Gastrocnemius: plantarflexion and knee flexion

The gastrocnemius is the most superficial muscle of the calf (the back of the lower leg) and gives it its rounded shape. It has a medial head and a lateral head, and a head is simply a separate origin of one muscle. The heads arise from the medial and lateral femoral condyles (the rounded ends of the thigh bone at the knee) and join into one muscle. This then meets the soleus, a flat muscle lying deep to it, to form the calcaneal (Achilles) tendon, which attaches to the calcaneus, the heel bone.

Pulling on the heel through that tendon produces plantarflexion, which means pointing the foot downwards at the ankle, as when you rise onto your toes. The muscle starts above the knee, so it passes behind the knee joint and can also help to bend the knee. Crossing two joints makes it a two-joint muscle.

The soleus arises from the tibia and fibula below the knee and acts only at the ankle, so it does not help to flex the knee.

The contribution of the gastrocnemius to plantarflexion depends on knee position. It contributes most when the knee is straight. When the knee is bent, the muscle is already shortened at the knee, so it has less length left to shorten at the ankle and pulls less effectively. The soleus then carries more of the load, so push-off (the ankle movement that drives the body forwards from the ground in walking) is reduced rather than lost.