Abductor Pollicis Brevis: Origin, Insertion, Nerve Supply, Action & MMT

The Abductor Pollicis Brevis (APB) is the most superficial muscle of the Thenar Eminence (the fleshy pad at the base of the thumb). It is a critical muscle for hand function, responsible for lifting the thumb away from the palm to grasp objects.

[Image of Abductor Pollicis Brevis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Flexor Retinaculum (Transverse Carpal Ligament), tubercles of the Scaphoid and Trapezium.
Insertion (Distal)	Lateral side of the base of the Proximal Phalanx of the Thumb (1st digit).
Nerve Supply	Median Nerve (Recurrent Branch) - C8, T1.
Blood Supply	Superficial Palmar Branch of the Radial Artery.
Primary Actions	Abduction: Moves the thumb anteriorly, perpendicular to the palm (lifting it up towards the ceiling). Opposition Assistance: Helps rotate the thumb medially.

Deep Dive: The Thenar Leader

The Thenar Eminence is composed of three muscles: Abductor Pollicis Brevis, Flexor Pollicis Brevis, and Opponens Pollicis. The APB sits on top of the others.

1. Palmar Abduction vs. Radial Abduction

It is important to distinguish the type of abduction. The Abductor Pollicis Brevis performs Palmar Abduction (moving the thumb 90° away from the palm). The Abductor Pollicis Longus (forearm muscle) performs Radial Abduction (moving the thumb out to the side, in the plane of the palm).

2. The "Millionaire" Nerve

The Recurrent Branch of the Median nerve, which supplies the thenar muscles, is sometimes called the "Millionaire Nerve." It is very superficial and can be easily cut during lacerations to the palm, which could cost a surgeon "a million dollars" in lawsuits due to the loss of thumb function.

Physio Corner: Clinical Relevance

💪 Functional Fact: You use the Abductor Pollicis Brevis every time you open your hand to grab a soda can or a glass of water. Without it, the thumb cannot lift out of the palm to accommodate large objects.

Palpation

Ask the patient to rest their hand palm up. Ask them to touch your finger held directly above their thumb (pointing to the ceiling). The APB is the muscle most lateral and superficial on the thenar pad that bulges during this action.

⚠️ Clinical Pathology: Carpal Tunnel Syndrome
The APB is often the first muscle to show weakness or atrophy in severe Carpal Tunnel Syndrome because the recurrent branch of the median nerve fibers are compressed under the flexor retinaculum. This leads to a flattened thenar eminence (Ape Hand Deformity).

Manual Muscle Testing (MMT)

We test the muscle's ability to lift the thumb perpendicular to the palm.

Testing Tip: Ensure the patient is lifting the thumb vertically (Abductor Pollicis Brevis), not sliding it out to the side (Abductor Pollicis Longus).

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Hand supinated (palm up), resting on table. Action: Patient lifts the thumb straight up towards the ceiling (Palmar Abduction). Resistance: Applied at the proximal phalanx, pushing the thumb down towards the palm. Grade 3: Full range against gravity. Grade 4/5: Holds against moderate/max resistance.
Grade 2 (Gravity Eliminated)	Position: Hand resting on its ulnar border (side), thumb on top. Action: Patient moves the thumb away from the palm horizontally. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the center of the thenar eminence. Cue: "Try to point your thumb to the ceiling." Grade 1: Trace contraction felt. Grade 0: No activity.

Frequently Asked Questions

What is "Ape Hand" deformity?

It is the inability to abduct or oppose the thumb due to median nerve palsy. The thenar muscles waste away, and the thumb falls back into line with the fingers (simian posturing).

Difference between APB and APL?

APB (Brevis): Intrinsic hand muscle, median nerve, moves thumb perpendicular to palm (grabbing a glass).
APL (Longus): Forearm muscle, radial nerve, moves thumb laterally in plane with palm (hitchhiking).

Does it originate on the Flexor Retinaculum?

Yes. This is why releasing the flexor retinaculum (surgery for Carpal Tunnel) requires care not to damage the origin of the thenar muscles.

Test Your Knowledge: Abductor Pollicis Brevis Quiz

1. Which nerve supplies the Abductor Pollicis Brevis?

2. What is the primary action of the APB?

3. The APB originates from the Flexor Retinaculum and which bones?

4. Which condition typically results in wasting of the APB?

5. Where does the APB insert?

6. To test MMT Grade 5, you resist the thumb moving:

7. Which is the deepest muscle of the Thenar eminence?

8. The APB is essential for grasping:

9. "Ape Hand" involves the loss of:

10. The recurrent branch of the median nerve is also known as:

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.