Extensor Digitorum Brevis
| Extensor Digitorum Brevis | |
|---|---|
| Muscle Type | |
| Origin | |
| Insertion | |
| Action | |
| Synergists | |
| Antagonists | |
| Spinal innervation | |
| Peripheral Innervation | |
| Vasculature | |
The extensor digitorum brevis (EDB) is a small muscle on the dorsum of the foot that extends toes 2, 3, and 4 at their metatarsophalangeal joints, aiding the long extensor in toe extension and helping with foot clearance during swing phase.
Muscle Type
EDB is an intrinsic dorsal foot muscle. It appears as a small fleshy bulge on the lateral dorsum of the foot just in front of the ankle. EDB is actually part of the same muscle mass as extensor hallucis brevis (they share origin), but by convention EDB refers to the portion sending tendons to the 2nd, 3rd, and 4th toes. It lies deep to the long extensor tendons (extensor digitorum longus) as they run over the foot.
Origin
EDB originates from the superolateral dorsal surface of the calcaneus, roughly in the area of the sinus tarsi (just anterior to the lateral malleolus). It also takes origin from the nearby inferior extensor retinaculum. The muscle belly then gives rise to four small tendons (the medial-most is often called extensor hallucis brevis, described above, and the other three are for toes 2–4).
Insertion
The extensor digitorum brevis has three tendons (excluding the EHB portion) that insert into the extensor expansion (dorsal aponeurosis) of toes 2, 3, and 4. Specifically, each EDB tendon joins the lateral side of the corresponding extensor digitorum longus (EDL) tendon on the dorsum of those toes. EDB does not typically send a tendon to the little toe (toe 5); extension of the little toe is solely by EDL. The medial slip (EHB) goes to the proximal phalanx of the big toe as discussed. Thus, EDB effectively contributes to the extensor mechanism of the middle three toes (2–4).
Action
EDB assists in extending toes 2, 3, and 4 at the metatarsophalangeal joints (and to a lesser extent at the interphalangeal joints via the extensor expansion). When these toes are being raised (dorsiflexed), EDB provides additional lift to the proximal phalanges. Its action is most significant in the early phase of toe extension or when EDL is slightly slack (as in mid-stance phase of gait). Overall, EDB contributes to toe clearance during swing and helps EDL in fine control of toe extension, especially of the middle toes which EDB targets.
Synergists
EDB works in tandem with the extensor digitorum longus (EDL) for extending the lateral four toes. For toes 2–4, EDL is the primary mover, and EDB is the assistant. EDB contracts at the same time as EDL during toe extension, making the movement smoother. The lumbricals and interossei of the foot, although primarily flexors at the MTP and extensors at IP (via extensor expansion), also interplay with EDB in coordinating toe movements. Additionally, tibialis anterior and extensor hallucis longus are synergistic in overall foot dorsiflexion, which complements toe extension by positioning the foot.
Antagonists
The antagonists of EDB are the toe flexors: the flexor digitorum longus (FDL) and flexor digitorum brevis (FDB) which curl toes 2–5 downward. Specifically, for toes 2–4, FDL (assisted by FDB at the PIP joints) plantarflexes those toe joints, opposing EDB’s extension. The lumbricals and interossei (which also flex the MTP joints) can act as antagonists at the MTP joint level by causing plantarflexion there, though they simultaneously extend the IP joints. In essence, any muscle that contributes to toe plantarflexion (like the small plantar interossei that assist flexion) will antagonize EDB’s dorsiflexion of those joints.
Spinal Innervation
EDB is innervated by nerve fibers from L5 and S1 (primarily L5) via the deep fibular nerve. This innervation is similar to EHB and the other dorsum muscles. Clinically, L5 nerve root function is partially assessed by dorsum foot muscles (with EDB as one component).
Peripheral Innervation
The deep fibular (peroneal) nerve (lateral terminal branch) innervates the extensor digitorum brevis. This nerve branch enters the muscle from its superolateral aspect. The same branch typically supplies both EDB and EHB. In neuropathies affecting the deep fibular nerve (e.g., compartment syndrome or compression at the ankle), EDB can be weak or atrophied.
Vasculature
EDB is supplied by the dorsalis pedis artery, especially its lateral tarsal artery branch. The lateral tarsal artery runs under the extensor retinaculum toward EDB, giving the muscle its blood supply. The arcuate artery (off dorsalis pedis) and fibular artery perforators can also contribute. The rich vascular plexus on the dorsum of the foot (dorsal metatarsal arteries) ensures EDB gets adequate blood. Venous drainage flows into the dorsal venous arch and anterior tibial vein.
Clinical Application
The extensor digitorum brevis muscle forms a palpable mass on the dorsum of the foot when a person extends their toes against resistance – clinicians sometimes use this as a landmark. For instance, EDB is tested by asking a patient to extend their toes; the muscle belly can be felt or observed contracting.
Deep fibular nerve injury or L5 radiculopathy may lead to atrophy of EDB, visible as a depression on the dorsum of the foot and weakness in toe extension. EDB is occasionally involved in injury during foot sprains (it can be contused in ankle inversion injuries due to its location). It can also rarely become hypertrophic and mimic a dorsal foot tumor – an enlarged EDB (especially in athletes) might be mistaken for a ganglion or mass, but it will contract with toe extension confirming it’s muscle.
Because EDB lies under the extensor retinaculum, anterior tarsal tunnel syndrome (compression of deep fibular nerve under the retinaculum) can cause weakness of EDB and EHB along with numbness in the first interdigital space. In clinical neurophysiology, the EDB is often used for nerve conduction studies of the deep peroneal nerve – it’s accessible for electrode placement.
Strengthening of EDB specifically is not a common focus (since EDL largely handles toe extension), but overall dorsiflexion exercises will engage it. Its main clinical significance is as a localizer for nerve or root problems and as a contributor to toe extension symmetry.
