Flexor Hallucis Longus

Description

Posterior lower leg anatomy is centered on the flexor hallucis longus (FHL), a deep posterior compartment muscle arising from the inferior two thirds of the posterior fibula and adjacent interosseous membrane. Its fusiform belly lies lateral to tibialis posterior and posterior to the tibia, then narrows into a tendon that descends toward the ankle between the tibia (medial) and fibula (lateral). Distally, the FHL tendon courses posterior to the talus, passes in the retromalleolar groove behind the medial malleolus, and turns plantarward beneath the sustentaculum tali en route to the base of the distal phalanx of the hallux. Bone landmarks include the partially visible distal femur at the knee, the tibia and fibula along the leg, and the tarsals, metatarsals, and phalanges of the foot. That tendon path matters. At the ankle it shares the tarsal tunnel region with flexor digitorum longus and tibialis posterior, so synovitis, scarring, or space-occupying lesions can produce painful stenosing tenosynovitis and “trigger hallux,” classically in ballet dancers and long-distance runners. Posterior ankle impingement, including an os trigonum, often irritates the FHL as it glides behind the talus, and surgeons use the FHL tendon as a landmark during posterior ankle endoscopy and as a transfer option for chronic Achilles tendinopathy or rupture. Small space, big consequences. Use this illustration for teaching deep posterior compartment anatomy in gross anatomy and kinesiology courses, for podiatry and sports medicine lectures on FHL tendinopathy and tarsal tunnel-related pain patterns, and for orthopedic or foot-and-ankle publications explaining posterior ankle approaches and tendon transfer planning. Anatomical accuracy verified by SciePro's Medical Advisory Board.