Extensor Digitorum: Origin, Insertion, Nerve, Action & The Extensor Hood

The Extensor Digitorum (also known as Extensor Digitorum Communis) is the primary extensor of the four fingers. Located in the superficial posterior forearm, its tendons are clearly visible on the back of the hand when you spread your fingers or "play the piano" in the air.

[Image of Extensor Digitorum muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Lateral Epicondyle of the Humerus (via the Common Extensor Tendon).
Insertion (Distal)	Extensor Expansions (Dorsal Hoods) of the medial four digits (Fingers 2-5).
Nerve Supply	Posterior Interosseous Nerve (Deep branch of Radial Nerve) - C7, C8.
Blood Supply	Posterior Interosseous Artery.
Primary Actions	Finger Extension: Extends the MCP, PIP, and DIP joints. Wrist Extension: Assists in extending the wrist joint. Finger Abduction: Spreads the fingers as they extend.

Deep Dive: The Complex Tendons

The Extensor Digitorum doesn't just attach to one bone; it has a sophisticated insertion mechanism.

1. The Extensor Expansion (Dorsal Hood)

As the tendon crosses the knuckle (MCP joint), it flattens into a "hood." This hood splits into three parts:
• Central Slip: Inserts onto the base of the Middle Phalanx.
• Two Lateral Bands: Rejoin to insert onto the base of the Distal Phalanx.
This mechanism allows the single muscle to extend all three joints of the finger.

[Image of Extensor Expansion dorsal hood]

2. Juncturae Tendinum

Have you ever noticed you cannot fully extend your ring finger if your other fingers are curled in a fist? This is due to the Juncturae Tendinum—fibrous bands that connect the tendons of the Extensor Digitorum on the back of the hand. They distribute force but also restrict independent finger movement.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Extensor Digitorum acts as the primary "release" muscle. Without it, you could grip objects but never let them go. It is vital for opening the hand to grasp large objects.

Palpation

Ask the patient to wiggle their fingers like they are playing the piano. Palpate the posterior forearm. You will feel the large muscle belly in the middle of the forearm moving. Follow the tendons down across the wrist and observe the connecting bands (juncturae) on the back of the hand.

⚠️ Clinical Pathology: Mallet Finger & Boutonnière Deformity
Injuries to the extensor mechanism cause specific deformities:
• Mallet Finger: Rupture of the lateral bands at the DIP joint (fingertip droops).
• Boutonnière Deformity: Rupture of the central slip at the PIP joint (middle knuckle bends, tip extends).

Manual Muscle Testing (MMT)

We test the extension of the MCP joints (knuckles) primarily.

Testing Tip: Keep the IP joints (fingers) slightly flexed during the test. If you allow full extension, the intrinsic muscles (Lumbricals/Interossei) participate too much. Focus on the knuckles!

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Forearm pronated, wrist in neutral, fingers relaxed (curled). Action: Patient extends the MCP joints (knuckles), lifting fingers off the table. Resistance: Applied at the proximal phalanges (just past the knuckles), pushing down into flexion. Grade 3: Full range against gravity. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Forearm in neutral (mid-position), resting on ulnar border. Action: Patient extends the fingers horizontally. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the tendons on the dorsum of the hand or the muscle belly in the forearm. Cue: "Try to lift your fingers." Grade 1: Tension felt. Grade 0: No activity.

Frequently Asked Questions

Does it extend the wrist?

Yes. Because it crosses the wrist joint, it assists the dedicated wrist extensors (ECRL, ECRB, ECU) in wrist extension, especially when the fingers are also extending.

Why can't I lift my ring finger independently?

This is due to the Juncturae Tendinum. The ring finger tendon is usually tethered to the middle and little finger tendons by these fibrous bands, making independent extension very difficult compared to the index finger.

Does it abduct the fingers?

Yes. The arrangement of the tendons causes the fingers to naturally spread (abduct) when the Extensor Digitorum contracts strongly. You can see this when you open your hand wide.

Test Your Knowledge: Extensor Digitorum Quiz

1. Where does the Extensor Digitorum originate?

2. The central slip of the extensor mechanism inserts onto the:

3. Which nerve supplies the Extensor Digitorum?

4. What structures connect the ED tendons on the back of the hand?

5. The Extensor Digitorum travels through which dorsal compartment?

6. A rupture of the lateral bands at the DIP joint causes:

7. Does Extensor Digitorum assist in finger abduction?

8. Which muscle lies deep to the Extensor Digitorum in the forearm?

9. To test MMT Grade 5, resistance is applied at the:

10. The Extensor Digitorum is often involved in which overuse condition?

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.