Extensor Indicis: Origin, Insertion, Nerve, Action & The "Pointing" Muscle

The Extensor Indicis (also known as Extensor Indicis Proprius) is a narrow, deep muscle in the posterior forearm. Its name literally translates to "Stretcher of the Index," and it gives humans the ability to point the index finger independently while the other fingers remain curled in a fist.

[Image of Extensor Indicis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Posterior surface of the distal third of the Ulna and the Interosseous Membrane.
Insertion (Distal)	Extensor Expansion (Dorsal Hood) of the Index Finger (2nd digit).
Nerve Supply	Posterior Interosseous Nerve (Deep branch of Radial Nerve) - C7, C8.
Blood Supply	Posterior Interosseous Artery.
Primary Actions	Independent Extension: Extends the index finger (MCP, PIP, DIP joints). Wrist Extension: Weakly assists in extending the wrist.

Deep Dive: The Spare Tire

The Extensor Indicis is located in the deep layer of the posterior forearm, medial to the Extensor Pollicis Longus.

1. Two Tendons for One Finger

The Index Finger is special because it has two extensor tendons:
1. Extensor Digitorum (ED): The "common" extensor shared with other fingers.
2. Extensor Indicis (EI): The "proper" or specific extensor.
The EI tendon usually lies on the ulnar side (medial side) of the ED tendon at the knuckle.

2. The Fourth Compartment

The EI tendon travels through the 4th Dorsal Compartment of the wrist, sharing the tunnel with the four tendons of the Extensor Digitorum. This makes the 4th compartment the most crowded tunnel in the extensor retinaculum.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Extensor Digitorum cannot extend a single finger independently if the others are fully flexed (try lifting your ring finger while making a fist). However, the Extensor Indicis can extend the index finger while in a fist. This is essential for typing, pointing, or pressing buttons.

Palpation

It is difficult to palpate the muscle belly as it is deep. However, you can feel the tendon. Have the patient make a fist and then extend only the index finger. Palpate the dorsum of the hand over the 2nd metacarpal. You will feel two tendons; the deeper, more medial one is the Extensor Indicis.

⚠️ Clinical Pathology: EIP Transfer
Because the index finger has two extensors (ED and EI), the Extensor Indicis is often considered "expendable." Surgeons frequently harvest the EI tendon to repair a ruptured Extensor Pollicis Longus (EPL) tendon (common in rheumatoid arthritis or distal radius fractures). This procedure is called an EIP to EPL transfer.

Manual Muscle Testing (MMT)

To isolate the Extensor Indicis, we must inhibit the Extensor Digitorum.

Testing Tip: Ask the patient to make a fist (flex fingers 3, 4, and 5). This puts the Extensor Digitorum at a mechanical disadvantage, forcing the Extensor Indicis to do the work of pointing.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Forearm pronated, wrist neutral. Fingers 3-5 flexed into a fist. Action: Patient extends the index finger (points). Resistance: Applied to the dorsal aspect of the proximal phalanx of the index finger, 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 index finger horizontally. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the tendon medial to the ED tendon at the 2nd MCP joint. Cue: "Try to point your finger." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

Does the pinky have a similar muscle?

Yes! The little finger has the Extensor Digiti Minimi. This is why you can make the "rock and roll" sign (horns) by extending the index and pinky while keeping the middle fingers curled.

Where does the EI tendon sit relative to the ED tendon?

The Extensor Indicis tendon is almost always found on the Ulnar (Medial) side of the Extensor Digitorum tendon at the level of the metacarpal head.

Does it adduct the index finger?

Yes, slightly. Because it approaches the index finger from the ulnar side of the forearm, it can assist in aducting the index finger (pulling it toward the middle finger) during extension.

Test Your Knowledge: Extensor Indicis Quiz

1. What is the origin of the Extensor Indicis?

2. Which nerve supplies the Extensor Indicis?

3. The EI tendon travels through which dorsal compartment?

4. What is a common surgical use for the Extensor Indicis?

5. At the metacarpal head, the EI tendon sits on which side of the ED tendon?

6. How many extensor tendons does the index finger typically have?

7. The Extensor Indicis is part of which muscle layer?

8. What is the primary advantage of having the Extensor Indicis?

9. Where is resistance applied during MMT?

10. True or False: The Extensor Indicis inserts into the Flexor Digitorum Profundus tendon.

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.