Flexor hallucis brevis muscle

Origin and insertion

The flexor hallucis brevis is a medial plantar muscle of the foot. It is composed of two muscle bellies that differ in origin due to the muscle arising from a bifurcate tendon. The lateral head arises from the medial part of the plantar surface of the cuboid bone, posterior to the groove for the fibularis longus tendon, and the adjacent surface of the lateral cuneiform bone. The medial head of flexor hallucis brevis arises from the lateral division of the tibialis posterior tendon and the middle band of the medial intermuscular septum.

The muscle also consists of lateral and medial bellies which run anteriorly and medially towards the great toe. The distal tendon of each belly terminates by inserting onto each side of the base of the proximal phalanx of the hallux. In the process, the tendon of the medial belly blends with the tendon of abductor hallucis muscle, while the tendon of the lateral belly blends with the tendon of adductor hallucis muscle.

Relations

The flexor hallucis brevis is found in the third layer of the medial plantar muscles of the foot, situated between the abductor hallucis medially and flexor digitorum brevis laterally. As the flexor hallucis brevis courses anteromedially towards the proximal phalanx of the great toe, the tendon of flexor hallucis longus passes in between its medial and lateral muscle bellies to attach at the base of the distal phalanx of great toe. The medial plantar nerve lies along the lateral aspect of flexor hallucis brevis.

Two small hallux sesamoid bones develop within the muscles’ distal tendons, near their attachment sites on either side of the hallux. These hallux sesamoid bones are small paired ovoid-shaped ossicles of the foot that are embedded within both the medial and lateral tendons of flexor hallucis brevis muscle bellies. As they lay on either side of the hallux, they are named the medial (tibial) and lateral (fibular) sesamoid bones of the first metatarsophalangeal joint. The hallux sesamoid bones articulate with the head of the first metatarsal, acting as a fulcrum to increase the leverage of flexor hallucis longus and flexor hallucis brevis.

Innervation

Flexor hallucis brevis is innervated by the medial plantar nerve (S1, S2), which is one of the terminal branches of the tibial nerve.

Blood supply

Flexor hallucis brevis muscle receives arterial blood supply from the first metatarsal artery, which branches off the convexity of the plantar arch. The plantar arch is a semicircular anastomosis formed by the medial and lateral plantar arteries. Flexor hallucis brevis muscle is also supplied by the superficial branch of the medial plantar artery that arises from the posterior tibial artery.

Function

The primary function of the flexor hallucis brevis is flexion of the great toe at the metatarsophalangeal joint. This muscle aids the flexor hallucis longus in the toe-off phase of locomotion, increasing the final push-off from the ground during activities such as walking, running and jumping.

The fact that the flexor hallucis brevis tendons blend with the adductor hallucis and abductor hallucis signifies its importance in providing stability of the great toe during the aforementioned activities, ensuring maximum force translation during the thrust phase.

Flexor hallucis brevis also plays a role in the maintenance of the medial longitudinal arch by acting as a bowstring between the proximal phalanx of the hallux and tarsal bones. Muscular contraction brings the bones closer together, thereby raising the arch.