Flexor Digitorum Longus
| Flexor Digitorum Longus | |
|---|---|
| Muscle Type | |
| Origin | Posterior surface of the tibia below the soleal line; fascia of tibialis posterior |
| Insertion | Base of the distal phalanges of toes 2–5 (plantar surface) |
| Action | Flexes lateral four toes (DIP, PIP, MTP), assists ankle plantarflexion and inversion, supports longitudinal arch |
| Synergists | |
| Antagonists | Extensor Digitorum Longus, Extensor Digitorum Brevis, Tibialis Anterior |
| Spinal innervation | L5, S1, S2 |
| Peripheral Innervation | Tibial nerve |
| Vasculature | Posterior tibial artery; fibular artery |
The flexor digitorum longus (FDL) is a deep posterior leg muscle that flexes the lateral four toes and assists in ankle plantarflexion, contributing to grip and balance of the foot during the toe-off phase of gait.
Muscle Type
FDL is a long, thin muscle in the deep posterior compartment of the leg, positioned medially (closer to the tibia) among the deep flexors. Its tendon runs behind the medial malleolus, then splits into four tendons in the sole of the foot, each going to one of the distal phalanges of the four lesser toes (toes 2–5). It is analogous in function to the flexor digitorum profundus in the hand, controlling the distal joints of the toes.
Origin
FDL originates from the posterior surface of the tibia, specifically the middle portion just medial to the vertical ridge (and below the soleal line). It does not originate from the fibula (in contrast to FHL which does), making it more medial in origin. Some fibers may also arise from the fascia of tibialis posterior.
Insertion
The tendon of FDL passes behind the medial malleolus in the tarsal tunnel. In the foot, it splits into four slips. Each tendon slip inserts into the base of the distal phalanx of toes 2, 3, 4, and 5 (plantar side). Before inserting, the FDL tendons perforate the tendons of the flexor digitorum brevis (which attach to middle phalanges), similar to the hand’s flexor digitorum profundus passing through the flexor digitorum superficialis. There is often a tendinous connection from FHL to the FDL tendon of the big toe (Master Knot of Henry area in the sole), aiding coordinated toe flexion.
Action
FDL flexes the lateral four toes at the distal interphalangeal (DIP), proximal interphalangeal (PIP), and metatarsophalangeal (MTP) joints. This curling of the toes helps in gripping the ground and maintaining balance, especially on uneven terrain. FDL also contributes to ankle plantarflexion (though weakly, as it’s not as large as gastro-soleus) and helps invert the foot (since it passes medially). During gait, FDL stabilizes the toes against the ground during the toe-off and helps support the longitudinal arches of the foot.
Synergists
FDL works with flexor digitorum brevis (FDB, the intrinsic muscle that flexes the PIP joints) to flex the toes. It also coordinates with quadratus plantae (which attaches to the FDL tendon in the sole to straighten its pull) to flex toes evenly. In plantarflexion, FDL contracts alongside gastrocnemius, soleus, tibialis posterior, FHL, and fibularis longus/brevis. Tibialis posterior and FDL together invert and plantarflex the foot, important for supination. Moreover, during toe-off, FDL’s toe flexion is synergistic with FHL’s big toe flexion to push the body forward.
Antagonists
The antagonists to FDL are the extensor digitorum longus (EDL) and the extensor digitorum brevis for the toes, which extend (dorsiflex) the lateral four toes. At the ankle, the dorsiflexors (tibialis anterior, EDL, extensor hallucis longus) oppose FDL’s plantarflexion action. In eversion, the peroneus tertius and lateral compartment muscles counter the inversion tendency of FDL.
Spinal Innervation
FDL is innervated by nerve fibers primarily from L5 and S1 (with some S2) via the tibial nerve. These segments are consistent with the innervation of the deep posterior compartment of the leg.
Peripheral Innervation
The tibial nerve innervates FDL. Branches to FDL usually arise in the proximal leg from the tibial nerve. The tibial nerve runs adjacent to FDL in the deep compartment and then behind the medial malleolus (tarsal tunnel) where it divides into plantar nerves.
Vasculature
The blood supply to FDL comes from the posterior tibial artery, which runs alongside the muscle in the leg. The fibular (peroneal) artery may also give contributions. In the foot, branches of the medial and lateral plantar arteries supply the FDL tendons and associated structures. Venous drainage flows through the posterior tibial veins.
Clinical Application
FDL plays a role in conditions like hammer toes or claw toes, where there is an imbalance between toe flexors and extensors leading to curled toes; overactivity or tightness of FDL can contribute to such deformities. Injury to the tibial nerve can weaken toe flexion – patients may notice difficulty gripping with toes or maintaining balance on tiptoe (though triceps surae weakness is more prominent). The FDL tendon can be affected in tarsal tunnel syndrome as it runs under the flexor retinaculum. In some flatfoot reconstructions, the FDL tendon may be transferred to replace the function of a diseased tibialis posterior tendon, because FDL can compensate for lost inversion and arch support. Rehabilitation exercises to strengthen FDL include towel curls or marble pickups with the toes, improving toe flexor strength and arch stability. Stretching the toe flexors (by dorsiflexing the ankle and extending the toes) can help relieve foot cramps or tightness. Quadratus plantae dysfunction can cause the FDL to pull the toes medially; this can be assessed in certain foot disorders. Overall, while FDL is less commonly injured in isolation, its function is integral to the fine balance and push-off strength of the foot.
