Flexor Digitorum Brevis: Origin, Insertion, Nerve, Action & The "Split" Tendon

The Flexor Digitorum Brevis (FDB) is the most superficial muscle in the center of the sole of the foot. It sits directly beneath the plantar fascia and corresponds anatomically to the Flexor Digitorum Superficialis in the hand. It is a primary supporter of the medial longitudinal arch.

[Image of Abductor Digiti Minimi Foot anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Medial process of the Calcaneal Tuberosity (Heel bone) and the Plantar Aponeurosis.
Insertion (Distal)	Splits into 4 tendons that insert on the sides of the Middle Phalanges of the lateral four toes (2-5).
Nerve Supply	Medial Plantar Nerve (S1, S2).
Blood Supply	Medial and Lateral Plantar Arteries.
Primary Actions	Toe Flexion: Flexes the Proximal Interphalangeal (PIP) and Metatarsophalangeal (MTP) joints of toes 2-5. Arch Support: Stabilizes the medial longitudinal arch during gait.

Deep Dive: The Perforated Tendon

The FDB resides in the First Layer of plantar muscles, sandwiched between the thick plantar fascia (superficial) and the Quadratus Plantae (deep).

1. The Split Mechanism

The insertion of the FDB is unique. As its tendons approach the toes, they split into two slips. This creates a tunnel or hiatus.

Why? To allow the tendons of the long flexor (Flexor Digitorum Longus) to pass through the FDB to reach the distal phalanges. This anatomical crossover allows both muscles to function without compressing each other.

2. Hand vs. Foot

In the hand, the equivalent muscle (Flexor Digitorum Superficialis) is in the forearm. In the foot, the FDB is an intrinsic muscle, meaning its origin and insertion are both contained within the foot itself.

Physio Corner: Clinical Relevance

💪 Functional Fact: The FDB is essential for balance. When you sway forward, the FDB grips the ground with the toes to push you back to neutral. It acts as a dynamic tie-rod for the foot arch.

Palpation

Palpate the center of the heel (plantar aspect). Move distally along the central band of the plantar fascia. Ask the patient to curl their toes. The muscle belly of the FDB can be felt contracting beneath the thick fascia in the mid-foot region.

⚠️ Clinical Pathology: Plantar Fasciitis
Because the FDB shares a common origin with the Plantar Fascia at the medial calcaneal tubercle, inflammation here (Plantar Fasciitis) often involves the muscle origin as well. Pain is typically worse with the first steps in the morning.

Manual Muscle Testing (MMT)

We test flexion of the PIP joints while stabilizing the MTP joints.

Testing Tip: It is hard to completely isolate FDB from FDL (Longus). However, if you stabilize the proximal phalanx and ask for flexion, you are biasing the PIP joint (FDB action).

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Supine or Sitting. Ankle neutral. Stabilization: Therapist holds the proximal phalanges of toes 2-5. Action: Patient curls the toes (flexes the PIP joints). Resistance: Applied to the plantar surface of the Middle Phalanges, pushing up into extension. Grade 3: Range of motion against gravity. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate the central sole of the foot. Cue: "Grip your toes." Grade 1: Deep tension felt under the fascia. Grade 0: No activity.

Frequently Asked Questions

Does FDB flex the tip of the toe?

No. The FDB inserts on the Middle Phalanx, so it flexes the PIP joint. The tip (Distal Phalanx) is flexed by the Flexor Digitorum Longus (FDL).

What passes through the FDB tendon?

The tendon of the Flexor Digitorum Longus (FDL) passes through the split (hiatus) in the FDB tendon to reach the distal phalanx.

Which layer of the foot is it in?

The First Layer (the most superficial layer), along with the Abductor Hallucis and Abductor Digiti Minimi.

Test Your Knowledge: FDB Quiz

1. Which nerve supplies the Flexor Digitorum Brevis?

2. Where does the FDB insert?

3. What unique feature does the FDB tendon have?

4. Which layer of the foot contains the FDB?

5. Which joint is primarily flexed by the FDB?

6. What is the origin of the FDB?

7. Which pathology often involves the origin of the FDB?

8. The FDB corresponds to which muscle in the hand?

9. Does the FDB act on the Big Toe (Hallux)?

10. During gait, the FDB helps to:

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.