The triceps surae, commonly known as the calf muscle, is primarily composed of two muscles: the gastrocnemius and the soleus.

Gastrocnemius

The gastrocnemius muscle has two heads, known as the lateral and medial heads. The lateral head originates from the lateral aspect of the femur, while the medial head originates from the medial aspect of the femur. Both heads come together to insert into the Achilles tendon. The gastrocnemius is responsible for flexing the ankle (pointing the toes away from the shin) and assisting in bending the knee. The gastrocnemius is involved in most standing movements, making it one of the most frequently used muscles during upright activities.

Soleus

The soleus muscle is known for its endurance. It is predominantly composed of Type I muscle fibers, which are slow to fatigue, making the soleus a critical muscle for sustained activities like walking or running. Without a predominantly Type I muscle fiber composition in the soleus, humans would struggle to walk or run for extended periods. The soleus originates from both the tibia and fibula and also inserts into the Achilles tendon. Like the gastrocnemius, the soleus is responsible for flexing the ankle.

Together, the gastrocnemius and soleus form a powerful muscle group that is active during ankle flexion, such as when stepping, standing, or jumping, making the calf muscles essential for a variety of movements and activities.