Extensor Carpi Radialis Brevis: Origin, Insertion, Nerve & Tennis Elbow

The Extensor Carpi Radialis Brevis (ECRB) is a central muscle in the posterior forearm. Though shorter ("Brevis") than its neighbor the Longus, it is centrally located and is the primary muscle implicated in Lateral Epicondylitis (Tennis Elbow).

[Image of Extensor Carpi Radialis Brevis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Lateral Epicondyle of the Humerus (via the Common Extensor Tendon).
Insertion (Distal)	Dorsal surface of the base of the 3rd Metacarpal bone.
Nerve Supply	Deep Branch of Radial Nerve (C7, C8).
Blood Supply	Radial Artery and Radial Recurrent Artery.
Primary Actions	Extension: Extends the wrist (Main Action). Radial Deviation: Abducts the wrist (moves it toward the thumb side).

Deep Dive: The "Tennis Elbow" Muscle

While the Extensor Carpi Radialis Longus arises from the supracondylar ridge, the Brevis arises directly from the Lateral Epicondyle. This anatomical difference is crucial.

1. The Common Extensor Tendon

The ECRB is the most central component of the Common Extensor Tendon (which also includes Extensor Digitorum, Extensor Digiti Minimi, and Extensor Carpi Ulnaris). Because it lies underneath the Longus, it is subject to friction and shearing forces against the radial head during rotation.

2. Insertion Specifics

It anchors firmly to the 3rd Metacarpal (middle finger). Since the middle finger is the axis of the hand, the ECRB provides a pure, centralized extension force compared to the Longus (2nd metacarpal) or Ulnaris (5th metacarpal).

Physio Corner: Clinical Relevance

💪 Functional Fact: You cannot grip strongly with a flexed wrist. The ECRB's main functional role is to hold the wrist in extension while you make a fist. This prevents the flexor muscles from actively insufficient shortening, allowing maximum grip strength.

Palpation

Ask the patient to make a fist and extend the wrist against resistance. Palpate just distal to the lateral epicondyle. The ECRB belly is felt just medial/posterior to the ECRL belly. Following the tendon down, it leads to the base of the middle finger.

⚠️ Clinical Pathology: Tennis Elbow (Lateral Epicondylitis)
Micro-tearing of the ECRB origin is the primary cause of Tennis Elbow.
Cozen's Test: Have the patient make a fist, pronate, and radially deviate. Resist their wrist extension. Pain at the lateral epicondyle = Positive for ECRB pathology.

Manual Muscle Testing (MMT)

To test the ECRB, we focus on extension with a bias toward the radial side.

Testing Tip: To differentiate from the Longus, perform the test with the elbow extended. The Longus works better with the elbow flexed; the Brevis works consistently.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Sitting. Forearm pronated (palm down) and supported on a table. Wrist hanging off the edge. Action: Patient extends the wrist upwards towards the ceiling, with slight deviation towards the thumb. Resistance: Applied at the dorsal hand (2nd and 3rd metacarpals), pushing down and slightly towards the ulnar side. Grade 3: Full range against gravity. Grade 4/5: Holds against strong resistance.
Grade 2 (Gravity Eliminated)	Position: Forearm in neutral (mid-position, thumb up) resting on the table. Action: Patient extends the wrist (sliding the back of the hand along the table). Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the dorsum of the wrist at the base of the 3rd metacarpal. Cue: "Try to lift your hand back." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

Does ECRB extend the fingers?

No. It inserts on the Metacarpal bone, so it only acts on the wrist joint. Finger extension is performed by the Extensor Digitorum.

What is the difference between ECRL and ECRB?

ECRL (Longus): Originates higher (supracondylar ridge), inserts on 2nd metacarpal, acts more on radial deviation.
ECRB (Brevis): Originates lower (epicondyle), inserts on 3rd metacarpal, acts more on pure extension.

Why is ECRB prone to injury?

Its tendon is undersurface to the ECRL and rubs against the capitulum of the humerus. It also has a relatively poor blood supply near its origin ("watershed zone"), making healing difficult.

Test Your Knowledge: ECRB Quiz

1. What is the primary insertion of the ECRB?

2. Which nerve supplies the ECRB?

3. ECRB pathology is the primary cause of which condition?

4. Where does the ECRB originate?

5. What is the main function of ECRB during gripping?

6. Which test is used to diagnose ECRB tendinopathy?

7. Besides extension, what other movement does ECRB assist with?

8. The ECRB is part of which compartment?

9. The Common Extensor Tendon attaches to:

10. True or False: ECRB is shorter than ECRL.

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.