The largest, most prominent muscle at the back of the lower leg, forming the bulge of the calf, is the gastrocnemius.
The gastrocnemius is a superficial, two-headed muscle located behind the knee and above the heel. Its medial and lateral heads attach to the femur, then join the soleus through the calcaneal, or Achilles, tendon. Because it crosses both the knee and ankle, it helps bend the knee and point the foot downward.
Its name comes from Greek words meaning “belly of the leg,” a reference to the rounded shape visible beneath the skin. Together, the gastrocnemius and soleus are often called the triceps surae, or three-headed calf muscle. The soleus lies deeper, so it contributes to calf thickness but is not usually the muscle creating the upper calf bulge.
A common mix-up is calling the entire calf the gastrocnemius. Anatomically, the calf includes several muscles, while the gastrocnemius is one distinct muscle. It is especially important for running, jumping and other explosive movements, and it is a frequent site of calf strains.