Leg and Foot

Similar to the thigh, the leg also has three fascial compartments; anterior, posterior, and lateral. The posterior compartment is divided into superficial and deep muscle groups. The muscles in the sole of the foot are described in 4 layers, from superficial to deep. There is a heavy aponeurosis in the sole of the foot, the plantar aponeurosis.

Leg

Overview

There are three or four fascial compartments of the leg: lateral, anterior, and posterior (the posterior compartment is sometimes divided into superficial and deep compartments). The lateral compartment is small, and contains only 2 muscles. The anterior compartment contains 3 muscles, including the tibialis anterior m., which is most famous for causing shin splints. The posterior compartment contains 7 muscles, 3 in a superficial group (“calf muscles”), and 4 in a deep group. The compartments of the leg are encased in strong connective tissue, similar to the thigh.

The popliteal a. splits to form the anterior tibial a. and the posterior tibial a. The anterior tibial a. supplies blood to the anterior compartment. The posterior tibial a. supplies blood to the posterior compartment, and gives off a branch (the fibular a. or peroneal a.) that supplies the lateral compartment.

The sciatic n. splits to the form the common peroneal n. and the tibial n. The tibial n. innervates the 7 muscles of the posterior compartment. The common peroneal n. splits to form the superficial peroneal n and the deep peroneal n. The superficial peroneal n. innervates the 2 muscles of the lateral compartment. The deep peroneal n. innervates the 3 muscles of the anterior compartment.

Figure 2.1 Cross-section through the leg

Posterior compartment of the leg, superficial layer

The most superficial muscle of the posterior compartment is the gastrocnemius m. The gastrocnemius m. has a medial and a lateral head that form the inferior borders of the popliteal fossa. Both heads insert into the calcaneal tendon, or achilles tendon. The gastrocnemius m. is the muscle that forms a bulge in the superior calf when you stand on your toes.

Deep to the gastrocnemius m. is the much larger soleus m. The soleus m. also inserts into the calcaneal tendon. The soleus is a postural muscle, constantly active while you are standing to counter the force of gravity and prevent dorsiflexion at the ankle.

Between the gastrocnemius m. and the soleus m. is the plantaris m., a very small muscle belly with a long tendon that joins the calcaneal tendon. The plantaris m. is analogous to the palmaris longus m. of the forearm.

Figure 2.2 Superficial muscles of the posterior compartment of the leg. The gastrocnemius muscle has been removed in the illustration on the right to view the underlying soleus, plantaris, and popliteus.

Posterior compartment of the leg, deep layer

Reflect the superficial muscles to view the deep muscles of the posterior compartment of the leg. Take care to preserve the tibial n. and posterior tibial a., both run between the superficial and deep muscles in the posterior compartment.

Dissect all the way down to the bone posterior to the knee and you’ll find the popliteus m. The popliteus m. runs obliquely from the lateral femur to the medial tibia.

The remaining 3 muscles in the deep posterior compartment have elongate muscle bellies and tendons that pass medial to the ankle and insert in the foot. At the ankle their tendons pass under a thick flexor retinaculum that you’ll have to cut through to follow the tendons into the foot.

The most medial muscle belly is the flexor digitorum longus m. The most lateral muscle belly is the flexor hallucis longus m. Between those two muscles, and deep to them, is the tibialis posterior m.

You’ll see the tendons of the flexor digitorum longus m. and the flexor hallucis longus m. in the sole of the foot. The tibialis posteior m. inserts more superficially on the tarsal bones.

Figure 2.3 Deep muscles of the posterior leg. The superficial muscles have been reflected.

Lateral compartment of the leg:

The lateral compartment of the leg contains two muscles. Both originate on the lateral aspect of the fibula.

The fibularis longus m. is the more superficial of the two muscles, and lying just deep to it is the fibularis brevis m.

The blood supply to these two muscles is the fibular a., a branch of the posterior tibial a. Go back to the posterior compartment and follow the posterior tibial a. superiorly to find the fibular a. where it branches off.

The common peroneal n. dives into the substance of the peroneus longus m., and soon after it divides into the superficial peroneal n. and the deep peroneal n. Scissor-spread into the muscle along the nerve to find the split. The superficial peroneal n. innervates the two peroneus mm. You’ll see the deep peroneal n. in the anterior compartment of the leg.

Figure 2.4 The two muscles of the lateral compartment of the leg.

Anterior compartment of the leg:

The muscles of the anterior compartment of the leg are just lateral to the sharp anterior border of the tibia. Their tendons pass under an extensor retinaculum as they pass through the ankle and onto the dorsal surface of the foot. You may need to transect the retinaculum to follow the tendons. Remove the skin from the dorsal surface of the foot to follow the tendons of the three anterior compartment muscles.

The tibialis anterior m. attaches to the overlying deep fascia, which attaches to the anterior border of the tibia. Overuse of this muscle irritates the deep fascia and its attachment to the bone, causing shin splints.

The most lateral muscle of the anterior compartment is the extensor digitorum longus m. The muscle is easily identified by following its tendon out onto the dorsal surface of the foot to the toes.

Between the tibialis anterior m. and the extensor digitorum longus m., and slightly deep to both, is the extensor hallucis longus m. You can follow the tendon of the extensor hallucis longus m. out to the big toe (hallux).

Some cadavers will have a 4th muscle in the anterior compartment, the fibularis tertius m. The tendon of this muscle attaches to the lateral aspect of the 5th metatarsal bone.

Within the anterior compartment find the anterior tibial a. and the deep fibular n.

Summary - Lower Leg:

Figure 2.5 Anterior view of the leg.

Figure 2.6 Muscles and arteries of the dorsal foot.

Dorsal surface of the foot:

There are two intrinsic muscles on the dorsal surface of the foot.

Deep to the tendons of the extensor digitorum longus m. find the small muscle belly of the extensor digitorum brevis m. The tendons of the extensor digitorum brevis m. are small, and run beside the tendons of the extensor digitorum longus m.

Medial to the extensor digitorum brevis m. is the extensor hallucis brevis m. The tendon of the extensor hallucis brevis m. typically joins the tendon of the extensor hallucis longus m.

Even deeper on the dorsal surface of the foot are 4 dorsal interossei mm. They abduct the toes, just as the dorsal interossie of the hand abduct the fingers.

The anterior tibial a. leaves the anterior compartment of the leg, passes under the extensor retinaculum, and continues onto the dorsal surface of the foot as the dorsalis pedis a. (dorsal artery of the foot), where the pedal pulse is taken. The dorsalis pedis a. forms an arch that gives off dorsal metatarsal aa. and dorsal digital aa.

The deep fibular n. runs with the anterior tibial a. onto the dorsal surface of the foot and divides into a lateral and medial branch to supply motor and sensory innervation to the dorsal surface of the foot.

Plantar surface of the foot:

The skin on the lateral and plantar surfaces of the foot is tightly adherent to the underlying fascia, and is difficult to remove. You’ll probably need to scalpel through it, but be careful not to go too deep.

In the sole of the foot you’ll see the heavy connective tissue of the plantar aponeurosis superficial to the muscles, nerves, and arteries. Clean the superficial fascia from the plantar aponeurosis to get a clear view of it, and then use a scalpel or scissors to make a longitudinal incision through the middle of the aponeurosis and reflect it away from the underlying muscle. There are also bands of the aponeurosis that extend to the base of the digits. Reflect those bands as well. Again, take care to do as little damage as possible to underlying muscles, nerves and arteries.

Figure 2.7 Dorsum of the foot with the skin and superficial fascia removed to expose the plantar aponeuronsis

Figure 2.8 First layer of muscles in the dorsum of the foot, and the lateral and medial plantar aa. and nn.

As you remove the aponeurosis and superficial fascia from the sole of the foot you’ll start to see muscles, nerves, and blood vessels. Clean things up as much as you can before you start to dissect through the layers of muscle in the dorsal foot. The muscles are organized into 4 layers, with the first layer most superficial, and the fourth layer deepest. As you move through the 4 layers you’ll identify not only the intrinsic muscles of the foot, but also the tendons of the extrinsic muscles of the foot that you identified during dissection of the leg.

There are 3 muscles in the first layer of the foot.

On the medial aspect of the foot is the abductor hallucis m.

On the lateral aspect of the foot is the abductor digiti minimi m.

Just deep to the reflected plantar aponeurosis, in the middle of the foot, is the flexor digitorum brevis m.

At this point you can also start to identify the nerves and arteries in the sole of the foot.

The nerves are continuations of the tibial n. that runs between the superficial and deep muscles of the posterior compartment of the leg. As the tibial n. passes through the ankle it splits for form a lateral plantar n. and a medial plantar n., which provide motor innervation to the muscles on the lateral and medial sides of the foot.

The arteries are continuations of the posterior tibial a. that runs with the tibial n. in the posterior compartment of the foot. As with the nerve, the posterior tibial a. splits to form a lateral plantar a. and the medial plantar a.

The plantar nn. and aa. continue to branch in the sole of the foot, to form metatarsal aa. and nn., and digital aa. and nn.

To access the second layer of muscles in the dorsum of the foot you’ll need to reflect the flexor digitorum brevis m. First be sure to separate it from the muscle just deep to it, and then cut it near the calcaneum and reflect it toward the toes.

Just deep to the reflected flexor digitorum brevis m. you’ll see the quadratus plantae m. This muscle originates from the calcaneum and inserts into the tendon of the flexor digitorum longus m., one of the deep muscles of the posterior compartment of the leg that you previously identified.

Distal to the attachment of the quadratus plantae m, look for the small lumbrical mm. that originate from the tendons of the flexor digitorum longus.

Also identify the tendon of the flexor hallucis longus m., another muscle of the deep posterior compartment of the leg.

Figure 2.9 Second layer of muscles in the dorsum of the foot, arteries and nerves are not shown here

Figure 2.10 Third layer of muscles in the dorsum of the foot.

You may be able to identify the muscles in the 3rd layer of the sole of the foot by separating and mobilizing the muscles you’ve already identified, but you may find it best to reflect the muscles in the second layer. Cut the quadratus plantae m. near the calcaneus, and leave it attached to the tendons of the flexor digitorum longus m. Then cut the tendon of the flexor digitorum longus m. where it is still a single tendon, before it splits into the 4 tendons that go to the lateral digits. Reflect the cut tendon of the flexor digitorum longus m., with the attached quadratus plantae m., toward the toes to view the deeper muscles in the 3rd layer.

On either side of the tendon of the flexor hallucis longus m. are the two heads of the flexor hallucis brevis m. Often the medial head of the flexor hallucis brevis m. is partially fused to the tendon of the abductor hallucis m.

Just medial to the abductor digiti minimi m. is the flexor digiti minimi m. Often the two muscles share a common tendon of origin.

Between the flexor hallucis brevis m. and the flexor digiti minimi m. is the adductor hallucis m. The adductor hallucis m. has two heads, an oblique head just lateral to the flexor hallucis brevis, and a transverse head that runs across the distal ends of the metacarpal bones. You’ll have to reflect the tendon of the flexor digitorum longus significantly toward the toes to view the transverse head.

The fourth layer of muscles of the foot are the interossei, and several tendons of extrinsic muscles of the foot.

Both the plantar interossei mm. and the dorsal interossei mm. can be seen from the dorsal surface. The 3 plantar interossei mm. attach to the medial surface of digits of metatarsals 3-5, and adduct digits 3-5. The 4 dorsal interossei are attached to the lateral surface of metatarsals 3 and 4, and to both sides of metatarsal 2.

The tendon of the fibularis longus m. can now be traced from the lateral aspect of the foot to its insertion at the base of the first metacarpal bone on the medial side of the foot.

The insertion of the tibialis posterior m. into multiple tarsal bones can also be seen on the medial aspect of the dorsum of the foot.

Also be sure to view the insertions of the tibialis anterior m. into the base of the first metatarsal.

Figure 2.11 Fourth layer of muscles in the dorsum of the foot

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