Flexor digitorum brevis muscle

Origin and insertion

Flexor digitorum brevis muscle originates from three sites; the medial process of calcaneal tuberosity, plantar aponeurosis and intermuscular septum. Its large muscle belly spans across the medial side of the plantar aspect of the foot. Approximately halfway across the sole of the foot, the muscle fibers divide into four slits, each of them giving off its own tendon for one of the lateral four toes. The tendons course over the respective plantar surfaces of lateral four metatarsal bones and proximal phalanges. 

At the level of the corresponding proximal phalanx, each tendon of flexor digitorum brevis divides into two terminal slits that insert onto the base of middle phalanx of the corresponding digit. By inserting to the medial and lateral sides of middle phalanx, each terminal slit bounds a triangular passage with the dorsal surface of its middle phalanx. These passages allow the tendons of flexor digitorum longus to reach their way to their attachment on distal phalanges.

Relations

Flexor digitorum brevis is situated lateral to abductor digiti minimi and medial to abductor hallucis muscles. This muscle runs deep to the thick layer of the plantar aponeurosis. Its tendons run medially to the common plantar digital nerves and vessels. Deep to flexor digitorum brevis you can find quadratus plantae and lumbrical muscles together with the tendons of flexor digitorum longus muscle. 

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Innervation

The innervation for flexor digitorum brevis muscle comes from the medial plantar nerve (S1 - S3) which is the larger of the two terminal branches of the tibial nerve. 

Blood supply

Flexor digitorum brevis muscle is vascularized by the branches of the posterior tibial artery; medial plantar and lateral plantar arteries. The additional blood supply comes from the branches of the anastomotic network of anterior tibial and posterior tibial arteries; plantar metatarsal arteries and common plantar digital arteries.

The majority of blood from flexor digitorum brevis muscle is drained by medial plantar vein that drains into the anterior and posterior tibial veins. Additional drainage is performed by deep plantar venous arch. 

Function

The main action of flexor digitorum brevis is the flexion of second to fifith digits at the metatarsophalangeal and proximal interphalangeal joints. This order of actions is different from flexor digitorum longus muscle which also acts as a flexor of phalanges, but starts with flexion in distal interphalangeal joins. The coordination between these two muscles is crucial for the gait cycle because in synergy they maintain balance by keeping the toes in firm contact with the ground. This muscle also supports the longitudinal arch of foot and stabilizes the foot while walking or running.To expand your knowledge check out the following videos and quizzes and learn the muscles of the foot.