Plantar Interossei (Foot): Origin, Insertion, Action, PAD Mnemonic & Testing

The Plantar Interossei are three small, unipennate muscles located deep in the sole of the foot, between the metatarsal bones. They are the direct antagonists to the Dorsal Interossei, working to bring the toes together and stabilize the forefoot during the stance phase of gait.

[Image of Dorsal Interossei Foot anatomy]

Quick Anatomy Snapshot

Count	Three muscles (acting on the 3rd, 4th, and 5th toes).
Origin (Proximal)	Medial sides of the bases and shafts of the 3rd, 4th, and 5th Metatarsals.
Insertion (Distal)	Medial sides of the bases of the Proximal Phalanges of the 3rd, 4th, and 5th toes.
Nerve Supply	Lateral Plantar Nerve (S2, S3) - Deep branch.
Blood Supply	Plantar Metatarsal Arteries.
Primary Actions	Adduction: Moves toes 3-5 toward the midline (2nd toe). MTP Flexion: Flexes the metatarsophalangeal joints. IP Extension: Extends the interphalangeal joints.

Deep Dive: The "PAD" Concept

Just like in the hand, understanding the foot interossei relies on the classic mnemonic.

1. The "PAD" Mnemonic

Plantar interossei ADduct.
Dorsal interossei ABduct (DAB).

2. The Midline Rule

In the foot, the 2nd Toe is the midline.
• The 2nd toe has NO Plantar Interosseous muscle attached to it, because it cannot adduct towards itself.
• The 1st toe (Hallux) has its own powerful adductor (Adductor Hallucis), so it doesn't need an interosseous muscle either.
• Therefore, Plantar Interossei are only found on the 3rd, 4th, and 5th toes, pulling them inward toward the 2nd toe.

Physio Corner: Clinical Relevance

💪 Functional Fact: While "squeezing the toes together" seems trivial, these muscles are critical for Metatarsal Stability. They prevent the metatarsal bones from splaying apart (spreading) when your foot hits the ground, maintaining the transverse arch.

Palpation

These muscles are very deep (4th plantar layer) and cannot be distinctly palpated through the thick plantar fascia and superficial muscles. Tenderness between the metatarsal heads, however, can indicate interosseous strain or intermetatarsal bursitis.

⚠️ Clinical Pathology: Hammer Toes
Just like the Dorsal Interossei, weakness in the Plantar Interossei disrupts the "Intrinsic Plus" position (flexed knuckle, straight toe). Without their stabilizing force, the long extensors pull the MTP joint back into hyperextension, and the long flexors curl the toes, leading to Hammer Toe or Claw Toe deformities.

Manual Muscle Testing (MMT)

We test the ability to hold the toes together against resistance.

Testing Tip: The "Paper Grip Test" is the most effective way to test adduction strength clinically.

Step-by-Step Procedure (Paper Grip Test)

Grade	Patient Action & Resistance
Functional Test	Position: Sitting or Supine. Toes extended and relaxed. Action: Place a piece of paper or a thin card between the 3rd and 4th (or 4th and 5th) toes. Command: "Squeeze your toes together and don't let me pull the paper out." Result: If the paper slides out easily with minimal resistance, the Plantar Interossei are weak.

Frequently Asked Questions

How many Plantar Interossei are there?

Usually three. They attach to the 3rd, 4th, and 5th metatarsals. The big toe and second toe do not have them.

Are they Unipennate or Bipennate?

Unipennate. They arise from a single metatarsal bone (the one they act upon). In contrast, the Dorsal Interossei are Bipennate (arising from two adjacent bones).

What is the antagonist muscle?

The Dorsal Interossei and the Abductor Digiti Minimi (for the 5th toe) are the antagonists, as they abduct (spread) the toes.

Test Your Knowledge: Plantar Interossei Quiz

1. What is the mnemonic for the action of Plantar Interossei?

2. How many Plantar Interossei are normally present?

3. The midline of the foot for adduction/abduction is the:

4. Which nerve supplies the Plantar Interossei?

5. Are Plantar Interossei Unipennate or Bipennate?

6. Which toe does NOT have a Plantar Interosseous muscle?

7. Besides adduction, what do they do at the IP joints?

8. In which layer of the foot are they found?

9. Where do they originate?

10. Which test assesses the strength of these muscles?

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.