 Ever wondered why you feel all stiff the day after doing lots of exercise? Well this video will explain why but first let's try a little experiment. Put your left hand as high above your head as you can, stretch your fingers towards the sky as far as you can and then clench your fist as tight as you can. Now I want you to keep doing that throughout the whole of the video if you can. Stretch and clench, stretch and clench, keep going. In order for you to contract your muscles they need the energy that is released in respiration. However when you are 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 an aerobic 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 with aerobic respiration and also it produces a nasty waste product called lactic acid. So how is your arm feeling now? It's probably started to ache. The muscles in your forearm which control the fingers are now carrying out anaerobic respiration. After a while the lactic acid will start to build up in the muscles and it causes pain. You can stop now if you like or maybe carry on to the end of video if you're as hard as nails. When you stop exercising you will need to break down this lactic acid. To do this the lactic acid needs to travel in the blood to the liver. Here it is broken down using oxygen into carbon dioxide and water. After vigorous exercise you may find you are still breathing very heavily for a time afterwards. This is to get the oxygen into the blood needed to break down the lactic acid. We say after doing anaerobic respiration that you're an oxygen debt. That you need to pay back the oxygen to the body. So to answer the question at the start of the video why do you feel stiff after exercise? It's all to do with with lactic acid. If you stop moving straight after exercise then the blood does not return to the liver and the lactic acid stays in the muscles. This is what makes them stiff the next day so it's very important to warm down after exercise with a gentle job and stretching. Okay you can really put your arm down now.