Anaerobic respiration occurs in your body when there is not enough oxygen to completely break down glucose for your energy needs. When you exercise vigorously, you force your muscles to work harder, until they eventually reach a point where your lungs can no longer supply sufficient oxygen.
Anaerobic respiration takes place in muscles of human beings. this happens when we do a more amount of physical work. and sometimes sufficient amount of oxygen is unable to reach the muscles hence anaerobic respiration occurs in muscles and lactic acid is formed.