The posterior compartment of the leg contains six or seven muscles, depending on whether or not the person has a plantaris muscle. Just like in the forearm, there’s a variable muscle in the leg. How are they linked to the anterior compartment? And, more importantly, how do they contribute to our movement?
Gastrocnemius: The Most Prominent Muscle of the Leg
The most obvious muscle of the posterior leg is the gastrocnemius—it makes up the prominence that most people call the calf.
Gastrocnemius is a two-headed muscle that crosses both the knee and the ankle—its lateral and medial heads originate off the respective posterior aspects of the femoral condyles and popliteal surface of the femur.
The heads combine to insert on the posterior aspect of the calcaneus by the calcaneal tendon—which most people call the Achilles tendon—now there’s an eponym that takes us to a mythical level. Because it crosses the posterior knee, the gastrocnemius assists in knee flexion—along with the hamstrings and other muscles in the thigh—and because it crosses the posterior ankle, gastrocnemius mainly acts in plantar flexion.
Deep to the gastrocnemius is the soleus muscle; it takes origin off the soleal line of the tibia and the posterior fibula and inserts along with the gastrocnemius by the calcaneal tendon. Because soleus is a wider and slightly more distal muscle, its belly can typically be seen on either side of gastrocnemius. Soleus doesn’t cross the knee, but plantarflexes the ankle with the gastrocnemius.
Within the calcaneal tendon, you really can’t differentiate gastrocnemius and soleus; the tendons literally fuse. Though both muscles aid in running and jumping, the soleus is more of an endurance muscle and also aids balance when on one foot; soleus is really the prime mover in plantar flexion when the knee is bent, since gastrocnemius is slack in that position.
And here’s a fun bit of anatomical history, related to our rule of adjectives: We know that the triceps brachii is in the posterior arm, but is there another triceps? Well, the two heads of gastrocnemius and the soleus were sometimes referred to as triceps surae, meaning a three-headed muscle of the leg, though that has fallen out of style—who knew there was style in anatomy!
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
The last muscle that might be in the superficial region of the posterior leg is the plantaris. It is a variable muscle that has a tiny little belly that takes origin from the femur’s lateral supracondylar ridge. It has such a long tendon that it gets the nickname “the freshman nerve”. That’s because gross anatomy is typically taken during first year of med school, and newbies often mistake the plantaris tendon for a nerve.
The tendon of plantaris sometimes fuses with the calcaneal tendon, but other times stays separate to attach to the calcaneus.
Most people have a plantaris—an estimated 80% or more, depending on population—and when present, it is said to weakly flex the knee and weakly plantarflex the ankle. But as with palmaris longus of the forearm, the plantaris tendon can be important clinically, in an autologous graft, when it can be sacrificed for a tendon replacement or repair.
Deep Posterior Leg Muscles
These two or three muscles in the superficial posterior leg are separated from the four deeper muscles in the posterior leg by the transverse intermuscular septum. The deep posterior leg includes tibialis posterior, flexor hallucis longus, and flexor digitorum longus—with adjectives that mirror three of the four muscles of the anterior leg—namely tibialis anterior, and extensors hallucis and digitorum longus. The fourth muscle of the deep posterior leg is popliteus.
Popliteus crosses only the knee, originating off the lateral femoral condyle and inserting on the proximal posterior tibia. It is said to unlock the knee joint by slight rotation at the knee joint.
The Anterior-Posterior Compartment Link
The three deep posterior muscles that mirror their anterior counterparts have an unusual relationship in the posterior leg—the belly of tibialis posterior is intermediate, while flexor hallucis longus is lateral—on the little toe side—and flexor digitorum longus is medial—on the big toe side.
When you think of their functions, that is odd—but the tendons of flexor hallucis longus and flexor digitorum longus make an X at the medial ankle and then approach their respective insertions.
Tibialis posterior originates on the posterior tibia, posterior fibula, and interosseous membrane; it passes posteromedially and inserts on most of the tarsals and metatarsals. Tibialis posterior functions to plantar flex and invert the foot.
Flexor hallucis longus takes origin from the posterior fibula and interosseous membrane and inserts on the plantar surface of the distal phalanx of the big toe. Flexor digitorum longus originates off the posterior tibia and inserts on the distal phalanges of the other toes.
Why Deep Muscles Are Important
Besides flexing their respective toes, since both flexor hallucis longus and flexor digitorum longus cross the posteromedial ankle, they assist in plantar flexion and inversion of the foot. They are also important to the arches of the foot, and for attachments of some muscles in the sole of the foot.
It must be remembered that all muscles of the posterior leg, whether superficial or deep, are supplied by the tibial branch of the sciatic nerve.
Common Questions about Muscles in the Posterior Compartment of the Leg
The posterior compartment of the leg contains six or seven muscles, depending on whether or not the person has a plantaris muscle. Just like in the forearm, there’s a variable muscle in the leg.
The deep posterior leg includes tibialis posterior, flexor hallucis longus, and flexor digitorum longus—with adjectives that mirror three of the four muscles of the anterior leg—namely tibialis anterior, and extensors hallucis and digitorum longus. The fourth muscle of the deep posterior leg is popliteus.
The deep posterior leg muscles assist in plantar flexion and inversion of the foot. They are also important to the arches of the foot, and for attachments of some muscles in the sole of the foot.