During hard exercise, not enough oxygen can reach your muscle cells. So, aerobic respiration is replaced with anaerobic respiration. This does not need oxygen for it to happen. This process produces much less energy than aerobic exercise and a waste product of lactic acid in the muscles, which causes pain and tiredness. The lactic acid is broken down when aerobic respiration is resumed. One way to think of anaerobic respiration in humans is as an emergency top up
