The Energy Systems – Ep. 3 “The Anaerobic glycolytic pathway”

Last week, we described the mechanisms that drive the phosphocreatine pathway. In this week’s episode, we will take a look at the pathway that kicks in right after: Anaerobic glycolysis. To be as complete as possible we have to describe this pathway as anaerobic or nonaerobic glycolysis/glycogenlolysis – which means “the breakdown of glucose or glycogen in the absence of oxygen. The mechanisms of this pathway kick in after several second and last up to minutes.
During high intensity exercise, the preferred substance for muscles to regenerate ATP is glucose (sugar). There are two ways in which the muscle can get the sugar it needs: 1. Glucose is available in the bloodstream (the amount is highly regulated by hormones such as insulin and glucagon, that can for example stimulate the release of glucose from the liver). 2. Glycogen (glucose linked together) in the muscle itself.

When intensity is extremely high, the muscle prefers glycogen. When the intensity is relatively high, the muscle can thrive on regular glucose as well.
Sugar is broken down through several systems. At first the anaerobic pathway that we discuss in this episode, but this system is incomplete in the breakdown – to realize the full breakdown of glucose, the end product of the anaerobic pathway have to go through the aerobic (with oxygen) pathway.
Now what exactly happens in the anaerobic pathway? As explained in last weeks episode, your muscles need ATP to contract. This anaerobic glycolytic pathway does this through a series of reactions called “glycolysis” – which is a reaction that happens in the cytosol of the cell. Demonstrated in the picture (from Khan academy, a great resource for clear explanations of complex topics) is the breakdown from glucose to pyruvate. Now, in the not simplified version there are 9 steps in this breakdown. For the sake of keeping this topic more approachable, we will  not look at the individual steps. What matters most is that, as drawn in the picture, during some steps ADP is converted to ATP, and during others NAD is converted to NADH.
The ATP can be used immediately as you know by now. But what is NADH? NADH is a cool molecule, because inside of it is a lot of energy. To release that energy the NADH has to get into the “Electron transport chain” which is a complex located on the inner membrane of the mitochondria (the energy factory of the muscle). Through multiple steps that are based on electrical charge, 2,5 ATP per NADH is created.  In total, one turn of the anaerobic glycolytic pathway produces 5 ATP. Now, that is less than what the aerobic (with oxygen) can produce: 20 ATP. However, the speed at which the anaerobic system turns, makes that it can produce so much power.
Very important to realize is that when not enough oxygen is present, NADH can not be broken down to NAD and energy. However, the NAD is absolutely necessary for the anaerobic glycolytic pathway to run, so without it, it slows down. Combine this with the release of H+ throughout the pathway (burning sensation) and you understand why this pathway is short of duration. Training the anaerobic pathway is best done through high intensity interval training, with different results from different interval proportions.  
In the next episodes we will take a look at the aerobic pathways and we will clarify some myths around lactate. Stay tuned!