Flexor Hallucis Brevis: Origin, Insertion, Nerve & The Sesamoids

The Flexor Hallucis Brevis (FHB) is a powerful intrinsic muscle of the foot located in the third plantar layer. It divides into two heads (medial and lateral) which contain two small but crucial bones called sesamoids. It works to flex the big toe, playing a pivotal role in the "toe-off" phase of walking.

[Image of Flexor Hallucis Brevis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Plantar surfaces of the Cuboid and Lateral Cuneiform bones.
Insertion (Distal)	Medial Head: Medial side of base of Proximal Phalanx (via medial sesamoid). Lateral Head: Lateral side of base of Proximal Phalanx (via lateral sesamoid).
Nerve Supply	Medial Plantar Nerve (S1, S2).
Blood Supply	Medial Plantar Artery.
Primary Actions	MTP Flexion: Flexes the Metatarsophalangeal joint of the Big Toe. Arch Support: Maintains the Medial Longitudinal Arch.

Deep Dive: The Sesamoid Mechanism

The most defining feature of the Flexor Hallucis Brevis is its relationship with the sesamoid bones.

1. The Two Heads

The muscle belly splits into a "Y" shape near the ball of the foot.
• The Medial Head tendon contains the Medial (Tibial) Sesamoid.
• The Lateral Head tendon contains the Lateral (Fibular) Sesamoid.

2. The Central Tunnel

The tendon of the Flexor Hallucis Longus (the long flexor from the calf) runs directly through the gap between the two heads of the FHB. The sesamoids act as a pulley system, lifting the FHB tendons away from the joint to increase leverage and protecting the FHL tendon from being crushed against the ground during walking.

Physio Corner: Clinical Relevance

💪 Functional Fact: During "Toe-Off" (propulsion), the entire body weight is transferred through the big toe. The FHB and its sesamoids absorb this massive load, protecting the metatarsal head and generating the final push.

Palpation

Palpate the ball of the foot (1st Metatarsal Head). Ask the patient to gently flex their big toe. You can feel the two heads of the FHB contracting on either side of the central flexor tendon. The sesamoids can be felt as hard, bony nodules within the tendons on the plantar surface.

⚠️ Clinical Pathology: Sesamoiditis & Turf Toe
Sesamoiditis: Inflammation of the sesamoid bones due to repetitive impact (dancers, runners).
Turf Toe: Hyperextension injury of the big toe. This sprains the plantar plate and the FHB tendon complex, often causing tearing near the sesamoids.

Manual Muscle Testing (MMT)

To test the FHB, we must distinguish it from the Longus (FHL).

Testing Tip: The FHL flexes the tip (IP joint). The FHB flexes the knuckle (MTP joint). To isolate FHB, apply resistance to the proximal phalanx, NOT the tip.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Supine or Sitting. Ankle neutral. Action: Patient flexes the big toe at the MTP joint (keeps the IP joint straight). Resistance: Applied to the plantar surface of the Proximal Phalanx of the big toe, pushing up into extension. Grade 3: Full range against gravity. Grade 4/5: Holds against strong resistance.
Grade 0, 1 (Palpation)	Action: Palpate the ball of the foot (plantar aspect of 1st Metatarsal). Cue: "Try to bend your big toe at the base." Grade 1: Contraction felt on either side of the central tendon. Grade 0: No activity.

Frequently Asked Questions

Which layer of the foot is FHB in?

It is in the Third Layer of plantar muscles, along with the Adductor Hallucis and Flexor Digiti Minimi Brevis.

Can the sesamoids fracture?

Yes. Sesamoid stress fractures are common in athletes. Because the sesamoids are embedded in the FHB tendon, every step pulls on the fracture fragments, making healing slow.

Does it flex the tip of the toe?

No. The FHB inserts on the Proximal Phalanx. Therefore, it acts only on the MTP joint. The tip (Distal Phalanx) is flexed by the Flexor Hallucis Longus.

Test Your Knowledge: FHB Quiz

1. Which nerve supplies the Flexor Hallucis Brevis?

2. The FHB tendons contain which unique structures?

3. What runs between the two heads of the FHB?

4. Where does the FHB insert?

5. Which layer of the plantar foot muscles is FHB in?

6. "Turf Toe" involves hyperextension of which joint?

7. Where does the FHB originate?

8. Which muscle lies lateral to the FHB in the 3rd layer?

9. To test MMT Grade 5, resistance is applied to the:

10. True or False: FHB originates from the Calcaneal Tuberosity.

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.