Extensor Carpi Ulnaris: Origin, Insertion, Action, Nerve & Rehabilitation

The Extensor Carpi Ulnaris (ECU) is a superficial muscle located on the ulnar (pinky) side of the posterior forearm. It is unique because it acts on the wrist in two planes: extending it and deviating it towards the ulnar side. It is a frequent source of ulnar-sided wrist pain in racket sports and golf.

[Image of Extensor Carpi Ulnaris muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	1. Humeral Head: Lateral Epicondyle of Humerus (via Common Extensor Tendon). 2. Ulnar Head: Posterior border of the Ulna.
Insertion (Distal)	Medial side of the base of the 5th Metacarpal bone.
Nerve Supply	Posterior Interosseous Nerve (Deep branch of Radial Nerve) - C7, C8.
Blood Supply	Ulnar Artery.
Primary Actions	Extension: Extends the wrist joint. Ulnar Deviation: Adducts the wrist (moves it towards the pinky).

Deep Dive: The Ulnar Stabilizer

The ECU is anatomically critical for the stability of the ulnar side of the wrist.

1. The Sixth Compartment

At the wrist, the extensor tendons pass through six discrete tunnels (dorsal compartments). The ECU tendon travels alone in the 6th Dorsal Compartment. It sits in a distinct groove on the dorsal ulna, held in place by its own sub-sheath.

2. Dynamic Stability

When the forearm is supinated (palm up), the ECU tendon pulls vertically and extends the wrist effectively. When the forearm is pronated (palm down), the tendon angle changes, and it acts more as an ulnar stabilizer. This variability makes it susceptible to snapping or subluxation (popping out of its groove) during twisting motions.

Physio Corner: Clinical Relevance

💪 Functional Fact: The ECU works in perfect synergy with the Flexor Carpi Ulnaris (FCU) to perform pure Ulnar Deviation (like chopping wood or using a hammer). Without the ECU balancing the FCU, the wrist would flex every time you tried to deviate ulnarly.

Palpation

Palpate the ulnar Styloid process (the bump on the pinky side of the wrist). Ask the patient to extend and ulnar deviate their wrist against resistance. The ECU tendon will pop up prominently just dorsal (back) to the ulnar styloid. You can trace the muscle belly up along the ulnar border of the forearm.

⚠️ Clinical Pathology: ECU Subluxation
Often seen in tennis players (double-handed backhand) or golfers. The sheath holding the tendon tears, causing the tendon to snap over the ulnar styloid with a painful "click" during supination and ulnar deviation.

Manual Muscle Testing (MMT)

Testing extension combined with ulnar deviation isolates the ECU from the radial extensors.

Testing Tip: Ensure the patient's fingers are relaxed. If they extend their fingers, the Extensor Digitorum might mask weakness in the ECU.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Sitting. Forearm fully pronated (palm down) on a table. Wrist hanging off edge. Action: Patient extends the wrist and deviates towards the pinky (up and out). Resistance: Applied at the dorsum of the 5th metacarpal, pushing down (flexion) and towards the thumb (radial deviation). Grade 3: Full range against gravity. Grade 4/5: Holds against strong resistance.
Grade 2 (Gravity Eliminated)	Position: Forearm in neutral (thumb up) resting on table. Action: Patient extends the wrist (sliding back of hand on table). Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate just proximal to the base of the 5th metacarpal or near the ulnar styloid. Cue: "Try to lift your hand up and out." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

Does ECU act on the elbow?

Very weakly. Since it originates from the lateral epicondyle, it crosses the elbow joint, but its leverage for elbow extension is negligible compared to the Triceps or Anconeus.

What compartment is the ECU in?

It is in the Posterior Compartment of the forearm (superficial layer). At the wrist, its tendon occupies the 6th Dorsal Compartment.

Is ECU involved in Tennis Elbow?

Yes, but less commonly than the Extensor Carpi Radialis Brevis (ECRB). Pain at the origin (lateral epicondyle) can involve the ECU, but pain at the wrist (insertion) is more specific to ECU tendinopathy.

Test Your Knowledge: ECU Quiz

1. Where does the Extensor Carpi Ulnaris insert?

2. Which nerve supplies the ECU?

3. What are the primary actions of the ECU?

4. Which dorsal compartment does the ECU tendon occupy?

5. ECU subluxation typically causes pain over the:

6. Where does the ECU originate?

7. Which muscle is the antagonist to the ECU in terms of deviation?

8. To MMT the ECU, you resist which motion?

9. True or False: The ECU has an ulnar head of origin.

10. The ECU aids the FCU in producing which movement?

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.