Flexor Digitorum Longus Muscle Under the Skin, Medial View

Description

Running along the deep posterior compartment, the flexor digitorum longus (FDL) is exposed from a medial aspect beneath the skin of an adult male lower leg and foot. Its muscle belly arises from the posterior surface of the tibia, positioned lateral to the tibial crest and posterior to the medial border of the tibial shaft, then narrows into a tendon that passes posterior to the medial malleolus. Medial and slightly anterior to the Achilles tendon, the FDL lies superficial to the tibialis posterior proximally, and in the retromalleolar region it travels with the posterior tibial artery and tibial nerve, with flexor hallucis longus (FHL) positioned more laterally. Medial visualization of the FDL tendon is the teaching view for the tarsal tunnel, where the tendon runs in a fibro-osseous canal bounded by the flexor retinaculum and the medial malleolus and calcaneus. Space is tight. Swelling from tenosynovitis, post-traumatic scarring, or ganglion formation can alter tendon gliding and contribute to tibial nerve compression, and the relative order of structures (tibialis posterior, FDL, posterior tibial vessels, tibial nerve, FHL) matters when interpreting medial ankle pain or planning release. Distally, the FDL crosses the plantar foot toward the lateral four toes and intersects the FHL at the knot of Henry, a common landmark during tendon transfer and reconstruction for posterior tibial tendon dysfunction and adult-acquired flatfoot. Use this asset to illustrate deep posterior compartment anatomy in gross anatomy, podiatric medicine, and orthopedic foot and ankle modules, or to support surgical atlases describing tarsal tunnel decompression and tendon transfer pathways. It also fits radiology teaching files that correlate medial ankle MRI or ultrasound with the course of the FDL tendon beneath the flexor retinaculum. Anatomical accuracy verified by SciePro's Medical Advisory Board.