The calves are composed of two muscles, the gastrocnemius and the soleus. Both muscles are active every time the ankle joint flexes. In other words, the calves are involved every time you take a step, stand up, or jump.
The gastrocnemius is composed of two heads, the lateral and the medial. The lateral head originates from the outer part of the femur, while the medial head orginates from the inner portion of the femur. Both heads insert into the Achilles tendon.
The gastrocnemius flexes the ankle and assists in knee flexion. Most standing human movement involves the gastrocnemius.
The soleus is built for endurance. A type 1 fiber muscle refers to a muscle which fatigues slowly, and the soleus is the most dominant type of this muscle in the body. If the soleus were not so type 1 dominant, then humans would not be able to walk or run for long periods of time. The soleus originates from the tibia and fibula, and inserts into the Achilles tendon.
Like the gastrocnemius, the soleus is responsible for ankle flexion.