Flexor Hallucis Longus
| Flexor Hallucis Longus | |
|---|---|
| Muscle Type | |
| Origin | Inferior two-thirds of the posterior fibula and interosseous membrane |
| Insertion | Base of the distal phalanx of the great toe (plantar surface) |
| Action | Flexes the great toe (MTP and IP joints), assists plantarflexion, supports longitudinal arch |
| Synergists | |
| Antagonists | Extensor Hallucis Longus, Extensor Hallucis Brevis, Tibialis Anterior |
| Spinal innervation | L5, S1, S2 |
| Peripheral Innervation | Tibial nerve |
| Vasculature | Fibular artery; posterior tibial artery; medial plantar artery (distally) |
The flexor hallucis longus (FHL) is a deep posterior leg muscle that flexes the great toe and contributes to ankle plantarflexion, playing a key role in the push-off phase of gait (propelling the body forward by the big toe).
Muscle Type
FHL is a unipennate muscle located in the deep posterior compartment of the leg. It lies lateral to the other deep flexors (flexor digitorum longus and tibialis posterior) in the leg but, intriguingly, crosses to the medial side at the ankle to reach the big toe. Its tendon runs behind the medial malleolus of the ankle (in the tarsal tunnel) and then along the plantar aspect of the foot to the distal phalanx of the hallux (great toe).
Origin
Flexor hallucis longus originates from the inferior two-thirds of the posterior surface of the fibula and the adjacent interosseous membrane of the leg. It also arises from the fascia over the tibialis posterior muscle. This origin on the fibula is lateral among the deep flexors.
Insertion
The FHL tendon courses behind the medial malleolus, then enters the sole of the foot, passing between the sustentaculum tali (a bony shelf on the calcaneus) and the other tendons. It runs in a groove on the talus and calcaneus. FHL inserts on the base of the distal phalanx of the great toe (plantar aspect). Before insertion, it runs between two tiny sesamoid bones under the first metatarsophalangeal joint, which help protect the tendon.
Action
FHL powerfully flexes the great toe at the metatarsophalangeal and interphalangeal joints. This action is crucial for the push-off in walking, running, and jumping, as the big toe is the last part of the foot in contact with the ground. FHL also assists in plantarflexion of the ankle, since it crosses the ankle joint (though its contribution to ankle plantarflexion is secondary to the triceps surae). Additionally, FHL helps support the longitudinal arch of the foot during weight-bearing (by virtue of its tendon under the foot).
Synergists
For great toe flexion, there is no other muscle as primary as FHL (the flexor hallucis brevis in the foot flexes the big toe at the MTP joint, but not the IP joint). However, FHL works with the flexor digitorum longus (FDL) and flexor digitorum brevis in general toe flexion for balance and grip. In ankle plantarflexion, FHL contracts synergistically with the gastrocnemius-soleus complex, tibialis posterior, flexor digitorum longus, and fibularis longus/brevis to point the foot downward. During the push-off phase, tibialis posterior and FHL together stabilize the medial longitudinal arch.
Antagonists
The main antagonist to FHL for big toe movement is the extensor hallucis longus (EHL), which extends (dorsiflexes) the great toe. Extensor hallucis brevis (on the dorsum of the foot) also assists EHL in extending the big toe at the MTP joint. For ankle plantarflexion, the antagonists are the dorsiflexors of the ankle (tibialis anterior, extensor digitorum longus, and EHL), which lift the foot upward.
Spinal Innervation
FHL is innervated by nerve fibers from L5, S1, and S2 (primarily S1–S2) through the tibial nerve. These are the same spinal segments serving most of the deep posterior compartment muscles.
Peripheral Innervation
The tibial nerve provides the motor innervation to FHL. In the lower leg, the tibial nerve gives off a branch specifically to FHL (as well as branches to FDL and tibialis posterior) in the deep posterior compartment. The nerve reaches FHL on its medial aspect as it runs down behind the tibia.
Vasculature
Blood supply to FHL comes from the peroneal (fibular) artery, which runs near the origin of FHL along the posterior fibula, and from branches of the posterior tibial artery. In the foot, the FHL tendon region is supplied by branches of the medial plantar artery. Venous return is via the accompanying veins into the posterior tibial and fibular veins.
Clinical Application
Tightness of FHL can contribute to conditions like hallux rigidus or flexor hallucis longus tendinopathy (sometimes seen in ballet dancers from excessive great toe use, termed “dancer’s tendinitis”). Conversely, paralysis or weakness of FHL (for example, from tibial nerve injury) impairs the push-off strength of the great toe, causing a less powerful gait propulsion. The FHL tendon is sometimes involved in tarsal tunnel syndrome, as it passes through the tarsal tunnel; inflammation of this tendon can contribute to nerve compression. Rupture of FHL is rare, but its tendon can be used in grafting (though less commonly than plantaris). Clinically, the strength of great toe flexion can be tested by asking the patient to flex the big toe against resistance. Also, because FHL supports the arch, weakness may slightly affect arch stability. In a surgical context, FHL tendon transfer is sometimes done to replace a dysfunctional tibialis posterior to restore inversion and arch support. Stretching and strengthening the FHL (via great toe flexion exercises and resisted toe curls) can be important in athletes to ensure strong push-off and prevent injury.
