The flexor digitorum longus is one of three muscles in the lower leg bearing this name: the flexor digitorum longus, flexor digitorum brevis, and flexor digitorum superficialis. Responsible for flexing or curling all toes except the big toe, this muscle is found deep on the inner side of the back of the leg beneath the gastrocnemius and soleus, the large muscles of the calf. It runs downward over the tibia bone, crosses the ankle, and inserts at the base of each toe where it pulls downward on the bottom side of the distal phalange bones during muscle contraction.
Originating on the upper quarter of the posterior or rear side of the tibia, the flexor digitorum longus descends alongside the tibialis posterior, which is more centrally located in the back of the leg. Its fibers insert obliquely along most of its length into a tendon on the muscle’s posterior that crosses the ankle to the rear of the medial malleolus, the large bony protrusion of the tibia felt at the inside of the ankle. From there the tendon passes above the calcaneus, or heel bone, traverses the underside of the foot, and inserts alongside the fibers of the quadratus plantae muscle in the form of four distinct tendons at the bottoms of the distal phalanges. The distal phalanges are the last bones in the toes; the four receiving the tendons of the flexor digitorum longus are those of the second through fifth toes.
In addition to this muscle’s role in flexing the last four toes, it is involved in plantarflexion of the entire foot at the ankle. Primarily the responsibility of the larger gastrocnemius, soleus, and tibialis posterior muscles, plantarflexion is the action of pointing the foot downward at the ankle as seen in a dancer pointing the toes. The flexor digitorum longus aides in this movement largely because it is so intrinsically connected to the latter muscle, the tibialis posterior, in its path across the ankle joint.
As the flexor digitorum longus originates not only on the body of the tibia bone but also on the fascia of the tibialis posterior — the sheath of fibrous tissue enveloping the muscle — and as its tendon runs concurrently with that of the tibialis posterior across the ankle joint, it becomes inherently involved with the action of the tibialis, that of plantarflexion. Also, the tendon is a major connective structure in the medial or inside arch of the foot along with the tendon of the flexor hallucis longus, which flexes the big toe. As such both are involved in supporting and maintaining the arch of the foot.