The Oxidative System
TL; DR:
1) The oxidative system requires oxygen to function.
2) It is capable of breaking down carbohydrates and fat and has the highest capacity for ATP production of the three energy systems.
3) It is the slowest to ramp-up, requiring upwards of 2 minutes to become fully functional.
4) It is most useful for aerobic exercise and providing energy at rest.
5) The mitochondria is the powerhouse of the cell.
6) Magnets!
The oxidative system is responsible for the complete breakdown of carbohydrate and fat molecules and yields the highest sustained output of ATP. It is more commonly known as the aerobic system, and as such, is reliant upon oxygen to function.
While the phosphagen and glycolytic systems required one-step and ten-step chemical reactions respectively, the oxidative system undergoes a much more complex series of reactions. It requires a full cycle of slow glycolysis before utilizing those products in an additional intermediary series of reactions called the citric acid cycle. The products of the citric acid cycle finally enter a process called oxidative phosphorylation which (re)produces ATP.
While superficially complex, simplicity can be found in the chaos.
In the initial post explaining the phosphagen system, we outlined that the action of breaking a phosphate bond off of adenosine triphosphate is what generates usable energy to drive muscle contraction. Therefore, it would be reasonable to assume that it requires energy to re-attach a phosphate group to an ADP molecule in order to make it usable once more. This is important as it relates to understanding the oxidative system moving forward because this additional energy must come from somewhere (hint – not more ATP).
The oxidative system is fueled by “food” molecules. The skeletal structure of carbohydrate and fat molecules are considered hydrocarbons (repeating chains of hydrogen and carbon) - which is what makes them much more potent, sustained sources of energy. The function of the oxidative system is to essentially strip-mine these long-chain hydrocarbons and stockpile the hydrogen found within them. These hydrogen atoms are later harnessed to power the conversion of ADP back into ATP; thus, the longer the hydrocarbon chain, the greater amount of bonds available to be strip-mined.
While a muscle exhausts its available ATP stores it simultaneously accumulates pools of ADP and free-floating phosphate. If breaking that phosphate bond produced usable energy, as previously inferred, it will require energy in order to reassemble. That’s where the stockpile of hydrogen atoms comes in.
In a specialized organelle (mini organ) within your muscles (the mitochondria), a concentration gradient of hydrogen is created. On one side of the mitochondrial wall, progressively more hydrogen atoms accumulate as they are deposited from the strip-mining process; however, with only one side accumulating these hydrogen atoms, there is a strong counter pressure (energy) driving the atoms to more evenly distribute.
Here are some analogies that may help this point become clearer.
Imagine you are holding two magnets. When you try to make the same pole from each magnet touch, they repel. Now instead of magnets, insert hydrogen atoms. A more visual example would be tossing a bath bomb into the tub. At first the bath bomb is primarily concentrated where you placed it, but very rapidly the contents of the bomb will evenly disperse throughout the water. In combination, these examples roughly explain what happens when the hydrogen concentration gradient is established.
There is energy being produced by the hydrogen atoms repelling one another and funneling themselves into a more even distribution. It is this energy that is being exploited to attach the free-floating phosphate molecules to ADP – thus reforming usable ATP. This is oxidative phosphorylation.
The entire process understandably requires a ramp up time in order to fully function, and as such, can only provide timely energy during rest and for exercise that is at a low enough intensity to accommodate the speed of production. While the oxidative system may not be the most useful while actually completing a resistance training set, it is actually quite complimentary in helping with recovery after the set; thus, completing the trifecta of energy systems and fully establishing the continuum during exercise.
Attached is a simple table to act as a quick reference tool for the training parameters and uses for each system.
Best,
Eric
