Anterior Compartment of Leg

The anterior compartment of leg, or dorsiflexor (extensor) compartment, is located anterior to the interosseous membrane, between the lateral surface of the tibia shaft and the medial surface of the fibula shaft.

The anterior compartment of leg is bounded anteriorly by the deep fascia of the leg and skin. The deep fascia overlying the anterior compartment is dense superiorly, providing part of the proximal attachment of the muscle immediately deep to it. With unyielding structures on three sides (the two bones and the interosseous membrane) and a dense fascia on the remaining side, the relatively small anterior compartment is especially confined and therefore most susceptible to compartment syndromes.

Inferiorly, two band-like thickenings of the fascia form retinacula that bind the tendons of the anterior compartment of leg muscles before and after they cross the ankle joint, preventing them from bowstringing anteriorly during dorsiflexion of the joint:

1. The superior extensor retinaculum is a strong, broad band of deep fascia, passing from the fibula to the tibia, proximal to the malleoli.
2. The inferior extensor retinaculum, a Y-shaped band of deep fascia, attaches laterally to the anterosuperior surface of the calcaneus. It forms a strong loop around the tendons of the fibularis tertius and the extensor digitorum longus muscles.

Muscles of Anterior Compartment of Leg

The four muscles in the anterior compartment of leg are:

1. The tibialis anterior,
2. Extensor digitorum longus,
3. Extensor hallucis longus,
4. Fibularis tertius.

See Also: Extensor hallucis longus
See Also: Fibularis tertius

These muscles pass and insert anterior to the transversely oriented axis of the ankle (talocrural) joint and, therefore, are dorsiflexors of the ankle joint, elevating the forefoot and depressing the heel. The long extensors also pass along and attach to the dorsal aspect of the digits and are thus extensors (elevators) of the toes.

Although it is a relatively weak and short movement – only about a quarter the strength of plantarflexion, with a range of about 20° from neutral – dorsiflexion is actively used in the swing phase of walking, when concentric contraction keeps the forefoot elevated to clear the ground as the free limb swings forward. Immediately after, in the stance phase, eccentric contraction of the tibialis anterior controls the lowering of the forefoot to the floor following heel strike. The latter is important to a smooth gait and is important to deceleration (braking) relative to running and walking downhill.

During standing, the dorsiflexors reflexively pull the leg (and thus the center of gravity) anteriorly on the fixed foot when the body starts to lean (the center of gravity begins to shift too far) posteriorly. When descending a slope, especially if the surface is loose (sand, gravel, or snow), dorsiflexion is used to “dig in” one’s heels.

See Also: Lateral Compartment of Leg

Tibialis Anterior

The tibialis anterior (TA), the most medial and superficial dorsiflexor of anterior leg muscles, is a slender muscle that lies against the lateral surface of the tibia. The long tendon of TA begins halfway down the leg and descends along the anterior surface of the tibia. Its tendon passes within its own synovial sheath deep to the superior and inferior extensor retinacula to its attachment on the medial side of the foot.

In so doing, its tendon is located farthest from the axis of the ankle joint, giving it the most mechanical advantage and making it the strongest dorsiflexor. Although antagonists at the ankle joint, TA and the tibialis posterior (in the posterior compartment) both cross the subtalar and transverse tarsal joints to attach to the medial border of the foot. Thus, they act synergistically to invert the foot.

To test the TA, the person is asked to stand on their heels or dorsiflex the foot against resistance; if normal, its tendon can be seen and palpated.

Extensor Digitorum Longus

The extensor digitorum longus (EDL) is the most lateral of the anterior leg muscles. A small part of the proximal attachment of the muscle is to the lateral tibial condyle; however, most of it attaches to the medial surface of the fibula and the superior part of the anterior surface of the interosseous membrane.

The EDL becomes tendinous superior to the ankle, forming four tendons that attach to the phalanges of the lateral four toes. A common synovial sheath surrounds the four tendons of the EDL (plus that of the fibularis tertius) as they diverge on the dorsum of the foot and pass to their distal attachments.

Each tendon of EDL forms a membranous extensor expansion (dorsal aponeurosis) over the dorsum of the proximal phalanx of the toe, which divides into two lateral bands and one central band. The central band inserts into the base of the middle phalanx, and the lateral slips converge to insert into the base of the distal phalanx. To test the EDL, the lateral four toes are dorsiflexed against resistance; if acting normally, the tendons can be seen and palpated.

Fibularis Tertius

The fibularis tertius (FT) is a separated part of EDL, which shares its synovial sheath. Proximally, the attachments and fleshy parts of the EDL and FT are continuous; however, distally, the FT tendon is separate and attaches to the 5th metatarsal, not to a phalanx. Although FT contributes (weakly) to dorsiflexion, it also acts at the subtalar and transverse tarsal joints, contributing to eversion (pronation) of the foot. It may play a special proprioceptive role in sensing sudden inversion and then contracting reflexively to protect the anterior tibiofibular ligament, the most commonly sprained ligament of the body. FT is not always present.

Extensor Hallucis Longus

The extensor hallucis longus (EHL) is a this muscle that lies deeply between the TA and EDL at its superior attachment to the middle half of the fibula and interosseous membrane. EHL rises to the surface in the distal third of the leg, passing deep to the extensor retinacula. It courses distally along the crest of the dorsum of the foot to the great toe. To test the EHL, the great toe is dorsiflexed against resistance; if acting normally, its entire tendon can be seen and palpated.

Muscle	Proximal Attachment	Distal Attachment	Innervation	Main Action
Tibialis anterior	Lateral condyle and superior hall of lateral surface of tibia and interos­seous membrane	Medial and inferior surfaces of medial cuneiform and base of 1st metatarsal	Deep fibular nerve (L4. L5)	Dorsiflexes ankle and Inverts foot
Extensor dlgitorum longus	Lateral condyle of tibia and superior three quarters of medial surface of fibula and interosseous membrane	Middle and distal phalanges of lateral four digits	Deep fibular nerve (L4. L5)	Extends lateral four digits and dorsiflexes ankle
Extensor hallucis longus	Middle part of anterior surface of fibula and interosseous membrane	Dorsal aspect of base of distal phalanx of great toe (hallux)	Deep fibular nerve (L4. L5)	Extends great toe and dorsiflexes ankle
Fibularis tertius	Inferior third of anterior surface of fibula and interosseous membrane	Dorsum of base of 5th metatarsal	Deep fibular nerve (L4. L5)	Everts foot and weakly plantarflexes ankle

Anterior Compartment of Leg Nerves

The deep fibular (peroneal) nerve is the nerve of the anterior compartment of leg. It is one of the two terminal branches of the common fibular nerve, arising between the fibularis longus muscle and the neck of the fibula. After its entry into the anterior compartment of leg, the deep fibular nerve accompanies the anterior tibial artery, first between the TA and EDL and then between the TA and EHL. The deep fibular nerve then exits the compartment, continuing across the ankle joint to supply intrinsic muscles (extensors digitorum and hallucis brevis), and a small area of the skin of the foot. A lesion of this nerve results in an inability to dorsiflex the ankle (footdrop).

Nerve	Origin	Course	Distribution in Leg
Saphenous	Femoral nerve	Descends with femoral vessels through femoral triangle and adductor canal and then descends with great saphenous vein	Supplies skin on medial side of ankle and foot
Sural	Usually arises from branches of both tibial and common fibular nerves	Descends between heads of gastrocnemius and becomes superficial at middle of leg; descends with small saphenous vein and passes inferior to lateral malleolus to lateral side of foot	Supplies skin on posterior and lateral aspects of leg and lateral side of foot
Tibial	Sciatic nerve	Forms as sciatic bifurcates at apex of popliteal fossa: descends through popliteal fossa and lies on popliteus; runs inferioriy on tibialis posterior with posterior tibial vessels; terminates beneath flexor retinaculum by dividing into medial and lateral plantar nerves	Supplies postenor muscles of leg and knee joint
Common fibular (peroneal)	Sciatic nerve	Forms as sciatic bifurcates at apex of popli­teal fossa and follows medial border of biceps femoris and its tendon; passes over poste­rior aspect of head of fibula and then winds around neck of fibula deep to fibularis longus. where it divides into deep and superficial fibular nerves	Supplies skin on lateral part of posterior aspect of leg via the lateral sural cutane­ous nerve; also supplies knee joint via its articular branch
Superficial fibular (peroneal)	Common fibular nerve	Arises between fibularis longus and neck of fibula and descends in lateral compartment of leg; pierces deep fascia at distal third of leg to become subcutaneous	Supplies fibularis longus and brevis and skin on distal third of anterior surface of leg and dorsum of foot
Deep fibular (peroneal)	Common fibular nerve	Arises between fibularis longus and neck of fibula; passes through extensor digitorum lon­gus and descends on interosseous membrane; crosses distal end of tibia and enters dorsum of foot	Supplies anterior muscles of leg. dor­sum of foot, and skin of first interdigital cleft; sends articular branches to joints it crosses

Anterior Compartment of Leg Artery

The anterior tibial artery supplies structures in the anterior compartment of leg. The smaller terminal branch of the popliteal artery, the anterior tibial artery, begins at the inferior border of the popliteus muscle (i.e., as the popliteal artery passes deep to the tendinous arch of the soleus). The artery immediately passes anteriorly through a gap in the superior part of the interosseous membrane to descend on the anterior surface of this membrane between the TA and EDL muscles. At the ankle joint, midway between the malleoli, the anterior tibial artery changes names, becoming the dorsalis pedis artery (dorsal artery of the foot).

References

1. Lezak B, Summers S. Anatomy, Bony Pelvis and Lower Limb, Leg Anterior Compartment. [Updated 2021 Aug 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539725/
2. Clinically Oriented Anatomy,8th Edition (2017)