The calf is made up of two main muscles, the gastrocnemius and the soleus, and a smaller one called the plantaris. The calf muscles are at the back of your lower leg—from your knee to your ankle.Usually when you have a calf strain it affects the gastrocnemius muscle. This muscle helps the knee flex and the foot move downward. When the knee is extended outward and the ankle is flexed upward (toes toward the head), the muscle is stretched and pulled and can easily tear, especially during a sudden movement.