Lateral head of flexor hallucis brevis

At a glance

English

Lateral head of flexor hallucis brevis

Latin (TA2)

Caput laterale flexoris brevis hallucis

System

Muscle

Region

Lower limb

Side

Paired, left and right

Arises from

The first metatarsal bone

About the lateral head of flexor hallucis brevis

Flexor hallucis brevis is a short muscle in the third layer of the sole, lying against the underside of the first metatarsal. It begins as one belly and splits in front into medial and lateral heads, which pass either side of the long flexor tendon of the big toe. The lateral head is the outer of the two.

The muscle arises from the plantar surface of the cuboid and the lateral cuneiform, and from the tendon of tibialis posterior where it fans out in the sole. It runs forwards along the first metatarsal, and the lateral head then blends with the tendon of adductor hallucis and attaches to the outer side of the base of the big toe's proximal phalanx. A sesamoid bone, the lateral sesamoid, lies in the tendon under the first metatarsal head, and the medial head carries its own sesamoid on the other side. Because of its close link with the adductor, the lateral head is sometimes called the first plantar interosseous.

Both heads flex the big toe at the metatarsophalangeal joint and press the toe down during the push-off of walking and running. The sesamoids act like a kneecap, lifting the tendons away from the joint and so improving their leverage, and they take much of the load under the ball of the foot. The muscle also helps support the medial longitudinal arch.

The tendon of flexor hallucis longus runs between the two heads and between the sesamoids, so the short flexor guards it. The lateral sesamoid can fracture, become inflamed (sesamoiditis), or shift outwards in hallux valgus. Most texts give the medial plantar nerve as its supply, with the lateral head sometimes served by the lateral plantar nerve.

Attachments, action and supply

Origin

Plantar surface of the cuboid and lateral cuneiform and the slips of the tibialis posterior tendon, in common with the medial head.

Insertion

Lateral side of the base of the big toe's proximal phalanx, through the lateral sesamoid and with the adductor hallucis tendon.

Action

Flexes the big toe at the metatarsophalangeal joint; supports the medial longitudinal arch.

Nerve supply

Medial plantar nerve (S1, S2); the lateral head sometimes receives a branch of the lateral plantar nerve.

Blood supply

Medial plantar artery and the first plantar metatarsal artery.