Muscles of the Leg

Knee joint | Main Anatomy Index | Tibiofibular joints

Muscles of the Leg

Compartments of the Leg (pp. 443-4)

• The tibia, fibula, the interosseous membrane, and the crural intermuscular septa divide the leg into three crural compartments; anterior (extensor), lateral (fibular or peroneal), and posterior (flexor).

• The fascial septa, called crural intermuscular septa, are attached superficially to the ensheathing deep fascia and the fibula.

• The anterolateral part of the leg contains the anterior and lateral crural compartments, which are separated by the anterior crural intermuscular septum.
• The muscles in the lateral crural compartment are separated from muscles in the posterior crural compartment by the posterior crural intermuscular septum.

• The much larger posterior compartment of the leg, often called the calf, is subdivided by a broad intermuscular septum (formed by the deep transverse fascia of the leg) into superficial and deep crural muscles, respectively.

• In summary, the leg muscles are arranged in three compartments: the anterior compartment, between the tibia and the anterior crural septum; the posterior compartment, between the tibia and the posterior crural septum; the lateral compartment, between the anterior and posterior crural septa.

• The muscles in a compartment share the same general function, the same nerve supply, and the same blood supply.

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Anterior Compartment of the Leg (p. 444)

• This extensor compartment, located anterior to the interosseous membrane, is located between the lateral surface of the tibia and the anterior crural intermuscular septum.

• The four muscles of the anterior compartment are extensor (dorsiflexor) muscles: tibialis anterior, extensor hallucis longus, extensor digitorum longus, and the fibularis (peroneus) tertius muscles.
• These muscles are mainly concerned with dorsiflexion of the ankle joint and extension of the toes.

• They are supplied by the deep fibular (peroneal) nerve, which is derived from the sciatic via the common fibular (peroneal), and by the anterior tibial artery. The muscles of this group are true extensors of the foot, although their action is usually referred to as dorsiflexion.

The Tibialis Anterior Muscle (p. 444-5)

• This long, thick muscle lies against the lateral surface of the tibia, where it is easy to palpate.

• Proximal attachments are: lateral condyle and superior half of lateral surface of tibia.
• Distal attachments are: medial and inferior surfaces of medial cuneiform bond and base of first metatarsal bone.
• Innervation: deep fibular (peroneal) nerve (L4 and L5)

• It dorsiflexes and inverts the foot.

Paralysis of the tibialis anterior muscle

• It is paralysed when there is injury to the common fibular (peroneal) nerve or to its branch, the deep fibular (peroneal) nerve, resulting in the condition known as foot-drop.

Shin Splits

• This is the common name for the painful condition of the anterior compartment of the leg that follows vigorous and/or lengthy exercise. Often people who lead sedentary lives develop pains in the anterior part of their legs when they take long walks.
• Their anterior tibial muscles swell from sudden overuse and the swollen muscles in the anterior compartment reduce the blood flow to the muscles. Cramps may develop if use of the muscles is continued.

The Extensor Hallucis Longus Muscle (p. 444-5)

• This thin muscle lies between and partly deep to the tibialis anterior and extensor digitorum longus muscles.

• Proximal attachments are: middle part of anterior surface of fibula and interosseous membrane.
• Distal attachments are: dorsal aspect of base of distal phalanx of the great toe (hallux).
• Innervation: deep fibular (peroneal) nerve (L5 and S1)

• It extends the great toe and dorsiflexes the foot.

The Extensor Digitorum Longus Muscle (p. 445-6)

• This pennate (feather-like) muscle lies laterally to the tibialis anterior and can be easily palpated.
• Its tendons may be seen and felt when the toes are dorsiflexed.

• A common synovial sheath surrounds its four tendons, which diverge on the dorsum of the foot as they pass to their distal attachments.
• Each tendon forms a membranous extensor expansion over the dorsum of the proximal phalanx, which divides into two lateral slips and one central slip.
• The central slip inserts into the base of the middle phalanx and the lateral slips converge to insert into the base of the distal phalanx.

• Proximal attachments are: lateral condyle of tibia, superior three-fourths of anterior surface of fibula, and interosseous membrane.
• Distal attachments are: middle and distal phalanges of lateral four digits.
• Innervation: deep fibular (peroneal) nerve (L5 and S1)

• It extends the lateral four digits and dorsiflexes foot.

The Fibularis (Peroneus) Tertius Muscle (p. 445-6)

• This is a small variable muscle and a partially separated part of the extensor digitorum longus muscle.
• The two muscles are fused at their proximal attachments, but distally the tendon of the fibularis tertius does not attach to a digit.

• Proximal attachments are: inferior third of anterior surface of fibula and interosseous membrane.
• Distal attachments are: dorsum of base of fifth metatarsal bone.
• Innervation: deep fibular (peroneal) nerve (L5 and S1)

• It dorsiflexes the foot and aids in eversion.

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Lateral Compartment of the Leg

• This compartment is bounded by the lateral surface of the fibula, the anterior and posterior crural fascia.
• The lateral (fibular or peroneal) compartment contains the fibularis (peroneus) longus and brevis muscles, which plantarflex and everts the foot.
• The superficial fibular (peroneal) nerve, a branch of the common fibular (peroneal) nerve, supplies them.

The Fibularis (Peroneus) Longus Muscle (p. 448)

• This is the longer and more superficial of the two fibularis (peroneal) mm. and it arises much more superiorly on the body of the fibula.
• The fibularis longus is a narrow muscle that extends from the head of the fibula to the sole of the foot.
• Its tendon can be palpated and observed proximal and posterior to the lateral malleolus.
• When standing on one foot, the fibularis longus also helps to stead the leg on the foot.

• The fibularis longus is enclosed in a common synovial sheath with the fibularis (peroneus) brevis muscle.
• It passes inferior to the fibular (peroneal) trochlea on the calcaneus to enter a groove on the anteroinferior aspect of the cuboid bone.
• It then crosses the sole of the foot, running obliquely and distally to reach its distal attachment to the first metatarsal and medial cuneiform bones.

• Proximal attachments are: head and superior two-thirds of lateral surface of fibula.
• Distal attachments are: base of first metatarsal bone and medial cuneiform bone.
• Innervation: superficial fibular (peroneal) nerve (L5, S1, and S2).

• It everts foot and weakly plantarflexes it.

The Fibularis (Peroneus) Brevis Muscle (p. 448)

• This is a fusiform muscle and it lies deep to the fibularis longus.
• As its name indicates, it is shorter than its partner in the lateral compartment of the leg is.

• Its tendon grooves the posterior aspect of the lateral malleolus, where it lies in a common tendon sheath with the fibularis (peroneus) longus.
• The tendon of the fibularis brevis can be easily traced to its distal attachment to the base of the fifth metatarsal bone.

• Proximal attachments are: interior two-thirds of lateral surface of fibula.
• Distal attachments are: dorsal surface of tuberosity on lateral side of base of fifth metatarsal bone.
• Innervation: superficial fibular (peroneal) nerve (L5, S1, S2)

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Posterior Compartment of the Leg

• From medial to lateral, this compartment lies posterior to the tibia, interosseous membrane, fibula, and the posterior crural intermuscular septum.
• The calf muscles in this compartment are divided into superficial and deep groups by the transverse crural intermuscular septum formed by the deep transverse fascia of the leg.

• The superficial group of muscles forms a powerful mass in the calf of the leg, which plantarflexes the foot.
• The large size of these muscles is a human characteristic, which is directly related to our upright stance.
• They are strong and heavy because they support and move the weight of the body.

• The tibial nerve and posterior tibial vessels supply both divisions of the posterior compartment.

The Superficial Group

• The gastrocnemius, soleus, and plantaris comprise the superficial group. The gastrocnemius and soleus form a tripartite muscle that is referred to as the triceps surae, which forms the prominence of the calf.
• These muscles act together in plantarflexing the foot at the ankle joint. They raise the heel against the weight of the body.

• It is the gastrocnemius that produces the rapid movements occurring during running and jumping. "One strolls along with the soleus, but one wins the long jump with the gastrocnemius."

The Gastrocnemius Muscle (pp. 449, 451)

• This is the most superficial of the muscles in the posterior compartment; it forms most of the prominence of the calf.
• It is a fusiform, two-headed, two-joint muscle.

• Its medial head is slightly larger and extends a little more distally than does its lateral head.
• The heads come together at the inferior margin of the popliteal fossa where they form the inferolateral and inferomedial boundaries of this fossa.

• As its fibres are mainly vertical, contraction of this muscle produces rapid movements during running and jumping.
• These muscles help to steady the legs; consequently they are active during standing, even when at ease.
• When standing on toes or when high heels are worn, these muscles are especially active.
• Although the gastrocnemius acts on both the knee and ankle joints, it is unable to exert its full power on both joints at the same time.

• The lateral head of the gastrocnemius often contains a sesamoid bone called a fabella (L. bean), which is close to its proximal attachment.
• The distal attachment of the gastrocnemius to the calcaneus via the tendo calcaneus or calcaneal tendon (Achilles tendon) is shared with the soleus muscle. The inferior expanded end of the tendo calcaneus attaches to the middle of the posterior surface of the calcaneus.
• A tendo calcaneus bursa separates the tendo calcaneus from the superior part of this bony surface.

• Proximal attachments are: lateral head: lateral aspect of lateral condyle of femur; medial head: popliteal surface of femur, superior to medial condyle.
• Distal attachments are: posterior surface of calcaneus via tendo calcaneus.
• Innervation: tibial nerve (S1 and S2)

• Its main actions are plantarflexion of the foot, raising of heel during walking, and flexion of the knee joint.

The Soleus Muscle (pp. 449, 451)

• This broad, flat, fleshy, multipennate muscle was named because of its resemblance to sole, a flat fish.
• It lies deep to the gastrocnemius and can be palpated on each side of it, inferior to the midcalf, when a person is standing on tiptoes.
• The soleus has a horseshoe-shaped proximal attachment to the tibial and fibula.

• It acts with the gastrocnemius in plantarflexing the ankle, but it does not act on the knee joint. It is also concerned with the maintenance of posture by steadying the leg on the foot, e.g., it prevents the body from falling anteriorly when standing.
• Because it is broad and multipennate, it is a very powerful muscle, but its contractions are slower than those of the gastrocnemius.

• Proximal attachments are: posterior aspect of head of fibular, superior fourth of posterior surface of fibula, soleal line, and medial border of tibia.
• Distal attachments are: posterior surface of calcaneus via tendo calcaneus.
• Innervation: tibial nerve (S1 and S2)

• It plantarflexes the foot and steadies the leg on the foot.

The Plantaris Muscle (pp. 449, 451)

• This small muscle is variable in size and extent. It may be absent; sometimes it is double.
• It has a small, fusiform fleshy belly and a long slender tendon, which runs obliquely between the gastrocnemius and soleus muscles.
• This feeble muscle, the rudiment of a large muscle, is of no practical importance, but is clinically significant.

• Proximal attachments are: inferior end of lateral supracondylar line of femur and oblique popliteal ligament.
• It weakly assists gastrocnemius in plantarflexing the foot and the knee joint.

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The Deep Group of Muscles in the Posterior Compartment of the Leg

• Four muscles comprise in this group. These muscles are the popliteus, flexor digitorum longus, flexor hallucis longus, and tibialis posterior.
• The popliteus acts on the knee joint, whereas the other muscles act on the ankle and foot joint.

The Popliteus Muscle (pp. 452, 455)

• This is a flat triangular muscle that forms the floor of the inferior part of the popliteal fossa.
• The proximal attachment of the popliteus is inside the fibrous capsule of the knee joint, deep to the fibular collateral ligament; thus its deep surface is covered by synovial membrane.

• Proximal attachments are: lateral surface of lateral condyle of femur and lateral meniscus.
• Distal attachments are: posterior surface of tibia, superior to soleal line
• Innervation: tibial nerve (L4, L5, and S1)

• The popliteus weakly flexes the knee, but its important action is in unlocking the knee.
• It unlocks the extended leg by rotating the femur laterally on the fixed tibia (e.g., when the foot is set on the ground).
• As the lateral condyle of the femur moves posteriorly, the lateral meniscus of the knee joint is also drawn posteriorly so it will not be injured.

• When the femur is fixed, the popliteus muscle unlocks the locked knee by rotating the tibia medially on the femur.

Flexor Retinaculum of the Ankle

• The tendons of the three deep muscles (flexor hallucis longus, flexor digitorum longus, and tibialis posterior) pass deep to the flexor retinaculum of the ankle.

• The flexor retinaculum is a thickening of the deep fascia of the leg, which passes from the medial side of the calcaneus to the medial malleolus.

The Flexor Hallucis Longus Muscle (pp. 452-457)

• This is the long, powerful flexor of the great toe is the largest of the four deep muscles.
• It lies laterally and is closely attached to the fibula.

• Its tendon passes posterior to the distal end of the tibial and deep to the flexor retinaculum.
• The tendon occupies a shallow groove on the posterior surface of the sustentaculum tali.
• This tendon then crosses deep to the tendon of the flexor digitorum longus in the sole of the foot, giving a tendinous slip to its tendon.
• As it passes the great toe, the tendon runs between two sesamoid bones in the tendons of the flexor hallucis brevis.
• These bones protect the tendon from the pressure of the head of the first metatarsal bone.

• The flexor hallucis longus is the powerful "push-off" muscle during walking, running, and jumping.
• It provides much of the spring to the step.
• The flexor hallucis longus is also important in holding the leg in the normal position on the foot.

• Proximal attachments are: inferior two-thirds of posterior surface of fibula and inferior part of interosseous membrane.
• Distal attachments are: base of distal phalanx of great toe (hallux).
• Innervation: tibial nerve (S2 and S3)

• It flexes the great toe at all joints and plantarflexes foot; and supports the longitudinal arch of foot.

The Flexor Digitorum Longus Muscle (pp. 455, 457)

• This is the long flexor of the lateral four toes and it lies medially and closely attached to the tibia.
• It is smaller than the flexor hallucis longus, even though it moves four digits.

• Its tendon runs inferiorly, passing posterior to the tibialis posterior tendon and the medial malleolus.
• It then passes diagonally in the sole of the foot, superficial to the tendon of the flexor hallucis longus.
• As the tendon reaches the middle of the foot, it divides into four tendons, which pass to the distal phalanges of the four lateral digits.

• Proximal attachments are: medial part of posterior surface of tibia, inferior to soleal line, and by a broad aponeurosis to the fibula.
• Distal attachments are: bases of distal phalanges of lateral four digits.
• Innervation: tibial nerve (S2 and S3)

• It flexes the lateral four digits and plantarflexes foot and also supports the longitudinal arch of the foot.

The Tibialis Posterior Muscle (pp. 455, 457)

• This is a large, fusiform muscle and is the deepest one in the posterior crural compartment.
• It lies between the flexor digitorum longus and the flexor hallucis longus in the same plane as the tibia and fibula.
• Its tendon can be seen and felt posterior to the medial malleolus, especially when the foot is inverted against resistance.

• Proximal attachments are: interosseous membrane, posterior surface of tibia inferior to the soleal line, and posterior surface of fibula.
• Distal attachments are: tuberosity of navicular, cuneiform, and cuboid bones, and bases of second, third, and fourth metatarsal bones.
• Innervation: tibial nerve (L4 and L5)

• It plantarflexes and inverts the foot.

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