Anatomical Structure and Location of the Flexor Hallucis Longus

Description

Running deep in the posterior compartment of the male leg, the flexor hallucis longus (FHL) forms a fusiform muscle belly on the posterior fibula and adjacent interosseous membrane, lateral to tibialis posterior and posterior to the distal tibia. Distally, its tendon narrows and courses inferiorly behind the medial malleolus within the tarsal tunnel, lying lateral to the flexor digitorum longus tendon and closely related to the posterior tibial artery and tibial nerve. A lateral view also places the fibularis (peroneus) longus superficially along the fibular shaft, contrasting the FHL’s deeper, more medial trajectory as it turns toward the plantar foot. Orientation matters. Following the tendon past the ankle clarifies why FHL is a common source of posteromedial ankle pain and snapping: it glides under the flexor retinaculum, then traverses the posterior talus and enters the plantar midfoot where it can cross the flexor digitorum longus at the chiasma plantare (knot of Henry). Tenosynovitis from repetitive plantarflexion is frequent in ballet dancers and soccer players, and hypertrophy or synovial thickening can contribute to tarsal tunnel symptoms. Surgeons also use this anatomy when planning endoscopic FHL release, harvesting the FHL for Achilles reconstruction, or avoiding iatrogenic injury during posteromedial ankle approaches. Use this illustration for lower limb anatomy teaching in gross anatomy and kinesiology courses, for podiatry and orthopaedic foot and ankle modules on the deep posterior compartment, and for surgical texts explaining tarsal tunnel contents and hallux flexor tendon pathology. It also fits patient-education materials on FHL tendinopathy and posterior ankle impingement. Anatomical accuracy verified by SciePro's Medical Advisory Board.