Lateral head of gastrocnemius

At a glance

English

Lateral head of gastrocnemius

Latin (TA2)

Caput laterale musculi gastrocnemii

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the lateral head of gastrocnemius

Gastrocnemius is the superficial two-headed muscle that forms the bulge of the calf; its name comes from Greek words meaning the belly of the leg. Its lateral head is the outer and usually the smaller and shorter of the two heads, the medial head reaching further down the calf. With soleus beneath, gastrocnemius makes up triceps surae, and the three bellies share the calcaneal, or Achilles, tendon.

The lateral head arises from an impression on the outer surface of the lateral femoral condyle, just above and behind the epicondyle, and from the back of the knee joint capsule nearby. Its fleshy fibres run down and in, forming the upper outer border of the popliteal fossa together with biceps femoris, then join the medial head on a broad aponeurosis in the middle of the calf. This narrows into the calcaneal tendon, which attaches to the middle of the back of the calcaneus. A small sesamoid bone, the fabella, is found in the tendon of the lateral head in some people, and can be mistaken on an X-ray for a loose body in the knee.

Because gastrocnemius crosses both knee and ankle, it plantarflexes the foot and helps flex the knee. It is built mainly of fast fibres and does most of its work in running, jumping and other quick, forceful movements, while soleus, with more slow fibres, keeps the body upright during standing. The common fibular nerve passes just behind and outside the lateral head, near the fibular neck, so the two lie close at the back of the knee.

Both heads are supplied by the tibial nerve and by sural branches of the popliteal artery, each head usually having its own artery. Calf cramps are common in this muscle. A sudden forced stretch of the ankle can tear the junction of muscle and tendon, more often in the medial head. Unusual attachments of the medial head can trap the popliteal artery, but the lateral head is rarely involved.

Attachments, action and supply

Origin

Outer surface of the lateral femoral condyle, above and behind the epicondyle, and the adjacent knee joint capsule.

Insertion

Through the calcaneal (Achilles) tendon, shared with the medial head and soleus, on the middle of the heel bone's back surface.

Action

Plantarflexes the foot at the ankle and helps flex the knee; mainly active in running and jumping.

Nerve supply

Tibial nerve (S1, S2).

Blood supply

Lateral sural artery from the popliteal artery.