Extensor Hallucis Brevis: Anatomy, Function, and Clinical Significance

Where is the Extensor Hallucis Brevis?

The Extensor Hallucis Brevis (EHB) is a small intrinsic muscle located on the dorsal (top) aspect of the foot, specifically within the lateral portion of the Extensor Digitorum Brevis muscle belly, responsible for extending the great toe.

Anatomy and Location

The Extensor Hallucis Brevis (EHB) is an intrinsic foot muscle, meaning it originates and inserts entirely within the foot itself. It is considered a part of the dorsal group of intrinsic foot muscles, often described as the most medial slip of the Extensor Digitorum Brevis (EDB) muscle.

• Origin: The EHB originates from the superior (dorsal) surface of the calcaneus (heel bone), specifically from the anterior part of its superior surface, just lateral to the calcaneal sulcus. This origin point is shared with the Extensor Digitorum Brevis.
• Course: From its origin, the muscle belly of the EHB travels anteromedially (forward and towards the midline of the body) across the dorsum of the foot.
• Insertion: Its tendon inserts onto the dorsal aspect of the base of the proximal phalanx of the great toe (hallux). Unlike the Extensor Hallucis Longus, it does not typically extend to the distal phalanx.

Due to its small size and deep location relative to the extensor tendons of the lower leg, the EHB is not easily palpable as a distinct muscle belly, but its function contributes to the overall movement of the great toe.

Function and Role

The primary action of the Extensor Hallucis Brevis is:

• Extension of the great toe (hallux) at the metatarsophalangeal (MTP) joint. This means it helps to lift the great toe upwards, away from the ground.

While the Extensor Hallucis Longus (EHL), a larger extrinsic muscle originating in the lower leg, is the primary and more powerful extensor of the great toe, the EHB plays an important synergistic role. It contributes to fine motor control and stability of the great toe during various phases of gait, particularly during the swing phase, helping to clear the toe from the ground. It also assists in maintaining the longitudinal arch of the foot during extension.

Relationship to Other Muscles

The Extensor Hallucis Brevis does not act in isolation. It works in concert with, and is anatomically related to, several other muscles:

• Extensor Digitorum Brevis (EDB): The EHB is often considered the most medial part of the EDB. The EDB itself extends the lateral four toes. Both muscles share a common origin on the calcaneus and are innervated by the same nerve.
• Extensor Hallucis Longus (EHL): This is the major extrinsic muscle responsible for great toe extension. The EHL originates from the fibula and interosseous membrane in the lower leg. While the EHL provides the bulk of the power for great toe extension, the EHB fine-tunes the movement and provides additional stability.
• Dorsal Interossei: These muscles also contribute to toe extension, particularly at the MTP joints.

Innervation: The Extensor Hallucis Brevis, along with the Extensor Digitorum Brevis, is innervated by the deep fibular (peroneal) nerve (L5, S1). This nerve branches from the common fibular nerve and supplies the muscles of the anterior compartment of the lower leg and the dorsal intrinsic muscles of the foot.

Clinical Significance and Injury

While direct injuries to the Extensor Hallucis Brevis are less common than those affecting larger, more superficial muscles, its function is vital for normal foot mechanics. Dysfunction or weakness can contribute to:

• Gait abnormalities: Impaired great toe extension can affect the push-off phase of walking and running, potentially leading to compensatory movements.
• Toe deformities: Although less directly implicated than intrinsic plantar muscles, imbalances in extensor strength can contribute to issues like "claw toe" or "hammer toe" over time, especially in conjunction with other muscular imbalances.
• Overuse syndromes: Repetitive dorsiflexion of the ankle and extension of the toes, common in activities like running or jumping, can lead to inflammation or strain in the extensor tendons on the dorsum of the foot, which can sometimes involve the EHB or its surrounding structures.
• Nerve entrapment: Compression of the deep fibular nerve, though rare, can affect the function of the EHB and EDB, leading to weakness or numbness on the top of the foot.

Strengthening and Maintaining Foot Health

Directly isolating and strengthening the Extensor Hallucis Brevis is challenging due to its small size and synergistic action with the Extensor Hallucis Longus. However, general exercises that promote overall foot muscle health and great toe mobility will indirectly benefit the EHB:

• Toe Extension Exercises: Simply lifting the great toe independently from the other toes, or extending all toes upwards, can engage the extensor muscles.
• Foot Intrinsic Muscle Strengthening: Exercises like "toe spreading" or "short foot" exercises (where you try to shorten the foot without curling the toes) can improve overall foot arch control and intrinsic muscle strength, benefiting the entire foot musculature.
• Barefoot Training: Walking or performing light exercises barefoot on varied surfaces can naturally engage and strengthen the intrinsic muscles of the foot, including the EHB, by requiring the foot to adapt and stabilize more actively.
• Ankle Dorsiflexion Exercises: While primarily targeting the tibialis anterior, maintaining good ankle dorsiflexion range of motion is crucial for overall foot function and can indirectly support the extensor muscles.

Maintaining strong and mobile foot musculature, including the often-overlooked Extensor Hallucis Brevis, is fundamental for efficient movement, balance, and preventing common foot pathologies.