Plantarflexion is a movement required in jumping, running and walking. The gastrocnemius is a large muscle in the posterior compartment of the leg, and is the powerful muscle that enables plantarflexion, as well as knee flexion. In this article we will discuss the anatomy of the gastrocnemius muscle, together with the clinical relevance of this muscle. We will conclude with some review questions to test the reader’s understanding of the article content.
The leg consists of a posterior, anterior and lateral compartments. The posterior compartment also known as flexor compartment is divided into a deep and superficial components. The gastrocnemius is a large muscle in the superficial compartment and lies superficial to the soleus muscle, the plantaris muscle runs between both of them. There are four muscles that lie deep and form the deep flexor group, they are popliteus, tibialis posterior, flexor hallucis longus, flexor digitorum longus.
The gastrocnemius muscle is a large bipennate muscle that lies in the superficial part of the posterior compartment of the leg. The gastrocnemius has two heads. The lateral head arises from the lateral surface of the lateral femoral condyle, and the medial head which is larger arises from the non-articular surface (upper and posterior part) of the medial femoral condyle behind the adductor tubercle and from the slightly raised area on the popliteal surface of femur shaft. Both heads also have their attachment from the capsule of the knee joint. There is present a bursa between each head and capsule of the knee joint. The gastrocnemius muscle crosses the knee joint, and hence is also a weak flexor of the knee. The two heads of the muscle join into a broad aponeurosis, and will unite with the tendon of the soleus muscle, and insert onto the posterior surface of the calcaneus as the Achilles tendon. The Achilles tendon inserts onto the middle third of the calcaneus. Two bursae lie here at the site of insertion. The gastrocnemius lies superficial to the soleus muscle, which is also a plantar flexor of the ankle. Many anatomists consider the two heads of gastrocnemius and soleus to be the triceps surae of the leg.
The plantaris muscle lies superficial to gastrocnemius, and is a weak plantarflexor. It is considered to act in supporting the gastrocnemius. It arises above and lateral to the lateral head of the gastrocnemius, and descends in an inferomedial direction. The muscle occasionally inserts onto the calcaneus via a separate insertion, medial to the Achilles tendon.
The two heads of the gastrocnemius muscle form the inferior medial and lateral boundaries of the popliteal fossa in the upper part of the posterior leg. Almost the entire superficial surface of the muscle is covered by deep fascia over which lies the short saphenous vein. The common peroneal nerve passes posterior to the lateral head.
Blood Supply and Innervation
Both heads are supplied by medial and lateral sural arteries (branches of popliteal artery). The muscle is innervated by the tibial nerve (ventral rami of L4-S2), a branch of the sciatic nerve (ventral rami of L4-S3). The tibial nerve also supplies the posterior compartment of the thigh and leg (apart from the short head of biceps femoris, which is innervated by the common fibular nerve).
Gastrocnemius is a weak flexor of the knee joint and main plantar flexors along with soleus. The fleshy bellies of gastrocnemius provide rapid contraction necessary for propulsion during running, fast walk and jumping.
Tennis leg- This is an injury that commonly occurs in tennis and squash players. It is sudden sharp pain at the back of the calf and usually follows rupture of the myotendinous junction of the medial head of the muscle, or rupture of the plantaris muscle (a less common cause).
Achilles Tendon tendonitis- This is pain and thickening of the Achilles tendon at its insertion and most commonly occurs in middle and old age. The disease follows repetitive strain of the muscle that results in cartilaginous and bony metaplasia. The patient commonly presents with pain at the back of the heel and bony enlargement at the insertion of the tendon.
Achilles Tendon rupture- This is often misdiagnosed as a simple ankle sprain and usually follows a sporting injury to the region. The rupture usually occurs 4-6cm above the calcaneal insertion of the tendon, which a relatively avascular. The patient may recall a pop sound upon rupturing of the tendon, and the feeling of the muscle rolling up their leg.
- The gastrocnemius muscle lies in the posterior compartment of the leg.
- The muscle gains its motor innervation from the tibial nerve.
- The muscle gains its blood supply from the posterior tibial artery, a primary branch of the popliteal artery.
- It is a plantarflexor of the ankle, and a weak flexor of the knee.
- It lies superficial to soleus, and deeper still is tibialis posterior, flexor digitorum longus and flexor hallucis longus.