Tendon of flexor digitorum longus

The muscle belly of flexor digitorum longus transitions to a tendon (tendon of the flexor digitorum longus (FDL)) in the distal third of the leg, which then passes posterior to the medial malleolus within the tarsal tunnel, deep to the flexor retinaculum. In the plantar aspect of the foot, the FDL tendon crosses superficial to the tendon of the flexor hallucis longus (FHL) at the level of the knot of Henry.

Distally, the FDL tendon divides into four slips, each inserting onto the base of the distal phalanges of the second through fifth toes. The quadratus plantae muscle inserts into the lateral margin of the FDL tendon, modifying its line of pull. Anatomical variations, such as accessory muscular slips or the presence of a flexor digitorum accessorius longus, may alter the course or insertion of the tendon and have clinical implications, including potential contribution to tarsal tunnel syndrome.