Flexor hallucis longus muscle

Origin and insertion

Flexor hallucis longus is found on the fibular side of the posterior aspect of the leg. The majority of the muscle fibers originate at the distal two-thirds of the posterior surface of the fibula. The rest of the flexor hallucis longus fibers arise from the lower part of the interosseous membrane and the posterior intermuscular septum of the leg, as well as the fascia covering the tibialis posterior muscle.

From these origin points, the flexor hallucis longus runs inferiorly towards the foot. Its muscle fibers are oblique and converge to attach on one side of its tendon. This tendon runs almost the entire length of the posterior aspect of the muscle, and grooves the posterior surface of the distal end of the tibia. The tendon passes inferiorly and laterally to the tendon of tibialis posterior muscle. On the medial aspect of the foot, the tendon of the flexor hallucis longus passes through the tarsal tunnel, covered by the flexor retinaculum.

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On its course through the tarsal tunnel, the muscle grooves the posterior surface of the talus, and the inferior surface of the sustentaculum tali of calcaneus. While passing over the sustentaculum tali, the tendon is stabilized by the annular ligament, a fibrous sheath that wraps the circumference of the tendon. Upon exiting the tarsal tunnel, flexor hallucis longus is crossed by the flexor digitorum longus that is passing anterolaterally. On the crossing point, the flexor hallucis longus often gives two slips to the medial two tendons of the flexor digitorum longus. On the rest of the plantar aspect, the flexor hallucis longus continues anteriorly to and enters the fibrous digital sheath of the great toe before inserting onto the plantar surface of the base of the distal phalanx.

The order of structures that pass through the tarsal tunnel, from anterior to posterior, can be easily remembered with a following mnemonic; 

Tom, Doug And Very Nervous Harry

• Tibialis posterior flexor
• Digitorum longus
• Artery (posterior tibial)
• Vein (posterior tibial)
• Nerve (tibial)
• flexor Hallucis longus

The foot is one of the most complex parts in the human body, containing many bones, muscles and neurovascular structures. If you want to learn more about it, check out the following resources:

Relations

The flexor hallucis longus muscle is situated on the fibular side of the leg, found deep to the gastrocnemius and soleus muscle (together called the triceps surae muscle) separated from them by the deep transverse fascia of the calf. Fibularis longus and fibularis brevis muscles are situated laterally to the flexor hallucis longus muscle. Medially, flexor hallucis longus is related to the posterior tibial vessels, the tibial nerve and the posterior tibial muscle, which it overlaps to some extent. Part of the flexor hallucis longus muscle fibers also attach on the tibialis posterior fascia. Deep to the flexor hallucis longus is the fibula, along with fibular artery and vein. 

On the medial aspect of the foot, the tendon of the flexor hallucis longus passes through the tarsal tunnel. The tarsal tunnel is a narrow space found on the medial side of the ankle, bound by the ankle bones medially and the flexor retinaculum laterally. The tendon of flexor hallucis longus is the most lateral structure in the tarsal tunnel, found adjacent to the tibial nerve. Other contents of the tarsal tunnel are found medially to the tibial nerve, and include posterior tibial vessels, tendon of flexor digitorum longus and tendon of tibialis posterior muscle.

Flexor hallucis longus is an important surgical landmark in the area of the tarsal tunnel, as it marks the lateral border of the aforementioned neurovascular structures, thereby staying lateral to the muscle can prevent injury to these structures. 

As it emerges from the tarsal tunnel into the plantar aspect of the foot, flexor hallucis longus is crossed by the flexor digitorum longus, that curves obliquely over it. The flexor hallucis longus continues anteriorly to insert at the distal phalange of the great toe, distal to the attachment of the soleus muscle. 

Test your knowledge of the foot muscles with this interactive quiz.

Innervation

Flexor hallucis longus is innervated by the tibial nerve, composed of spinal roots L4, L5, S1, S2, and S3. 

Blood supply

Flexor digitorum longus receives arterial supply from the branches of the posterior tibial and fibular arteries. Venous drainage is of this muscle is provided by the peroneal vein, a tributary of the popliteal vein.

Functions

The primary action of the flexor hallucis longus is flexion of all the joints of the great toe (hallux). When the foot is off the ground, this muscle, together with flexor digitorum longus, flexes the toes at the distal phalanges. When the foot is on the ground, the flexor digitorum muscle acts synergistically with small muscles of the foot, to keep the pads of the toes in firm contact with the ground. In addition, it can also contribute to plantar flexion and inversion of the foot, but this action is weak compared to the triceps surae. 

The activity in flexor hallucis longus muscle is usually not extensive during quiet standing, but becomes very active during toe-off and tiptoe movements. The muscle also acts as a bowstring for the medial longitudinal arch, thereby playing a role in its maintenance.