Extensor Hallucis Brevis
| Extensor Hallucis Brevis | |
|---|---|
| Muscle Type | |
| Origin | |
| Insertion | |
| Action | |
| Synergists | |
| Antagonists | |
| Spinal innervation | |
| Peripheral Innervation | |
| Vasculature | |
The extensor hallucis brevis (EHB) is a small muscle on the dorsum of the foot that extends the big toe at the metatarsophalangeal joint, essentially being the dorsal counterpart to the short toe flexors.
Muscle Type
Extensor hallucis brevis is one of the intrinsic dorsal foot muscles (actually the medial part of the extensor digitorum brevis muscle mass). It lies on the top of the foot, just in front of the ankle, and is visible as a small bulge on the dorsum when the toes are extended. It is essentially the portion of the extensor digitorum brevis that goes to the great toe, but anatomically it’s often named separately as EHB for the big toe.
Origin
EHB originates from the superolateral (dorsal) surface of the calcaneus, specifically just anterior to the lateral sinus tarsi region (in common with extensor digitorum brevis origin). It also arises from the adjacent inferior extensor retinaculum of the ankle. In essence, EHB shares origin with the extensor digitorum brevis on the calcaneus.
Insertion
The muscle fibers of EHB run medially toward the great toe. It inserts into the dorsal base of the proximal phalanx of the hallux (great toe). Some descriptions say it joins the long extensor tendon (extensor hallucis longus) on the dorsum of the big toe. Functionally, it provides an additional tendon on the dorsal aspect of the big toe’s first joint.
Action
Extensor hallucis brevis extends the great toe at the metatarsophalangeal (MTP) joint. It helps lift the proximal phalanx of the hallux. Unlike the extensor hallucis longus (which extends both the MTP and interphalangeal joints of the big toe), EHB acts only at the MTP joint (since it attaches to the proximal phalanx). Its action is most evident when initiating toe-off: it assists in dorsiflexing the big toe to clear the ground. Overall, it contributes to toe extension and helps the long extensor in controlling toe position during gait (especially in early stance and swing phases).
Synergists
EHB works directly with the extensor hallucis longus (EHL), which is the primary extensor of the big toe. During big toe extension (as in lifting the toe or when kicking or during the swing phase of gait), EHB and EHL contract together to dorsiflex the hallux. The extensor digitorum brevis (EDB) is essentially a lateral counterpart (for toes 2–4) and contracts alongside EHB when all toes are extended. The tibialis anterior is a synergist in dorsiflexing the foot, which indirectly aids toe extension by positioning the foot for clearance.
Antagonists
The antagonists of EHB are the flexors of the great toe, particularly the flexor hallucis brevis (which flexes the MTP joint of the big toe) and, to an extent, the flexor hallucis longus (which flexes the IP joint and assists at MTP). When EHB extends the toe upward, FHL and FHB would oppose by pulling the toe downward (plantarflexing it). Additionally, the intrinsic muscles like the abductor hallucis and adductor hallucis can oppose EHB in stabilizing the toe (though they are not direct flexors, they contribute to toe plantarflexion force at the MTP joint). Gravity and ground resistance also naturally oppose toe extension in weight-bearing.
Spinal Innervation
EHB is innervated by fibers from the L5–S1 spinal segments via the deep fibular (peroneal) nerve. Notably, the L5 root is often tested by big toe extension (primarily EHL, but EHB contributes), so weakness in EHB could be part of an L5 radiculopathy.
Peripheral Innervation
The deep fibular (peroneal) nerve innervates the extensor hallucis brevis. This is the same nerve that supplies the EDL, EHL, and EDB on the dorsum of the foot. Specifically, the lateral branch of the deep fibular nerve (sometimes called the extensor digitorum brevis nerve) innervates EHB and EDB on the dorsum of the foot.
Vasculature
EHB receives blood from the dorsalis pedis artery, particularly its lateral tarsal artery branch and arcuate artery on the dorsum of the foot. The dorsalis pedis (continuation of anterior tibial artery) runs near EHB and gives small branches to it. Venous drainage is via the dorsal venous arch of the foot into the anterior tibial veins.
Clinical Application
Injury or atrophy of extensor hallucis brevis alone is uncommon. However, in foot surgeries or trauma to the dorsum of the foot, the EHB can be damaged or inadvertently cut, which might slightly weaken big toe extension (though EHL typically compensates). During neurological exams, the clinician might palpate the dorsum of the foot for the contraction of EHB when asking the patient to extend the big toe – this can supplement observation of the EHL tendon to assess deep fibular nerve (and L5 root) integrity. Sometimes EHB can be hypertrophied or have accessory slips; rarely, a painful hypertrophy might cause dorsal foot pain or be mistaken for a dorsal foot mass. Because EHB lies under the extensor retinaculum, entrapment or pressure in that area (like tight boots) could theoretically affect its function or cause local discomfort. In flatfoot or certain midfoot injuries, the attachments of EHB (like to the calcaneus or to the hallux dorsum) can be landmarks in imaging. Overall, extensor hallucis brevis is small but functionally assists in toe clearance during gait – in deep fibular nerve injury, patients may have trouble fully extending the toes, and EHB paralysis accompanies EHL paralysis, contributing to a foot drop gait. Strengthening EHB specifically isn’t usually necessary since big toe extension exercises inherently engage both EHL and EHB. It is more relevant as an anatomical and neurological marker than as a standalone muscle of clinical concern.
