The calf muscle, on the back of the lower leg, is made up of two muscles:
THE GASTROCNEMIUS
This is the larger calf muscle, forming the bulge visible beneath the skin. The gastrocnemius has two parts or “heads,” which together create its diamond shape.
THE SOLEUS
This is a smaller, flat muscle that lies underneath the gastrocnemius muscle. The gastrocnemius and soleus muscles taper and merge at the base of the calf muscle. Tough connective tissue at the bottom of the calf muscle merges with the Achilles tendon. The Achilles tendon inserts into the heel bone (calcaneus). During walking, running, or jumping, the calf muscle pulls the heel up to allow forward movement.