Palmar Interossei: Origin, Insertion, Action, PAD Mnemonic & Testing

The Palmar Interossei (Voler Interossei) are small, unipennate muscles located on the palm side of the hand between the metacarpal bones. While the dorsal interossei spread the fingers apart, the palmar interossei bring them back together. They are key players in grip strength and fine motor coordination.

[Image of Dorsal Interossei Foot anatomy]

Quick Anatomy Snapshot

Count	Usually 3 muscles (acting on the 2nd, 4th, and 5th digits).
Origin (Proximal)	The side of the Metacarpal shaft facing the midline (3rd finger).
Insertion (Distal)	Base of the Proximal Phalanx (and extensor expansion) of the same finger.
Nerve Supply	Deep Branch of Ulnar Nerve (C8, T1).
Blood Supply	Palmar Metacarpal Arteries.
Primary Actions	Adduction: Moves fingers toward the midline (Middle Finger). MTP Flexion: Flexes the knuckles. IP Extension: Extends the interphalangeal joints (via extensor hood).

Deep Dive: The "PAD" Concept

Understanding the Palmar Interossei is all about knowing where the midline of the hand lies.

1. The "PAD" Mnemonic

Palmar interossei ADduct.
Dorsal interossei ABduct (DAB).

2. The Midline Rule

The axis of the hand is the 3rd Digit (Middle Finger).
• Since adduction means moving toward the midline, the Middle Finger cannot adduct (it is the midline). Therefore, the Middle Finger has NO Palmar Interossei attached to it.
• The 1st (Index), 2nd (Ring), and 3rd (Little) Palmar Interossei pull digits 2, 4, and 5 inward toward the middle finger.

3. Unipennate Structure

Unlike the Dorsal Interossei which are Bipennate (arise from two bones), the Palmar Interossei are Unipennate. They originate from only one metacarpal bone each—specifically the side that faces the middle finger.

Physio Corner: Clinical Relevance

💪 Functional Fact: The "Card Test" is a quick way to test these muscles. Ask the patient to hold a piece of paper or a playing card between their extended fingers (e.g., between Ring and Middle). If you can easily pull the card out, their Palmar Interossei (Adductors) are weak.

Palpation

These muscles are deep within the palm and covered by the flexor tendons and lumbricals, making direct palpation difficult. However, you can feel the bulk of the first palmar interosseous deep in the palm near the index finger when the patient forcefully adducts.

⚠️ Clinical Pathology: Ulnar Nerve Palsy
Since all interossei are supplied by the Ulnar nerve, a lesion (like at the Guyon's canal) results in Wartenberg's Sign. The little finger drifts into abduction because the 3rd Palmar Interosseous is paralyzed and cannot hold the pinky in, while the Extensor Digiti Minimi pulls it out.

Manual Muscle Testing (MMT)

Testing the ability to squeeze the fingers together.

Testing Tip: Ensure the fingers are fully extended. If the fingers are flexed, the long flexor tendons can simulate adduction.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Forearm pronated, hand flat, fingers extended and together. Action: Patient holds fingers together. Resistance: Therapist tries to pull the fingers apart. Test 1: Pull Index away from Middle. Test 2: Pull Ring away from Middle. Test 3: Pull Little away from Ring.
Grade 0, 1 (Palpation)	Action: Palpation is unreliable. Watch for the ability to slide the finger toward the middle finger on a table surface. Cue: "Bring your fingers together."

Frequently Asked Questions

Does the thumb have a Palmar Interosseous?

Usually no. The thumb is adducted by the powerful Adductor Pollicis muscle. However, some anatomical texts describe a rudimentary "1st Palmar Interosseous of Henle" at the thumb base, but functionally, the Adductor Pollicis does the job.

Do they extend the fingers?

They extend the Interphalangeal (IP) joints via the extensor hood mechanism, but they flex the Metacarpophalangeal (MCP) joints. This is the "Intrinsic Plus" position.

How many Palmar Interossei are there?

Classically, there are 3 (for the Index, Ring, and Little fingers). If you count the rudimentary one for the thumb, there are 4, but clinically we focus on the 3 that act on fingers 2, 4, and 5.

Test Your Knowledge: Palmar Interossei Quiz

1. What is the primary action of the Palmar Interossei?

2. Which finger has NO Palmar Interosseous muscle?

3. Are Palmar Interossei Unipennate or Bipennate?

4. Which nerve supplies the Palmar Interossei?

5. The Palmar Interossei originate from:

6. Which test specifically assesses Palmar Interossei strength?

7. What is the action on the MCP joint?

8. How many Palmar Interossei are typically described in standard anatomy?

9. Wartenberg's Sign involves which finger drifting into abduction?

10. True or False: Palmar Interossei are superficial to the Lumbricals.

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.