This post is part of an ongoing series about my learning process as I train to become a personal trainer. This is the second of three posts about my journey into the strange and wonderful world of energy systems. Last week’s post was about the aerobic system, this week’s post is about the anaerobic system and next week is about the creatine phosphate system.
***
Anaerobic respiration: the basics
Starting with the basics, anaerobic respiration is a way for us to produce usable energy, in the form of ATP, without oxygen. In the simplest of terms, it is respiration without oxygen. It is useful when we need to exert ourselves above and beyond the level at which we can operate aerobically. The three steps of respiration are:
- Glycolysis;
- The citric acid cycle; and
- The respiratory chain, or electron transport chain.
In order for the electron transport chain to function, a final electron acceptor must be present to take the electron away from the system after it is used. In aerobic respiration, this final electron acceptor is oxygen. In chemical terms, the oxygen has been “reduced”.
In anaerobic respiration, the final electron acceptor is pyruvate, which is another by-product of glycolysis. The pyruvate has been reduced instead of oxygen. This reduction of pyruvate forms lactate, often called lactic acid.
***
Anaerobic respiration: the more technical explanation
During glycolysis NAD+ is reduced, forming NADH. However, there is only a limited supply of NAD+ available in a cell. For glycolysis to continue, NADH must be oxidized (have electrons taken away) to regenerate the NAD+ so that the reaction can continue.
In anaerobic conditions, the NADH reduces the pyruvate molecules that are also formed during glycolysis. Since the NADH has lost electrons, NAD+ regenerates and is again available for glycolysis. Lactic acid is formed by this reduction of pyruvate.
***
So what’s the point of anaerobic respiration?
The point of anaerobic respiration is that we can exerts ourselves at much higher levels than we can maintain aerobically. Two qualifications apply to this statement, however.
- We can only maintain these higher levels for a short period of time. How long we can maintain these higher levels for is a matter of training. What causes us to stop is a matter of debate.
- We experience Excess Post-Exercise Oxygen Consumption (“EPOC”) following a period of anaerobic respiration. This is the feeling of breathlessness we get after a hard effort.
***
But I thought lactic acid build-up put a brake on performance?
Well, there are two major theories about the role of lactate in the body.
Theory #1: the metabolic by-product
Work done by the German biochemist Otto Meyerhoff in 1922 suggested the first theory, that lactic acid was used to replenish muscle glycogen, as experiments with frogs legs showed a decrease in lactate and a corresponding increase in glycogen it has since been discovered that amphibians function slightly differently from mammals).
After that, the theory was refined slightly but was left fundamentally unchanged for many years. In my textbook, for example, it says that during hard exercise lactic acid is produced and diffuses quickly into the bloodstream, where it is transported away from the site of the exercising muscles. The acidity of the blood increases and this is the reason for the burning sensation and the fatigue. During recovery, the lactic acid is converted back to glucose through gluconeogenesis in the liver.
Theory #2: the Lactate Shuttle Theory
However, in the 1980s, George Brooks came up with a different theory, called the Lactate Shuttle Theory (see more here). The theory has been reviewed recently by both Clarence Bass and the New York Times. (See here for more of Clarence Bass) Fundamentally, the theory is that lactate is a preferred fuel in the muscles and is processed through the mitochondria. Brooks says of his theory that:
“The theoretical construct is that, together with blood glucose, glycogen reserves in diverse tissues are mobilized to provide lactate, a glycolytic product that is either used within the cells of formation or transported through the interstitium and vasculature to adjacent and anatomically distributed cells for utilization. Hence lactate is a quantitatively important oxidizable substrate and gluconeogenic precursor, as well as a means by which metabolism in diverse tissues is co-ordinated.”
***
And what is EPOC, again?
EPOC is the increased level of oxygen we take in after stopping hard exercise. The first theory of EPOC came in 1920, where the concept of an “oxygen deficit” was introduced. This was measured as the difference between the required oxygen for the exercise and the measured VO2 Max.
Early exercise physiology researchers, including the great British Nobel Laureate, Archibald Vivian Hill, believed that the equation was as simple as saying that the oxygen that was required but not supplied had to be “paid back” in a 1:1 ratio. The idea of “paying back” led them to coin the phrase in financial terms as “oxygen debt” rather than ”oxygen deficit”.
Wait a moment, while I calculate your oxygen debt, please sir…
***
However, later work found that while the 1:1 ratio holds true for lighter exercise, it can be 1:2 or even 1:3 for very strenuous exercise. This seems odd. Why would we need to pay back a larger amount of oxygen than we needed the first time around? Has interest been charged? If anyone knows why this is, please point me in the right direction…
***
That’s all for this week. Next week will be a quick run through the creatine-phosphate system.
Pingback: Good Reads for the Week « Bret's Blog