Human Anatomy Lesson 26
The leg is the part of the lower limb between the knee and ankle joint, and is made up of two bones (tibia, fibula) and muscles in three compartments - anterior, lateral, posterior - that surround these two bones. As with the upper limb, we will cover the distal part of the limb (in this case, the leg) and the joint, and we will focus on the muscles that cross the ankle joint but not the intrinsic muscles of the foot.
Learning Objectives - By the end of this lesson, you should be able to ...
- Describe movements at the knee, ankle, and foot joints
- List and describe the bones in the leg, ankle, and foot, which includes the tibia, fibula, calcaneus, talus, tarsals, metatarsals, and phalanges
- List the origins, insertions, innervations, and actions of muscles in the posterior, lateral, and anterior compartments of the leg
- Describe the courses of major nerves and arteries in the leg, and list what they supply
As noted in the last lesson, the knee joint is largely limited to flexion and extension, although a small degree of medial and lateral rotation of the leg at the knee is possible. There is a wider range of motion in the foot, including dorsiflexion, plantarflexion, eversion, inversion, and circumduction, which in the ankle is a combination of the above movements. Movements of the foot are more circumscribed than those of the hand, and include flexion and extension of the toes at the metatarsophalangeal joints and the interphalangeal joints. There is limited to no movement possible between tarsal bones and the metatarsals, which make up the bulk of the foot.
- Dorsiflexion/plantarflexion: the "top" of the foot is the dorsum, and raising the dorsum superiorly is to dorsiflex the foot at the ankle joint. Plantarflexion of the foot at the ankle joint is to stand up on your toes, a movement important in "toe-off" during bipedal walking.
- Eversion/inversion: inversion is to move the sole of the foot toward the midline, and eversion is to point it outward, away from the midline.
- Flexion/extension of toes: the toes can be flexed and extended to some small extent, and a small amount of AB-duction and AD-duction is possible, but nothing like is possible in the hand.
Bones and Joints
The tibia is the weightbearing bone of the leg, and the fibula is a long spindly bone on the lateral side of the leg that participates in the ankle joint but not the knee joint.
- Shaft and distal end of tibia: the anterior and medial borders of the tibia are subcutaneous and easily palpable, and the interosseous border provides attachment area for an interosseous membrane that runs between the tibia and fibula along its entire length. The posterior surface is marked by the oblique soleal line, which marks the origin of the soleus muscle. The distal end of the tibia contains a bony protuberance called the medial malleolus, which articulates with the talus and participates in the ankle joint.
- Shaft and distal end of fibula: the medial surface of the fibula faces the anterior compartment of leg, the lateral surface faces the lateral compartment, and the posterior surface faces the posterior compartment. The distal end of the fibula is expanded, and is called the lateral malleolus, which forms part of the ankle joint. The posterior surface of the fibula is marked by a shallow groove for the tendons of fibularis longus and fibularis brevis.
- Interosseous membrane: as in the forearm, there is an interosseous membrane that spans the space between the two bones (in this case, the tibia and fibula) and provides increased surface area for muscle attachment.
- Foot bones: The foot can be broken down into tarsal bones, including a proximal row (talus, calcaneus) and a distal group including the cuboid and cuneiforms. Metatarsals are within the pad of the foot, and phalanges form the toes. The ankle joint is the joint between the medial malleolus, lateral malleolus, and talus, and is stabilized by medial and lateral stabilizing ligaments. Like in the hand, the names of joints in the foot are highly logical: tarsometatarsal, metatarsophalangeal, and interphalangeal. As noted above, the foot is capable of a more limited range of motion than is the hand. A number of cartilages and ligaments stabilize the foot and create a longitudinal arch and a transverse arch, which facilitate a spring-like mechanism during walking and running.
Posterior Compartment Muscles
Muscles on the posterior side of the leg generally plantarflex and invert the foot and flex the toes, and are innervated by the tibial nerve.
- Gastrocnemius: runs from the posterior surfaces of the medial and lateral femoral condyles to the posterior surface of the calcaneus via the Achilles tendon. Flexes the knee and plantarflexes the foot.
- Soleus: runs from the soleal line and medial border of the tibia, posterior surface of fibular head, neck, and shaft, and tendinous arch between tibial and fibular attachments to the posterior surface of the calcaneus via the Achilles tendon. Soleus is almost completely covered by the two heads of gastrocnemius. Plantarflexes the foot.
- Plantaris: runs from the inferior part of the lateral supracondylar line of the femur to the posterior surface of the calcaneus via the Achilles tendon. Flexes the knee and plantarflexes the foot. Gastrocnemius, soleus, and plantaris form a superficial layer of muscles in the leg.
- Popliteus: runs from the lateral femoral condyle to the posterior surface of the proximal tibia. Stabilizes the knee joint by resisting lateral rotation of the tibia on the femur and unlocks the knee joint by laterally rotating the femur on a fixed tibia.
- Flexor hallucis longus: runs from the posterior surface of the fibula and adjacent interosseous membrane to the plantar surface of the distal phalanx of the 1st digit. Flexes the big toe.
- Flexor digitorum longus: runs from the medial side of the posterior surface of the tibia to the plantar surfaces of the bases of the distal phalanges of digits 2 through 5. Flexes digits 2 through 5.
- Tibialis posterior: runs from the interosseous membrane and adjacent surfaces of the tibia and fibula to the navicular and medial cuneiform. Inverts and plantarflexes the foot, and supports the medial side of the arch during walking.
Lateral Compartment Muscles
The two muscles in the lateral compartment of the leg - fibularis longus and fibularis brevis - evert the foot, and both are innervated by the superficial fibular nerve, a branch of the common fibular nerve. These two muscles and their associated nerves were formerly called peroneus (so, peroneus longus, peroneus brevis, and the superficial and common peroneal nerves). You may see these names in other books or atlases, and in the figure from Gray's Anatomy reproduced below.
- Fibularis longus: runs from the head and upper lateral surface of the fibula, runs around the ankle joint, and attaches to the distal end of the medial cuneiform and first four metatarsals on their plantar sides. Everts and plantarflexes the foot and supports the arch.
- Fibularis brevis: runs from the lower 2/3 of the lateral surface of the fibular shaft, runs around the ankle, and attaches to the base of the 5th metatarsal. Everts the foot.
Anterior Compartment Muscles
The four muscles in the anterior compartment of the leg - tibialis anterior, extensor hallucis longus, extensor digitorum longus, fibularis tertius - dorsiflex the foot at the ankle, extend the toes, and invert the foot. They are all innervated by the deep fibular nerve, the other main branch of the common fibular nerve.
- Tibialis anterior: runs from the lateral surface of the tibia and adjacent interosseous membrane to the medial cuneiform and base of the first metacarpal. Dorsiflexes the foot at the ankle, inverts the foot, and supports the medial arch.
- Extensor hallucis longus: runs from the middle 1/2 of the medial surface of the fibula and adjacent surface of the interosseous membrane to the base of the distal phalanx of the 1st digit on its dorsal surface. Extends the big toe and dorsiflexes the foot.
- Extensor digitorum longus: runs from the proximal 1/2 of the medial surface of the fibula and related surface of the lateral tibial condyle to the bases of distal and middle phalanges of digits 2 through 5.
- Fibularis tertius: runs from the distal part of the medial surface of the fibula to the base of the 5th metatarsal on its dorsal and medial surfaces. Dorsiflexes and everts the foot.
The main nerves running through the leg include the tibial nerve and two branches of the common fibular nerve.
- Tibial nerve: this is the main nerve of the posterior compartment, and is itself the main continuation of the sciatic nerve from the gluteal region and thigh. The tibial nerve runs under the tendinous arch formed by the two heads of the soleus muscle and passes through the leg on the deep surface of the tibialis posterior muscle. It passes through the tarsal tunnel behind the medial malleolus and enters the foot to supply intrinsic muscles and skin. The tibial nerve has two cutaneous branches: the sural nerve and the medial calcaneal nerve. The sural nerve originates between the two bellies of gastrocnemius above the knee and runs inferiorly in a vertical direction superficial to the gastrocnemius muscle before piercing the deep fascia in the middle of the leg, at which point it passes around the medial malleolus and into the foot. It supplies skin on the posterolateral surface of the leg and the lateral side of the foot and little toe. The medial calcaneal nerve branches off the tibial nerve near the ankle and innervates skin on the medial surface and sole of the heel.
- Superficial fibular nerve: this nerve innervates the two fibularis muscles in the lateral compartment. It branches off the common fibular nerve which is itself from the sciatic nerve. It follows the medial margin of the biceps femoris tendon over the lateral head of the gastrocnemius muscle, giving off the sural communicating nerve and lateral sural cutaneous nerve, then penetrates deep fascia in the lower leg to enter the foot, where it divides into medial and lateral branches.
- Deep fibular nerve: the common fibular nerve enters the lateral compartment and gives off the superficial fibular nerve (see above) and the deep fibular nerve, which passes through the intermuscular septum between lateral and anterior compartments, where it supplies tibialis anterior, extensor hallucis longus, extensor digitorum longus, and fibularis tertius.
- Popliteal artery: passes into the posterior compartment between the gastrocnemius and popliteus muscles, passing through the tendinous arch formed by the two heads of soleus, and immediately splitting into anterior and posterior tibial arteries (see below).
- Anterior tibial artery: after branching off the popliteal artery, the anterior tibial artery runs through the aperture in the upper part of the interosseous membrane to supply the anterior compartment of the leg and dorsum of the foot. Distal branches include the anterior medial malleolar artery and anterior lateral malleolar artery.
- Posterior tibial artery: descends through the leg on the superficial surfaces of the tibialis posterior and flexor digitorum longus muscles, passes through the tarsal tunnel behind the medial malleolus, and supplies blood to the posterior and lateral compartments of the leg and sole of the foot. It gives off two major branches in the leg, the circumflex fibular artery and fibular artery.
© 2014 Robert McCarthy