Leg Anatomy: Gastrocnemius, Soleus, Plantaris

• The posterior compartment of the leg is commonly called the calf.
• The skin of the calf is supplied by the posterior femoral cutaneous nerve halfway to the ankle.
• Below that level, the sural and peroneal communicating nerves supply the back and lateral side.
• The saphenous nerve supplies the medial side.
• The small saphenous vein drains the lateral side of the dorsal venous arch and the lateral margin of the foot.
• It ascends with the sural nerve behind the lateral malleolus.
• It pierces the deep fascia from midcalf to the popliteal fossa roof and enters the popliteal vein.
• The small saphenous vein communicates with the great saphenous vein through several channels.
• The deep fascia is a continuation of the popliteal fascia downwards, attaching to the posterior border of the tibia.
• It surrounds the calf, peroneal and extensor compartments, attaching to the anterior border of the tibia.
• The deep fascia attaches to the fibula's periosteum where it becomes subcutaneous.
• The posterior intermuscular septum separates the calf from the peroneal compartment.
• The deep fascia thickens above the heel, attaching to the tibia and fibula across the Achilles tendon.
• This forms a pulley for the tendon separated by a bursa.
• A thickening of fascia, the flexor retinaculum, bridges the deep flexor tendons and neurovascular bundle, stretching from the medial malleolus to the calcaneus.
• The muscles of the posterior compartment are divided into superficial and deep groups.
• The superficial muscles include: gastrocnemius, plantaris, and soleus.
• The deep group includes: popliteus, flexor digitorum longus, flexor hallucis longus, and tibialis posterior.
• The superficial muscles converge on the Achilles tendon and are the main plantarflexors of the ankle joint.
• The deep muscles' tendons pass under the flexor retinaculum into the sole of the foot.
• The tibial nerve innervates the posterior compartment.
• The posterior tibial artery (from the popliteal artery) and its peroneal branch supply the posterior compartment.

Superficial Muscles of the Calf

• Gastrocnemius, plantaris arise from the lower end of the femur.
• The epiphyseal junction passes through the adductor tubercle transversely.
• The lateral head of gastrocnemius arises from the epiphysis.
• The medial head arises from both the epiphysis and the shaft.
• Plantaris arises from the shaft.

Gastrocnemius

• The lateral head arises from the lateral condyle's lateral surface, marked by a smooth pit above the popliteus pit.
• The epicondyle separates the pits.
• Some fleshy fibers arise from the condyle above the pit, near the lower end of the lateral supracondylar line.
• The medial head arises from a smooth shallow pit on the medial condyle at the lower end of the medial supracondylar line.
• Fibers also arise from a roughened area on the popliteal surface of the femur's shaft.
• In the fetus, the medial head originates entirely from the epiphysis.
• Postnatally, the origin extends upwards onto the popliteal surface of the femur's shaft.
• The two heads converge, with a dense aponeurosis beneath them, bearing on the soleus muscle.
• The medial head is longer at each end, extending below the lateral head.
• The flat aponeurosis blends with that of soleus at the lateral head's lower border.
• In the midline, it blends with the soleus aponeurosis via criss-cross fibers.
• The medial half of the aponeurosis remains separate from soleus down to the heel, with the plantaris tendon in between.
• A bursa sits between the medial head and the capsule over the femur's medial condyle.
• It communicates with the knee joint and may also connect with the semimembranosus bursa.

Achilles Tendon Formation

• Aponeurosis forms the Achilles tendon with that of the soleus.
• It inserts into a smooth transverse area on the middle third of the calcaneus's posterior surface.
• A bursa is located is between the tendon and the upper calcaneus.
• Another bursa lies between the tendon and the thickened deep fascia 2 inches above its insertion, where inflammation is often misdiagnosed as tenosynovitis.
• There is no synovial sheath around the Achilles tendon.

Plantaris

• A vestigial muscle, shows a short belly and long tendon that indicates phylogenetic degeneration.
• It arises from the femur's shaft at the lower part of the lateral supracondylar line, next to the lateral head of gastrocnemius.
• Its slender tendon runs deep to the medial head of gastrocnemius and down the calf's midline, between the aponeuroses of gastrocnemius and soleus.
• It inserts into the calcaneus on the medial side of the Achilles tendon.
• The tendon is flat between the aponeuroses but unravels into a wide, spirally twisted ribbon.

Soleus

• It has a morphological resemblance to flexor digitorum superficialis in the forearm.
• Structurally, flexor digitorum superficialis has parallel fibers for the range of movement, while soleus is multipennate for power.
• The flexor digitorum brevis muscle in the sole is considered the distal part of soleus, separated by the posterior projection of the calcaneus.
• The muscle arises from the fibula and tibia.
• The upper quarter of the fibula gives some origin to the muscle.
• A fibrous arch bridges over the popliteal vessels and tibial nerve and carries the muscle to the soleal line of the tibia and the fascia on popliteus above this line.
• It extends along the posterior border of the middle third of the tibia.
• The soleus muscle has a flat structure with a dense aponeurosis on either surface.
• The bulk of the soleus comprises muscle fibers sloping downwards from the anterior to the posterior lamella.
• Fleshy fibers are visible at the medial and lateral borders.
• The posterior lamella continues into the Achilles tendon.
• Muscle fibers of soleus insert into its deep surface down to near the calcaneus.
• The Achilles tendon inserts onto a smooth transverse area across the middle third of the calcaneus's posterior surface.
• Detaching soleus from its fibular origin reveals the anterior surface.
• A slender bipennate muscle belly attaches to the aponeurosis's center.
• This belly lies on the neurovascular bundle, in the groove between flexor digitorum longus and flexor hallucis longus.
• Branches from the posterior tibial vessels perforate the aponeurosis.
• Perforating veins from the great saphenous vein enter the soleus substance.
• The muscle contains a plexus of small veins that are emptied by muscle contraction, thus aiding venous return.
• Stagnation in these veins predisposes to deep venous thrombosis and pulmonary embolism.
• The soleal pump is aided by the "sole pump".

Nerve Supply and Action

• The tibial nerve (S1, 2) supplies all three muscles.
• Each head of gastrocnemius receives a branch from the nerve in the popliteal fossa, where the small branch to plantaris also arises.
• Soleus receives two branches, one from above in the popliteal fossa and one on its deep surface in the calf.
• Both soleus and gastrocnemius act together on the Achilles tendon; the action of plantaris is negligible.
• Soleus is a powerful antigravity muscle.
• Soleus contracts alternately with leg extensors to maintain balance while standing.
• Soleus is a strong but relatively slow plantar flexor of the ankle joint due to the obliquity of its multipennate fibers.
• Gastrocnemius fibers lie roughly vertical in the leg's long axis.
• Gastrocnemius provides the "whip-like" contraction that aids rapid propulsion.
• The whip-like contraction is increased if the flexed knee extends simultaneously because the muscle's origin is pulled upwards.
• In propulsion, the powerful soleus overcomes the inertia of the body weight, while the quicker-acting gastrocnemius increases the speed of movement.
• Gastrocnemius can assist in knee flexion, but then its maximum force cannot be applied to plantarflexion, and vice versa.

Test

• Foot plantarflexed is against resistance.
• The Achilles tendon and muscles can be seen and palpated.

Deep Muscles

• Flexor digitorum longus, flexor hallucis longus, and tibialis posterior.
• Their tendons pass under the flexor retinaculum into the sole of the foot.
• Tibialis posterior and flexor hallucis longus tendons run parallel, tibialis medial to hallucis.
• Flexor digitorum longus takes an oblique course superficial to both.
• In the calf, flexor digitorum longus is medial, crossing tibialis posterior to lie under the flexor retinaculum between the two.
• In the sole, it crosses flexor hallucis longus to pass to the lateral four toes.

Flexor Digitorum Longus

• A bipennate muscle that arises from both bones of the leg.
• Fibers arise from the posterior surface of the tibia below the soleal line.
• It also arises from the fibula by a broad aponeurosis.
• The tendon forms centrally and slopes downwards across the tendon of tibialis posterior in the lower leg.
• Passing beneath the flexor retinaculum, it enters the sole of the foot and crosses flexor hallucis longus.
• At this point, it divides into four tendons, the medial two receiving a strong slip from the tendon of flexor hallucis longus.
• The four tendons receive the insertion of the flexor accessorius muscle at their commencement.
• Each gives origin to a lumbrical muscle more distally.
• The tendons pass into the fibrous flexor sheaths of the lateral four toes, perforate the tendons of flexor digitorum brevis, and are inserted into the bases of the distal phalanges.
• The tibial nerve (S1, 2) supplies flexor digitorum longus.
• Flexor digitorum longus plantarflexes the lateral four toes and, secondarily, the ankle joint.
• Its tonus, with that of the other deep calf muscles, maintains the longitudinal arch of the foot.

Flexor Hallucis Longus

• The bulkiest and most powerful of the deep calf muscles.
• A multipennate muscle whose fibers arise from the flexor surface of the fibula and the adjoining aponeurosis of flexor digitorum longus.
• Below, the muscle arises from the fibula's whole flexor surface and the lower part of the interosseous membrane.
• The fibers spiral down to insert into a central tendon that escapes from the muscle just at the lower end of the tibia.
• The tendon grooves the posterior process of the talus and the under surface of the sustentaculum tali.
• It passes directly forwards beneath the arched medial border of the foot to insert into the distal phalanx base of the great toe.
• It is crossed in the sole by the tendons of flexor digitorum longus and gives a strong slip to the medial two.
• The peroneal artery runs down deep to it on the fibular aponeurosis of flexor digitorum longus and is covered by a fibrous roof.
• The tibial nerve (S1, 2) supplies flexor hallucis longus.
• Flexor hallucis longus flexes the great toe, the point to leave the ground in propulsion.
• It plantarflexes the ankle joint simultaneously.
• The pull of this muscle maintains the medial longitudinal arch of the foot.
• The terminal phalanges of the great toe are flexed against resistance for flexor hallucis longus for a test.

Tibialis Posterior

• Arises from the interosseous membrane and the adjoining surface of both leg bones below the origin of soleus and the fibular aponeurosis of flexor digitorum longus.
• It is pierced by the nutrient artery of the tibia.
• It is a unipennate muscle; the tendon emerges from its medial side and lies with the tibia's edge.
• The tendon grooves the back of the medial malleolus.
• It passes forward above the medial side of the sustentaculum tali and inserts into the tuberosity of the navicular.
• The expansions that pass from its insertion on the navicular to other tarsal bones are ligaments.
• Its blood supply derives from the peroneal artery through branches that pierce flexor digitorum longus.
• The tibial nerve (L4) supplies Tibialis posterior.
• Tibialis posterior inverts and adducts the forefoot and plantarflexes the ankle joint.
• With the foot in slight plantarflexion, it is inverted against resistance.
• The tendon can be seen and felt behind the medial malleolus.

Posterior Tibial Artery

• Arises at the lower border of the popliteus, where the popliteal artery divides into anterior and posterior tibial branches.
• It passes under the fibrous arch in the origin of soleus and runs down on the fibular aponeurosis of flexor digitorum longus, between the muscle bellies of that muscle and flexor hallucis longus.
• Ends under the flexor retinaculum by dividing into medial and lateral plantar arteries.
• It accompanies through its course by a pair of venae comitantes.
• Pulsation is felt at the level of the medial malleolus, 2.5 cm in front of the medial border of the tendo calcaneus.
• It can be exposed here to make an arteriovenous shunt with the great saphenous vein for hemodialysis.
• Its nutrient artery to the tibia is the largest in either limb.
• It pierces the fibular aponeurosis of flexor digitorum longus, passes medially beneath the muscle, and then downwards into the bone between the tibial origins of flexor digitorum longus and tibialis posterior.
• Muscular branches supply the overlying soleus, Achilles tendon, and skin.

Peroneal Artery

• Arises an inch below the posterior tibial commencement and runs into a tunnel whose fibrous roof gives attachment to muscle fibers of flexor hallucis longus.
• Gives muscular branches that perforate the fibular aponeurosis of flexor digitorum longus to supply tibialis posterior and wind around the fibula to supply peroneus longus and brevis.
• It gives a nutrient artery to the fibula.
• Ends by dividing into a perforating branch which pierces the interosseous membrane to enter the extensor compartment and a lateral calcanean branch.

Tibial Nerve

• Runs straight down the midline of the calf, deep to soleus, lying on the fibular aponeurosis of flexor digitorum longus.
• The posterior tibial artery is lateral to it, but passes deep to it at the origin of its peroneal branch and continues downwards on its medial side.
• The nerve ends under the middle of the flexor retinaculum by dividing into the medial and lateral plantar nerves.
• Its surface marking is from the middle of the popliteal fossa to behind the medial malleolus.
• It is the nerve of the flexor compartment, giving muscular branches to the deep surface of soleus, to flexors digitorum longus and hallucis longus, and tibialis posterior.
• It gives off several cutaneous twigs, the medial calcanean nerves, to the skin of the heel, including the weight-bearing surface.

Explore the anatomy of the gastrocnemius, soleus, and plantaris muscles in the leg. Learn about their origins, insertions, relative positions, and unique characteristics. Understand the relationship between these muscles and surrounding structures.