Triceps surae muscle
The triceps surae muscle is a three-headed muscle in the posterior compartment of the leg. It consists of two muscles, gastrocnemius and soleus. Along with the plantaris muscle, the triceps surae composes the superficial flexor group of the leg, which forms the bulk on the back of the calf.
The main function of the triceps surae is to plantar flex the foot. In addition, the gastrocnemius portion of the muscle aids the flexion of the leg, while the soleus assists in maintaining the steady posture while standing on the ground.
This article will discuss the anatomy and function of the triceps surae muscle.
Origin and insertion
The gastrocnemius muscle comprises the first layer of the superficial flexors of the leg. It contains two heads that originate by two strong tendons from the femoral condyles, respectively:
- The medial head originates from the posterior surface of the medial condyle of femur, posterior to the adductor tubercle and the popliteal surface of the femoral shaft.
- The lateral head originates from the superior part of the posterolateral surface of the lateral femoral condyle, and from the lower part of the lateral supracondylar line. A sesamoid bone called the fabella is often found embedded in the lateral head of the gastrocnemius.
The two heads of gastrocnemius converge below the popliteal fossa into a single large muscle belly that extends to the midpoint of the calf. As the muscle descends further, it begins to develop a broad aponeurosis on its anterior surface.
The aponeurosis receives the soleus tendon and gradually narrows towards the ankle to form a large tendon called the calcaneal (Achilles) tendon. The calcaneal tendon goes on to insert at the middle part of the posterior surface of the calcaneus.
The soleus muscle is a broad, flat, triangular muscle found deep to the gastrocnemius. It arises from the soleal line on the posterior surface of the tibia, medial border of the tibia and the posterior surface of the upper third of the fibula.
The muscle belly of the soleus descends until approximately mid-way of the calf, where the muscle fibers converge into a flat aponeurosis on the anterior surface of the muscle, similarly to the gastrocnemius. This aponeurosis extends distally into a tendon that merges with the tendon of the gastrocnemius and forms the calcaneal tendon together with it.
The triceps surae forms the bulk of the calf. The two heads of gastrocnemius lie superficial to the soleus and form the lower borders of the popliteal fossa. The lateral head of the gastrocnemius is partially overlapped by the tendon of biceps femoris, while the medial head is covered by the semimembranosus muscle. The superficial surface of the gastrocnemius is covered with a deep fascia that separates it from the small saphenous vein and the fibular communicating and sural nerves. The deep surface of the gastrocnemius is related to the oblique popliteal ligament, popliteus, soleus, plantaris, popliteal artery and vein and the tibial nerve.
The deep surface of the soleus is related to the flexor digitorum longus, flexor hallucis longus, tibialis posterior muscles, the posterior tibial artery and vein and the tibial nerve. These structures are separated from the triceps surae by the transverse intermuscular septum of the leg.
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The triceps surae muscle is innervated by the tibial nerve (S1-S2).
Triceps surae receives arterial supply from the branches of the popliteal artery. In addition, the soleus receives contributions from the branches of the posterior tibial or fibular (peroneal) arteries, while the gastrocnemius may receive supply from branches of the superior genicular arteries.
The triceps surae is the main plantar flexor of the foot on the ankle joint, enabling the elevation of the heel against gravity. This provides the force for propulsion during walking, leaping or jumping. In addition, the gastrocnemius muscle contributes to a small extent to the flexion of the leg in the knee joint.
As the body's center of gravity acts through a vertical line that passes anterior to the ankle joint, the body has a natural tendency to lean forward. This requires a great continuous pulling force behind the joint to maintain stability, which is found to be supplied mainly by the soleus. This is due to the fact that the soleus is in a constant state of contraction during standing or walking, which is also evident by its high content of slow-twitch, fatigue-resistant type of muscle fibers.
For more details about the posterior leg muscles, including triceps surae, take a look at the videos, articles, illustrations and quizzes in the following study unit:
Clinically, the triceps surae muscle is the reference muscle for the nerve root S1. It can be compressed due to a herniated disk or vertebral fracture. Classic symptoms are irritations and pain along the buttock and posterior leg and a weak or absent Achilles tendon reflex. Additionally the triceps surae muscle is often functionally restricted. The affected patients present with a so-called talipes calcaneus caused by the overbalance of the anterior muscles of the leg. Both walking on toes as well as rolling the foot from the heel becomes practically impossible.
The Achilles tendon is the strongest tendon of the human body. Its load bearing capacity amounts up to one tonne, which is why its rupture is followed by a loud “whipping” sound. It is usually associated with previous damages due to chronic false strain. The microtraumas disturb the blood supply of the tendon leading to a decrease of its strength. The region about 3 to 5 centimeters proximal to the tendon insertion is particularly vulnerable as it is relatively poorly supplied already. In adolescents, rupture of the Achilles tendon is often accompanied with a fracture of the calcaneus bone.