis?

3. Which muscle shares a conjoint tendon of origin with Coracobrachialis?

4. What are the primary actions of the Coracobrachialis?

5. Why is the elbow flexed during MMT for Coracobrachialis?

6. The Coracobrachialis prevents which type of shoulder dislocation?

7. Entrapment of the nerve passing through Coracobrachialis causes weakness in:

8. Where is the origin of Coracobrachialis located relative to the glenoid?

9. Which artery runs adjacent to the Coracobrachialis?

10. True or False: The Coracobrachialis is a prime mover for shoulder abduction.

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.
• Kendall, F. P. (2005). Muscles: Testing and Function, with Posture and Pain. 5th ed. Lippincott Williams & Wilkins.