Flexor Digitorum Accessorius: Origin, Insertion, Nerve & The "Corrector" Muscle

The Flexor Digitorum Accessorius (also commonly known as the Quadratus Plantae due to its square shape) is a unique intrinsic muscle of the foot. Unlike most muscles that move bones, this muscle inserts into a tendon. Its primary job is to correct the diagonal pull of the long toe flexors.

[Image of Quadratus Plantae muscle]

Quick Anatomy Snapshot

Origin (Proximal)	Two Heads: 1. Medial surface of the Calcaneus (larger head). 2. Lateral border of the plantar surface of the Calcaneus.
Insertion (Distal)	Posterolateral margin of the tendon of the Flexor Digitorum Longus (FDL).
Nerve Supply	Lateral Plantar Nerve (S2, S3).
Blood Supply	Lateral Plantar Artery.
Primary Actions	Assists Flexion: Helps flex toes 2-5. Vector Correction: Straightens the line of pull of the FDL.

Deep Dive: The Vector Corrector

This muscle resides in the 2nd Layer of the plantar foot, deep to the Flexor Digitorum Brevis but superficial to the Adductor Hallucis.

1. The Problem: Diagonal Pull

The Flexor Digitorum Longus (FDL) tendon enters the foot from the medial side (behind the medial malleolus). If it acted alone, it would pull the toes diagonally inward (medially) as it flexed them, causing them to curl sideways.

2. The Solution: Quadratus Plantae

The Flexor Digitorum Accessorius originates from the heel and pulls on the FDL tendon from the lateral side. This lateral pull cancels out the medial vector of the FDL, resulting in a straight, pure flexion of the toes. It effectively "squares up" the pull.

Physio Corner: Clinical Relevance

💪 Functional Fact: This muscle is essential for walking on uneven surfaces. It fine-tunes the tension in the long flexor tendons, allowing the toes to grip the ground effectively regardless of the ankle position.

Palpation

Deep palpation is required. Press into the center of the heel pad, just distal to the weight-bearing tuberosity. It is difficult to distinguish from the overlying Flexor Digitorum Brevis and plantar fascia, but tenderness here deep in the arch can indicate strain.

⚠️ Clinical Pathology: Heel Pain mimic
Strains of the Quadratus Plantae are often misdiagnosed as Plantar Fasciitis. If a patient has heel pain that is reproduced by resisted toe flexion (gripping) but not by simple dorsiflexion of the toes (stretching the fascia), suspect this muscle.

Manual Muscle Testing (MMT)

It is impossible to test this muscle in isolation from the Flexor Digitorum Longus.

Testing Tip: To infer weakness, observe the direction of toe flexion. If the toes curl medially (inward) rather than straight down during flexion, the "corrector" muscle may be weak or inhibited.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Supine or sitting. Ankle neutral. Action: Patient curls the lateral four toes. Resistance: Applied to the plantar surface of the distal toes, pushing up into extension. Note: This tests the composite strength of FDL, FDB, and FDA.
Grade 0, 1 (Palpation)	Action: Deep palpation in the proximal arch/heel. Cue: "Grip your toes hard."

Frequently Asked Questions

Why is it called "Accessorius"?

Because it acts as an "accessory" to the Flexor Digitorum Longus. It has no tendon of its own that reaches the bones; it simply attaches to the FDL tendon to help it work better.

Is it present in the hand?

No. There is no "Quadratus Palmae" in the human hand. The flexor tendons in the hand enter straight through the carpal tunnel, so no correction vector is needed. This muscle is unique to the foot.

Which plantar layer contains this muscle?

The Second Layer. It shares this layer with the Lumbricals and the tendons of the Flexor Digitorum Longus and Flexor Hallucis Longus.

Test Your Knowledge: Quadratus Plantae Quiz

1. What is the alternative name for Flexor Digitorum Accessorius?

2. Where does this muscle insert?

3. Which nerve supplies the Quadratus Plantae?

4. What is the primary function of the FDA?

5. Which plantar layer is this muscle found in?

6. Where does the Quadratus Plantae originate?

7. Without the FDA, the FDL would pull the toes in which direction?

8. Heel pain mimicking plantar fasciitis can be caused by this muscle because:

9. Does the human hand have a Quadratus Palmae?

10. The FDA helps flex which toes?

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.
• Brukner, P., & Khan, K. (2017). Clinical Sports Medicine. 5th ed. McGraw-Hill Education.