Flexor Digitorum Brevis
| |
| Flexor Digitorum Brevis | |
|---|---|
| Muscle Type | |
| Origin | Calcaneus |
| Insertion | Middle phalanges of toe 2-5 |
| Action | Flexion of the lateral four digits |
| Synergists | |
| Antagonists | Extensor digitorum longus and Extensor digitorum brevis |
| Spinal innervation | |
| Peripheral Innervation | Medial Plantar Nerve |
| Vasculature | Medial and lateral plantar arteries and plantar arch, plantar metatarsal and plantar digital arteries |
The flexor digitorum brevis (FDB) is a superficial sole muscle that flexes the lateral four toes (digits 2–5) at the proximal interphalangeal joints, aiding in gripping the ground and maintaining balance, and it lies just deep to the plantar fascia.
Muscle Type
FDB is an intrinsic muscle of the foot, specifically in the first (superficial) layer of the plantar muscles, centered in the sole. It is analogous to the flexor digitorum superficialis in the hand. The muscle has a broad, flat belly in the arch of the foot and splits into four tendons that run to toes 2–5. It is just deep to the plantar aponeurosis and shares a common origin area with abductor hallucis and abductor digiti minimi.
Origin
FDB originates from the medial process of the calcaneal tuberosity and from the deep surface of the plantar aponeurosis. Essentially, it starts at the heel bone and the adjacent plantar fascia. It may also have small contributions from the intermuscular septa dividing the foot compartments.
Insertion
Its four tendons pass forward and superficially to the flexor digitorum longus (FDL) tendons. Each FDB tendon splits into two slips at the base of a proximal phalanx to allow the FDL tendon to pass through (this splitting is similar to the hand’s Camper’s chiasm). The two slips then reunite and insert onto the middle phalanx of toes 2 through 5. Thus, FDB tendons attach to the sides of the middle phalanges of the four lesser toes. (They do not reach the distal phalanges; the distal phalanges are flexed by FDL.)
Action
Flexor digitorum brevis flexes the lateral four toes at the proximal interphalangeal (PIP) joints. It also aids in flexing the metatarsophalangeal (MTP) joints of those toes to a lesser degree. By curling the toes, FDB helps the foot grip the ground, which is important for balance, especially on uneven surfaces or when starting push-off. During the stance phase of gait, FDB contracts to help maintain contact of the toes with the ground, contributing to stabilization of the foot’s longitudinal arches.
Synergists
FDB works synergistically with the flexor digitorum longus (FDL) to flex the toes. FDL flexes the distal interphalangeal (DIP) joints and assists at PIP and MTP, while FDB flexes the PIP (and contributes to MTP) – together, they produce a coordinated curling of the toes. The quadratus plantae (flexor accessorius) is a synergist in that it adjusts the pull of FDL to be more in line with the toes, thereby indirectly aiding FDB’s action by ensuring efficient toe flexion. The lumbrical muscles also act synergistically by flexing the MTP joints and extending the IP joints; although lumbricals extend the IPs, their flexion at the MTP helps the overall toe flexion movement in gait. Additionally, during activities like tiptoeing, FDB works with the intrinsic toe flexors (FDL, interossei, flexor digiti minimi brevis) to stabilize and flex the toes.
Antagonists
The antagonists to FDB are the toe extensors: extensor digitorum longus (EDL) and extensor digitorum brevis (for toes 2–4). These muscles lift the toes up at the PIP and MTP joints, opposing the downward flexion by FDB. The lumbricals and interossei, while helping flex the MTP, actually extend the IP joints, so they oppose FDB’s action at the PIP joints (the lumbricals in particular extend the PIP and DIP, balancing FDB’s tendency to flex them too strongly). Thus, the interplay between lumbricals and FDB ensures smooth toe flexion without clawing. In essence, any muscle that extends the toes (like EDL, EDB) or that contributes to IP extension (like lumbricals) can antagonize the pure flexion action of FDB at the PIPs.
Spinal Innervation
FDB is innervated by S1 and S2 nerve fibers (with contributions from L5) via the medial plantar nerve. This innervation corresponds with the major portion of the medial plantar nerve distribution (which is similar to median nerve distribution in hand, covering the first lumbrical and medial muscles).
Peripheral Innervation
The medial plantar nerve (branch of tibial nerve) innervates flexor digitorum brevis. After the tibial nerve divides under the flexor retinaculum, the medial plantar nerve runs deep to abductor hallucis and sends a branch into FDB’s belly. The lateral plantar nerve, in contrast, supplies most of the deeper layer flexors and abductor digiti minimi.
Vasculature
FDB is primarily supplied by the medial plantar artery, with contributions from the lateral plantar artery. The medial plantar artery (from posterior tibial artery) runs with the nerve and sends branches to FDB. Venous return goes through the medial plantar veins into the posterior tibial vein.
Clinical Application
FDB often bears significant load during activities like walking barefoot on uneven surfaces, as it helps the toes grip. In cases of plantar fasciitis, FDB can be involved since it shares origin with the plantar fascia; heel pain sometimes affects this muscle’s origin. Tight or spasmodic FDB can contribute to toe clawing. Strengthening FDB (e.g., towel-curl exercises with the toes) can improve toe grip and arch support in flat-footed individuals. During foot surgery or deep lacerations of the sole, FDB is a key landmark (lying just under the plantar fascia). Its tendons can be targets in hammer toe corrections (though usually the distal long flexor is addressed). A medial plantar nerve lesion can weaken FDB, but the long flexor (FDL via lateral plantar nerve) may partially compensate in toe flexion. Overall, FDB’s health is important for maintaining a supple yet stable toe-off mechanism in gait.
