Most people still use the terms “aerobic” and “anaerobic” exercise to name intensity levels, referring to the way your body produces the required energy. However, the way you produce energy is one big continuous chain of reactions, and categorizing exercise in this way can lead to misunderstandings.
In an article in March 2015, Kamir Chamari and Johnny Padulo suggest using the terms “explosive efforts”, “high intensity efforts” and “endurance intensity efforts”.
Energy production: a complex chain of reactions
When you exercise, your body transforms glycogen, glucose, fats or some proteins into a specialised molecule called ATP (adenosine triphosphate), which can then be used by your muscle fibres.
There is some ATP available for immediate use to perform very intensive bouts of exercise, e.g. sprinting, which we should call “explosive efforts”. After about 6 sec however, it is gone and your body therefore immediately starts topping it up.
Glycogen or glucose is first broken down in the cytoplasm of the cells into pyruvic acid, producing about 3 molecules of ATP. This might not sound as very much, but the system is quick. It does not need any oxygen, even if oxygen is available, and it is therefore often called “anaerobic”. It is everything you need for short, intense bouts of exercise which Chamari and Padulo suggest calling “high intensity efforts”.
Pyruvic acid is then used by the mitochondria of your cells to produce about 32 molecules of ATP in a complex series of reactions. This part of the energy production chain is very productive but it is rather slow. As it requires oxygen, it is often called “aerobic” and the exercise intensity at which you rely most on it is “endurance intensity exercise”.
The bottle neck between high intensity and endurance intensity levels
As the first part of the chain is fast (up to pyruvic acid, without the need for oxygen) and the second slow, there will be a bottle neck between the two of them. If you go harder, the bottle neck will become bigger, and more of your energy will have to come from the first “anaerobic” part of the chain, even if there is plenty of oxygen available.
Whatever the intensity you are exercising at, you will always be using energy from both parts of the chain. The relative amounts will differ, obviously, but will be determined by the intensity of the effort and not by the presence or absence of oxygen. Labelling a workout as “aerobic” or “anaerobic” is therefore incorrect, and can lead to confusing and misunderstandings.
If you are going hard, pyruvic acid will be accumulating in your cells due to the bottle neck. It changes then into lactic acid and moves out of the cell. As lactic acid can very easily change back into pyruvic acid, which can used to produce a lot of energy, it is eagerly taken up by other tissues. It is therefore not a waste product at all, but a very important molecule.
However, if you produce more lactic acid than your tissues can take up, the amount in your blood will increase. Your brain uses this rise as a signal that you are going a bit too hard and it will slow you down by making your muscles ache.
As you can have this feeling even if you are doing an endurance workout, it is clear that you are getting energy via every part of the chain.
Chamari K and Padulo J. “Aerobic” and “anaerobic” terms used in exercise physiology: a critical terminology reflection. Sports Medicine- Open. 2015; 1:9. doi: 10.1186/s40798-015-0012-1.
Willmore JH, Costill DL and Kenney W L. Physiology of sports and exercise. Ed: Human Kinetics 2008.
photo credit: <a href=”http://www.flickr.com/photos/33442021@N00/1673932398″>spitting blood</a> via <a href=”http://photopin.com”>photopin</a> <a href=”https://creativecommons.org/licenses/by/2.0/”>(license)</a>
photo credit: <a href=”http://www.flickr.com/photos/65483692@N06/13191313253″>Limassol Marathon, #Cyprus 2014</a> via <a href=”http://photopin.com”>photopin</a> <a href=”https://creativecommons.org/licenses/by-nd/2.0/”>(license)</a>