The flexor digitorum longus runs down the posterior tibia, passes medial to the medial malleolus with the other deep posterior compartment tendons, and divides into four slips under the foot to reach the distal phalanges of the lesser toes. At the level of the midfoot it crosses over the FHL tendon at the master knot of Henry, where the lumbricals and flexor accessorius attach before the tendon continues to the toes.
| Origin | Posterior surface of the tibia below the soleal line |
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| Insertion | Plantar surfaces of the bases of the distal phalanges of toes 2 through 5 |
| Nerve Supply | Tibial nerve (L5, S1) |
| Blood Supply | Posterior tibial artery |
| Actions | Flexion of the lateral four toes at all joints; Assists plantarflexion of the ankle; Assists inversion of the foot; Supports the longitudinal arches during weight-bearing |
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It curls the tips of the lesser toes into the ground during the propulsive phase of gait and running, increasing traction during push-off. Together with the FHL it forms the active component of the toe-off mechanism that provides the final forward thrust in walking and running.
FDL tendinopathy is less common than FHL tenosynovitis but can produce medial ankle pain at the retro- and inframalleolar positions that requires ultrasound or MRI to distinguish from tibialis posterior and FHL pathology. Claw toe and hammer toe deformities result partly from FDL overactivity or contracture relative to the intrinsic muscles of the foot.
The FDL tendon is palpable just posterior to the tibialis posterior tendon at the medial ankle, lying in the middle of the three deep posterior compartment tendons behind the medial malleolus.
Overuse degeneration of the tendon behind the medial malleolus producing medial ankle pain that worsens with running and toe-off activities, distinguished from other medial ankle tendons by its position and the specific reproduction of symptoms with resisted toe flexion.
