Flexor Hallucis Longus – Anatomy, Nerve Supply, Function

Flexor Hallucis Longus Muscle/The flexor hallucis longus muscle (FHL) is one of the three deep muscles of the posterior compartment of the leg that attaches to the plantar surface of the distal phalanx of the great toe. The other deep muscles are the flexor digitorum longus and tibialis posterior; the tibialis posterior is the most powerful of these deep muscles. All three muscles are innervated by the tibial nerve which comprises half of the sciatic nerve.^[rx]

The flexor hallucis longus (FHL) muscle is one of the four muscles that compose the deep posterior compartment of the lower limb. The other three deep muscles are the flexor digitorum longus (FDL), tibialis posterior, and popliteus muscles. The flexor hallucis longus originates at the posterior surface of the fibula, inserting at the plantar aspect at the base of the distal phalanx of the great toe. The flexor hallucis longus’ primary function is flexion of the great toe, also serving to plantarflex and invert the foot. The antagonistic muscle is the extensor hallucis longus (EHL).

Anatomy of Flexor Hallucis Longus

The flexor hallucis longus is situated on the fibular side of the leg. It arises from the inferior two-thirds of the posterior surface of the body of the fibula, with the exception of 2.5 cm. at its lowest part; from the lower part of the interosseous membrane; from an intermuscular septum between it and the peroneus muscles, laterally, and from the fascia covering the tibialis posterior, medially.

The fibers pass obliquely downward and backward, where it passes through the tarsal tunnel on the medial side of the foot and end in a tendon which occupies nearly the whole length of the posterior surface of the muscle.

This tendon lies in a groove which crosses the posterior surface of the lower end of the tibia, between the medial and lateral tubercles of the posterior surface of the talus, and the undersurface of the sustentaculum tali of the calcaneus; in the sole of the foot it runs forward between the two heads of the flexor hallucis brevis, and is inserted into the base of the last phalanx of the great toe. The grooves on the talus and calcaneus, which contain the tendon of the muscle, are converted by tendinous fibers into distinct canals, lined by a mucous sheath.

As the tendon passes forward in the sole of the foot, it is situated above and crosses from the lateral to the medial side of the tendon of the flexor digitorum longus, to which it is connected by a fibrous slip.

• Origin: inferior two-thirds of the posterior fibula
• Insertion: The plantar surface of the base of the distal phalanx of the great toe
• Action: Flex the great toe but may minimally supinate and plantarflex the ankle
• Blood Supply: Peroneal and posterior tibial artery
• Innervation: Tibial nerve

Origin and Insertion of Flexor Hallucis Longus

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 the 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 function of Flexor Hallucis Longus

• Similar to the flexor digitorum longus and tibialis posterior muscles, the flexor hallucis longus muscle functions to plantarflex and invert the foot. However, it is unique in that it also functions to flex the great toe and helps supinate the ankle.^[rx]
• 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.

References

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