The Achilles tendon is formed by the combination of the two calf muscles: the gastrocnemius and the soleus. It is also the largest and strongest tendon in your body. When you have an abrupt increase of stress on your tendon that exceeds its capacity, it can cause a partial or complete rupture (tear) of the tendon. This often happens during quick, strenuous activities that place excessive strain on your tendon such as sports, running, sudden steps or falls. One of the classic feelings that patients experience is like something bit them on the back of the leg or they were kicked in the calf at the time of injury. You may also recall a popping or snapping sensation at the back of your leg. https://www.myfootspecialist.com/sports-medicine/achilles-tendon-rupture/