Aerobic vs Anaerobic
To breathe, or not to breathe; that is the question
There’s so much noise around the various parameters of run training that it’s easy to have the very basics lost in the confusion. What pace? Which zone? What heart rate? One of the very basic things which we think gets lost is the difference(s) between aerobic and anaerobic efforts. Follow along as we de-tangle myth from legend. We’ll try to keep things simple, but understand that while the What’s and How’s of these parameters may be relatively simple, the Why’s can get very complex.
Aerobic: from the Greek words “aero”, (air) + “bios” (life). The word is often credited to scientist Louis Pasteur who, in 1863, defined it as, “able to live or living only in the presence of oxygen, requiring or using free oxygen from the air," (Note: he was referencing certain bacteria)
Anaerobic: Same as above, but adding in the Greek “an” (without). Pasteur defined it as “capable of living without oxygen.” (Same note applies)
Without a doubt, we are aerobic beings. We need oxygen to live. So why is there so much discussion about anaerobic exercise? If not breathing means dying, why would we want to go anaerobic?
The simple answer is that nature designed us this way. There are VERY short periods of time when our muscles can contract without having to utilize oxygen in the process. These are quick, powerful bursts of energy, meant to maximize force production. So, for our running, the simplest way to think about this is:
Aerobic exercise: Training which primarily conditions the heart (which it is often referred to as “cardio”, short for cardiovascular), such as running or cycling. We can sustain this level of exercise for a prolonged period of time (many minutes up to hours), depending on the rate of exertion and the individual’s adaptation to the activity.
Anaerobic exercise: Strength and power, such as weight lifting or High Intensity Interval Training (HIIT). This level of exercise can only be sustained for very short durations (seconds to minutes), depending on rate of exertion and the individual’s adaptation to the activity.
Let’s set some ground work by doing a shallow dive into the chemistry of all of this. This can easily get confusing and overwhelming, but it’s important to understand the biological imperatives in order for us to make sense of the What’s and How’s of our training.
At its most basic, our muscles favor cellular aerobic respiration. Here’s what that looks like from a scientist’s point of view:
C6H12O6 + 6O2 → 6CO2 + 6H2O + 38 ATP (energy)
Here’s what that means: our muscle cells take one molecule of glucose (C6H12O6, the most elemental form of sugar), and six molecules of oxygen (O2), in order to create the energy needed to move the muscle fibers. This process is known as the Krebs Cycle. Of course, there’s a whole lot more going on in this process; feel free to follow that hyperlink to dive into the details if you so choose. A key to this process is the creation of the molecule called adenosine triphosphate, or ATP. ATP is the body’s energy fuel source of choice. It is a highly charged (ionized) molecule. Due to its negative charge, ATP’s chemical bonds can store a large amount of energy, which can be liberated easily within the muscle cell’s mitochondria, the “power factory” where all this ensorcelled chemistry occurs. In aerobic respiration, when each glucose molecule gets broken down, and when combined with those six oxygen molecules, the reaction creates 38 ATP molecules.
We’re keeping it simple, so we’re continuing on — the end result of this combustion is a release of six molecules of carbon dioxide (CO2) and six molecules of water (H2O), along with the energy (38 ATP) needed to actually do the work. This is considered a very metabolically efficient process, since the end “waste” products are nothing more than carbon dioxide and water, which our bodies are well equipped to get rid of. One of the nice things about this process is that while muscle glycogen (glycogen being two or more linked molecules of glucose) is relatively limited in quantity, our body has large reserves of glycogen stored away in the liver. When we’re operating in the aerobic zone, our body can tap into these reserves, and use these to fuel the operation. Additionally, it’s really easy for us to ingest something with a lot of carbohydrates (like a sports drink), which replenishes a proportion of the balance, allowing us to operate for long periods of time. Something to keep in mind, however, is that it takes a relatively long time to pull glycogen out of the liver and get it into the muscles.
Of course, there is a LOT more going on in the muscles than this simple formula, and this is why it’s so easy to get swamped with seemingly contradictory information. We’ll get into all of that in a moment. First, let’s talk about anaerobic respiration, a.k.a, Glycolysis. Here’s the chemical formula:
C6H12O6 + 2 NAD⁺ + 2 ADP + 2 Pᵢ → 2 Pyruvate (C3H4O3) + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O + energy
So again we’re starting with one molecule of glucose, but this time we’re adding in two molecules of nicotinamide adenine dinucleotide (NAD+), an essential “coenzyme”, or an organic non-protein compound that binds with an enzyme to catalyze a reaction; two molecules of adenosine diphosphate (ADP), another essential organic compound found in living cells which has an essential role in the energy flow of cells; and two molecules of inorganic phosphate (Pi), which is required by the body for things like energy metabolism, signal transduction and pH buffering. Notice that the only oxygen in this formula is that which is bound up within glucose. The mitochondria crunches these components together into an explosive amalgamation which results in two molecules of pyruvate, a transport molecule which carries carbon atoms to and from the mitochondria; two molecules of reduced nicotinamide adenine dinucleotide (a positively charged version (NAD⁺) goes in, and an uncharged version (NADH) comes out); two charged hydrogen atoms (also known as hydrogen ions); two molecules of water; and two molecules of ATP. There’s an additional two molecules of ATP created in this process (not shown), for a total of 4 ATP.
So simple, right? Here’s the take-away: *IF* we keep our effort levels relatively low and controlled, we can utilize the aerobic process and create 38 ATP, and clean up is easy. Once we start exercising at a level where the demands for energy in the muscles becomes so severe that they can no longer sit around and wait for the aerobic process to pull in all those energy-rich oxygen molecules and re-package them into clean little CO2 and water molecules, the mitochondria starts grabbing less energy-dense but more readily available organic compounds, smashes them together to break them into little firecrackers of energy. This is a messier process (as you may have noticed from the above chemical formula), which only gives us 4 ATP. Less bang for the buck, but the needs are exponentially higher, and the time it takes to get that energy is reduced, which is why this metabolic “short-cut” gets used. The resultant waste products get dumped out into the bloodstream faster than the liver, kidneys, and other cells can clean them up, and the increasing number of hydrogen ions acidifies the blood plasma.
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