Extensor Hallucis Longus
| Extensor Hallucis Longus | |
|---|---|
| Muscle Type | |
| Origin | Anterior surface of fibula (middle third) and interosseous membrane |
| Insertion | Dorsal aspect of base of distal phalanx of great toe |
| Action | Extends great toe; assists in dorsiflexing ankle |
| Synergists | |
| Antagonists | Flexor Hallucis Longus, Gastrocnemius, Soleus |
| Spinal innervation | |
| Peripheral Innervation | Deep fibular (peroneal) nerve (L5–S1) |
| Vasculature | Anterior tibial artery |
The extensor hallucis longus (EHL) is a thin muscle in the anterior compartment of the leg that extends to the great toe (hallux). It lies deep between the tibialis anterior and extensor digitorum longus. EHL dorsiflexes the ankle and extends the great toe at the metatarsophalangeal and interphalangeal joints. This muscle is crucial for actions like toe clearance during swing and contributes to balanced gait (preventing the toe from dragging). The tendon of EHL is visible on the dorsum of the foot leading to the big toe.
Structure
EHL is a pennate muscle in the anterior compartment of the leg. It is relatively thin and sandwiched between tibialis anterior and extensor digitorum longus in the mid-leg. Its tendon runs on the dorsum of the foot to the big toe.
Origin
It originates from the middle portion of the anterior surface of the fibula and the adjacent interosseous membrane. The origin is slightly distal and lateral to tibialis anterior’s origin. There is typically no tibial origin for EHL – it’s purely fibular.
Insertion
EHL’s tendon crosses anterior to the ankle and inserts on the dorsal base of the distal phalanx of the hallux (big toe). It also usually sends a slip to the base of the proximal phalanx (or via the extensor hood of the great toe). Thus, it spans both joints of the big toe.
Action
It extends (dorsiflexes) the great toe at the IP and MTP joints, meaning it lifts the toe upward. It also contributes to dorsiflexion of the foot at the ankle (though primary dorsiflexor is tibialis anterior). Because of its lateral origin, it can cause slight inversion of the foot along with dorsiflexion (not as much as tibialis anterior, but a minor inversion component). During gait, EHL helps clear the big toe during swing and aids in placing the foot for heel strike. It also is active in actions requiring toe extension like kicking with the dorsum of the foot or balancing on the heel.
Synergists
- For great toe extension: Extensor hallucis brevis (a small muscle on dorsum of foot) assists at the MTP joint.
- For ankle dorsiflexion: EHL works with tibialis anterior and extensor digitorum longus. These three collectively form the dorsiflexor group.
- In toe-off phase of gait, EHL contracts to lift the toe and also to stabilize the toe as it leaves the ground. It synergizes with the intrinsic toe extensors (extensor brevis) and the lumbricals/extensor hood mechanism to extend the big toe smoothly.
Antagonists
- The flexor hallucis longus (FHL) is the primary antagonist at the great toe, flexing the IP and MTP joints (like during push-off or toe gripping). They form a pair controlling toe position (one lifts, one pushes down)【72†L175-L179】.
- The gastrocnemius and soleus muscles antagonize the dorsiflexion at the ankle by plantarflexing. Similarly, tibialis posterior and flexor digitorum longus contribute to toe flexion or foot inversion against EHL's actions.
- When EHL dorsiflexes/inverts, the fibularis longus and brevis (evertors) can counter the inversion aspect to keep the foot balanced.
Spinal Innervation
L5–S1 (via deep fibular nerve).
Peripheral Innervation
Deep fibular (peroneal) nerve innervates EHL. This is the same nerve serving tibialis anterior and extensor digitorum longus, all located in the anterior compartment. With a common fibular nerve injury at the knee, EHL will be paralyzed along with other dorsiflexors, leading to foot drop with inability to extend the toes.
Vasculature
The anterior tibial artery supplies EHL via branches in the anterior compartment. Near the ankle, the dorsalis pedis (continuation of anterior tibial) runs lateral to EHL tendon. Clinically, you palpate dorsalis pedis just lateral to EHL's tendon at the foot dorsum.
Clinical Relevance
Foot Drop and Toe Drag: In deep fibular nerve palsy or L5 radiculopathy, EHL weakness means the patient cannot fully extend the big toe. They may catch the toe on the ground during swing (especially if tibialis anterior is working but EHL is not, the foot might clear but the toe droops). In a complete foot drop, EHL is part of the group of paralyzed muscles. Recovery is assessed by asking the patient to extend the great toe (since EHL is largely L5, it's a key test for L5 nerve root function).
Tendon Injury: The EHL tendon can be cut by lacerations on the dorsum of the foot (for example, someone stepping on broken glass). A severed EHL tendon results in inability to actively dorsiflex the big toe. Surgical repair is typically done, given the importance in gait (untreated leads to toe drag or reliance on extensor hallucis brevis which is not strong enough for IP joint).
Hammer Toe of Hallux: Although less common than lesser toe deformities, an imbalance where EHL overpowers the flexors can contribute to a cock-up deformity of the hallux (like an extension contracture). Typically though, hallux issues (hallux malleus) are more due to other causes (neurologic or compensation from a short first ray).
Gait Initiation: The EHL along with tibialis anterior fires at the end of stance (pre-swing) to initiate toe-off by dorsiflexing the toes and ankle, unweighting the forefoot. In some gait pathologies (like Parkinson's disease), one might see diminished heel strike and toe clearance, partly from reduced EHL activity.
Testing Strength: Clinicians test EHL by asking the patient to lift the big toe against resistance (like pushing the big toe upward). This isolates L5 nerve root often, as tibialis anterior (also L4-L5) might still be strong if there's an L5 partial lesion. In disc herniations, an L4-L5 disc impinging L5 nerve often presents as weak EHL (the patient can't resist downward pressure on the big toe) – it's a classic exam finding.
Exertional Compartment Syndrome: EHL (and EDL) may suffer in chronic anterior compartment syndrome; patients might complain of foot slapping or toe drag when running long distances due to transient nerve/muscle compression. If severe, they might even have transient weakness of toe extension. Conservative measures (rest, stretching) or fasciotomy can be considered.
Spasticity: In upper motor neuron lesions, EHL can contribute to a dystonic extensor response of the toe (a positive Babinski sign is extension of big toe – but that’s a reflex, not EHL per se). However, in conditions like ALS or stroke with spasticity, one might see a “triple extension” synergy: extended knee, plantarflexed ankle, extended hallux due to unopposed extensors. Managing this might involve targeting the long toe extensors with therapy or medications.
Interaction with Extensor Brevis: On the dorsum of the foot, the extensor hallucis brevis arises from calcaneus and inserts into the base of proximal phalanx of hallux, assisting EHL. Sometimes after foot injuries or surgeries, extensor brevis can overwork to compensate for a weak EHL or vice versa. In rehab, ensuring full EHL strength is regained helps reduce strain on the smaller brevis.
