Ever wondered why feel all stiff the day after doing lots of exercise, in order for you to contract your muscles they need the energy that is released in respiration. However, when you're doing heavy exercise for a long time the muscles can't always get the oxygen they need to carry out aerobic respiration. In this case the muscle cells can switch to carry out another type of respiration called anaerobic respiration. this is respiration without oxygen so why don't we just do this respiration all the time if we don't need oxygen that we don't need to bother breathing. Well unfortunately anaerobic respiration is not as good as aerobic firstly it only produces a small amount of energy compared to aerobic respiration and also it produces a nasty waste product called lactic acid.