The anterior muscles of the lower leg
Anatomy and supply
These anterior muscles are a group of extensors at the lower leg. They lie within the anterior compartment located at the ventrolateral region of the lower leg where they form its surface structure. Their tendons are particularly prominent on the dorsum of the foot. All anterior muscles are innervated by the deep fibular nerve (L4-S1). They include:
- Tibialis anterior muscle: runs mediocaudally from the lateral tibia, interosseous membrane and crural fascia. At the height of the lower tibia it merges into a tendon which is led by both extensor retinacula of the foot and finally inserts at the plantar side of the medial cuneiform and first metatarsal bone. The tibialis anterior muscle serves as the leading muscle for the neurovascular pathway running towards the ankle (deep fibular nerve, anterior tibial artery and vein).
- Extensor digitorum longus muscle: originates at the lateral tibial condyle, ventral side of the fibula and interosseous membrane and merges into a tendon above the superior extensor retinaculum. Below the inferior extensor retinaculum it now divides into four tendons inserting at the dorsal aponeurosis and the distal phalanges of the second through fifth toes. About 90% of the people have an additional fifth tendon running towards the lateral foot edge and inserting at the fifth metatarsal bone (fibularis tertius muscle).
- Extensor hallucis longus muscle: has its origin at the medial side of the fibula and interosseous membrane. It merges into a tendon above the superior extensor retinaculum as well. Its insertion is the distal phalanx of the big toe.
The canal-like extensor retinacula are thickenings of the crural fascia anchoring and leading the tendons of the anterior muscles. The superior extensor retinaculum (transverse crural ligament) runs between fibula and tibia proximal to the malleoli. The Y-shaped inferior extensor retinaculum (cruciate crural ligament) lies more distally and stretches between calcaneus, medial malleolus and plantar aponeurosis.
The main task of the anterior muscles of the lower leg is dorsiflexion of the foot. In addition both the extensor digitorum longus and extensor hallucis longus muscles are responsible for the lifting of the toes (extension of the first, middle and end joints). Due to the course of the tendons the anterior muscles also contribute to the movement of the lower ankle joint: The tibialis anterior muscle causes a weak inversion (supination), the extensor digitorum longus muscle a powerful eversion (pronation) and the extensor hallucis longus muscle both an inversion and eversion – depending on the initial situation.
The anterior muscles are indispensable for a natural gait. When paralyzed a dorsal extension of the foot and toe-lifting become practically impossible. In order to prevent the toes from dragging on the floor the affected patients lift up their legs unusually high and place their toes first (steppage gait).
Because of its relatively rigid and inflexible structure the anterior compartment of the lower leg is extraordinarily vulnerable to critical increases of pressure, e.g. due to bleeding or edemas (compartment syndrome). Often they occur after a heavy strain (e.g. a long hike) or trauma. Hereby the muscles swell compromising the nerves and vessels within the compartment which may quickly lead to ischemic muscle necrosis. In this emergency case the muscle compartment has to be immediately decompressed by a surgical incision of the fascias (fasciotomy).