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Muscles of the lower leg mainly act at the ankle joint, and some also act at the knee or the toes. They are innervated by branches of the sciatic nerve. The many muscles of the leg are grouped into anterior (front), lateral (outer) and posterior (back) compartments. Each compartment is wrapped in a sheet of connective tissue called fascia, and the muscles in a compartment share a nerve supply and, for the most part, a main action.
You will need the anatomical terms for movements and directions throughout this article. For more information, please check out our Anatomical Terminology Article.
Muscles in the anterior compartment act to dorsiflex the foot and are innervated by the deep fibular nerve (a branch of the common fibular nerve).
Tibialis Anterior is found on the lateral surface of the tibia, where it originates (its fixed starting point). It attaches (inserts, at its moving end) to the medial cuneiform (one of the bones in the foot) and the base of the first metatarsal (the long bone behind the big toe), and acts to dorsiflex the foot. It also acts to invert the foot (turn the sole inwards).
Extensor Digitorum Longus originates from the lateral condyle (a rounded bump at the top of the tibia), the fibula and the interosseous membrane (the sheet of connective tissue joining the two leg bones). It forms four tendons, each inserting into one of the four toes other than the hallux/big toe. As the name suggests (digitorum means of the toes, or digits, and longus means long), these act to extend the toes, whilst also aiding dorsiflexion.
Extensor Hallucis Longus originates from the medial fibula and inserts into the distal phalanx of the big toe. It is responsible for the extension of the hallux/big toe but also contributes to dorsiflexion of the foot.
Diagram - Anterior compartment of muscles in the leg, highlighted. Notice how deep the extensor hallucis longus is compared to the tibialis anterior and extensor digitorum longus
Creative commons source by OpenStax College, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The posterior compartment is slightly more complex, in that it is split into superficial and deep layers. Both layers are innervated by the tibial nerve. Most of the muscles act to plantarflex the foot. The exception is popliteus, which acts only at the knee.
Superficial Muscles of the Posterior Compartment
Each superficial muscle inserts into the calcaneus (heel bone) of the foot via the calcaneal tendon (also known as the Achilles tendon). Each of these muscles is innervated by the tibial nerve. There are three main muscles to know:
Gastrocnemius is the most superficial of the posterior leg muscles. It has a medial and lateral head (a separate starting point of the muscle), which originate from the medial and lateral femoral condyles (the rounded bumps at the lower end of the femur) respectively. These converge to form a single muscle belly, which then attaches to the calcaneus as described above. It acts to plantarflex the foot. It can also aid knee flexion as it crosses the knee joint.
A large, flat muscle located just deep to Gastrocnemius, Soleus gets its name from its resemblance to a sole (a flat fish, not the sole of the foot). It originates from the soleal line (a ridge on the back of the tibia) and the posterior surface of the fibula, and attaches to the calcaneus via the calcaneal tendon. It acts to plantarflex the foot.
Found just superficial to the soleus, it is a small muscle with a very long tendon. It originates from the lateral supracondylar line (a ridge of bone just above the lateral condyle) on the femur and attaches to the calcaneus, blending with the calcaneal tendon. Like the other muscles, it acts to plantarflex the foot.
Deep Muscles of the Posterior Compartment
The remaining muscles of the posterior compartment are deep. Notice the similarities with the anterior compartment of leg muscles. In the deep posterior compartment, there is a tibialis muscle which acts to plantarflex and invert the foot. There is also a muscle for the flexion of the toes and a muscle for flexion of the big toe. Apart from popliteus, the tendons of the muscles in the deep posterior compartment travel posterior to the medial malleolus (the bony bump on the inner ankle). This lets them reach the inferior surface of the foot, where they attach. Once again, each muscle is innervated by the tibial nerve. There are four main muscles to know:
This muscle lies posterior to the knee joint. It originates from the lateral femoral condyle and attaches just above the soleal line of the tibia. When we stand, the knee locks so that it can support the weight of the body. Popliteus 'unlocks the knee' by laterally rotating the femur on the tibia (while the foot is planted on the ground). This allows knee flexion to occur. When the foot is off the ground, popliteus instead rotates the tibia medially on the femur.
A large, deep muscle which originates between the tibia and fibula, via the interosseous membrane. It attaches to the inferior surface of the medial tarsal bones (the tarsal bones make up the back half of the foot), mainly the navicular and the medial cuneiform. This acts to plantarflex and invert the foot.
This muscle is located medially in the posterior compartment. It originates from the medial tibia and attaches to the inferior surface of each of the four digits (toes) other than the big toe. It acts to flex each toe (with exception of the hallux).
This muscle is located laterally in the posterior compartment, and originates from the fibula. It attaches to the inferior surface of the distal phalanx of the hallux. As the name suggests, it acts to flex the hallux.
Diagram - Superficial posterior compartment of the leg, highlighted. Notice how long the tendon of plantaris is
Creative commons source by OpenStax College, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The muscles in the lateral compartment of the leg are responsible for everting the foot. The muscles are innervated by the superficial fibular nerve (a branch of the common fibular nerve) and the tendons pass posterior to the lateral malleolus.
A long muscle that originates from the lateral surface of the fibula, the Fibularis Longus descends posterior to the lateral malleolus (the bony bump on the outer ankle) and attaches to the medial cuneiform and the base of the first metatarsal (the long bone behind the big toe). It everts the foot, whilst also aiding plantarflexion.
Fibularis Brevis is a small muscle that is found lower down and deep to (beneath) the fibularis longus muscle. It arises from the lateral fibula and attaches to the fifth metatarsal (the long bone behind the little toe). Its primary action is to evert the foot, and it also assists ankle plantarflexion.
Diagram - Deep posterior and lateral compartment of the leg, highlighted
Creative commons source by OpenStax College, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
For Pathologies and Conditions affecting the lower leg area, check out our Conditions of the Lower Leg article.
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
Edited by: Dr. Maddie Swannack
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