Understanding the Underlying Purpose of the Energy Systems

The most important thing for detectives trying to solve a case is to understand the motive of potential suspects. Training the energy systems of an athlete is one of the most important jobs of the strength and conditioning professional. To solve this case, you must understand the motive force behind why the energy systems are present in the body. I’m going to say the same thing a bunch of times in a row in the following sentences because I need to kick the absolute hell out of this dead horse to reinforce the point I’m going to try to make with the gravity it deserves. The purpose of the energy systems is to deal with the outcome of the hydrolysis reaction of ATP. Stated in another way, the purpose of the energy systems is to rephosphorylate ATP and to deal with the threat of hydrogen and heat that cellular and mechanical work imposes upon the organism. Stated in another way, the purpose of the energy systems is to allow you to perform sufficient levels of ATP hydrolysis to power your organism’s need to engage in behaviors in specific environmental circumstances. If you do not understand this underlying purpose and the ways in which this plays out in the body, then you do not truly understand energy system training. We all have our pet peeves. One of mine is that I can’t stand it when people say that energy systems create energy. Another one is any time I hear anyone say anything about lactic acid. Energy can neither be created nor destroyed. Energy is transferred from one state to another inside the body. Lactic acid does not exist inside the human body. Lactic acid never has existed inside the human body. Lactic acid never will exist inside the human body. These statements may sound like condescending, semantical remarks made by an exercise science nerd; however, I do not think they are, and I think that failing to address these concerns will continue to lead to erroneous thought processes in trying to develop energy system training. I think these pet peeve concepts of mine are related to the two biggest missing links in our field’s current view of developing the energy systems, which are both fundamentally tied with failure to appreciate the two-tiered purpose of the energy systems.

Threat Deterrence

We probably all know about the concept of ATP being the energy currency of the body. The ability to restock your supply of ATP is one of the two purposes of the energy systems. This is the most commonly discussed factor in regards to the science of energy systems, and I will surely address this here, but first I would like to discuss the second energy system purpose, which is threat deterrence.

Hydrogen is the most abundant material in the universe, with approximately 80% of the known universe being made up by hydrogen. Movement of hydrogen is what drives the universe. When viewing the internal universe of the human, hydrogen is both the driver of life and something that can kill you quickly if left unchecked. Entropy is the direction of the universe. The universe is expanding and the energy found within the universe is headed more and more towards a chaotic state. Heat is the expression of entropy most prominently displayed by life forms. Try living as a mammal without heating yourself. Hydrogen load and heat load are perhaps the two most fundamental things that the human body has to manage. If not kept within a careful window of appropriate levels, you will surely die. We have a variety of measures and systems that we use to regulate hydrogen and heat, and the energy systems are a powerful one when it comes to the hydrogen threat.

There is no lactic acid inside your body, therefore it is not a threat. Lactate production is an outlet for dealing with an acid threat, and is therefore not a threat (it’s a strategy). Hydrogen is real, and very present inside your body. Hydrogen is a threat, and hydrogen must be accounted for. Where does this hydrogen come from though? Hydrogen is a bi-product of the hydrolysis of ATP. Every time I do anything inside my body, I need to power that action via the hydrolysis of ATP. The potential energy that will power my bodily actions is found in the bonds between the phosphates making up the ATP molecule. I must break these bonds to release energy from a bound/potential state to make it available as free energy to perform work. The body uses a hydrolysis reaction to break these bonds. Hydrolysis reactions are those that require water to be present. When ATP combines with water in the presence of the enzyme ATPase, the bond between the second and third phosphate is broken, and stored energy is released. The reaction looks like this:

ATP + H2O (in the presence of ATPase) → ADP + P + Free energy + Hydrogen + Heat

We did this to gain the release of free energy. Free energy release is the purpose of the hydrolysis of ATP. The energy systems are in the body to deal with the outcomes of the hydrolysis of ATP.

Three Strategies


The energy systems put ATP back together again after it is broken down. We have three strategies of putting ATP back together again, a phosphagenic one, a glycolytic one, and an oxidative one. The phosphagenic and glycolytic strategies are the most primitive, and took place in cellular life forms prior to the evolutionary step of mitochondria creating a mutually symbiotic relationship with cellular organisms by moving into the cells of other creatures. The phosphagenic energy system can rephosphorylate a singular ATP through its one enzymatic step, but it cannot do anything to reduce hydrogen or heat levels inside the body. Here is the primary reaction used by the phosphagen system:

ADP + CP (in the presence of Creatine Phosphate) à ATP + Creatine

The phosphagenic energy system has low cost associated with it, since it does not cost any ATP to run the system. This lack of cost cannot be said about the glycolytic system.


The glycolytic energy system has the ability to rephosphorylate 4 ATP (you receive a net of 2 ATP, because you have to spend 2 ATP to power the glycolytic machinery) through 10 enzymatic steps. Glycolysis can also directly take two hydrogen ions out of circulation. To view the ATP rephosphorylation and hydrogen reduction capacity of glycolysis, the following image is helpful (note that the hydrogen is reduced at step 6, where NAD combines with a hydrogen).


The non-oxidative energy systems pale in comparison to the ability of the oxidative energy system to rephosphorylate ATP and reduce the hydrogen threat inside the body. One of the interesting things about the oxidative system is that it actually powers itself through the motion of hydrogen.


The oxidative energy system utilizes the Krebs Cycle and the Electron Transport Chain (ETC) to rephosphorylate ATP and to reduce the hydrogen threat inside the body. Very little ATP rephosphorylation takes place within the Krebs Cycle; however, the products of the Krebs cycle power the ATP rephosphorylation machinery of the ETC. The primary product of the Krebs Cycle that powers the ETC to rephosphorylate ATP is NADH and FADH2. Every NADH that enters the ETC allows the ETC to rephosphorylate 3 ATP, and every FADH2 allows the ETC to rephosphorylate 2 ATP. The Krebs Cycle churns out 8 NADH and 2 FADH2 molecules every time carbohydrates are the substrate being utilized to power the energy systems (note fats have the potential for many more NADH and FADH2 molecules). The following diagram depicts the NADH and FADH2 synthesizing steps of the Krebs Cycle (note that the Krebs Cycle spins twice when carbohydrate is the substrate):


It is fair to say that when it comes to the power of the oxidative energy system, the ability to shuttle NAD/NADH back and forth between the Krebs Cycle and the ETC is the show. If you have a super powered ability to load hydrogen onto NAD (which converts it into NADH), move NADH to the ETC, unload the hydrogen from NADH at the ETC (which converts it into NAD), and then return that NAD to Krebs to repeat the procedure, you will have a monster aerobic system. It is probably also fair to say that NADH is the show inside the show, and the thing that nobody is talking about. Finally, it is tremendously fair to say that the purpose of the Krebs Cycle is not to rephosphorylate ATP directly, but to power the reduction reaction that results in NADH, which powers the ETC.

Electron Transport Chain

The ETC is the engine that is the big bang in the rephosphorylation of ATP. The ETC is also the best strategy for reducing (both literally and figuratively if you appreciate redox humor) the hydrogen threat. The ETC is a multi-enzymatic intra-mitochondrial machine that has the potential to rephosphorylate 28 ATP from the products of the Krebs Cycle when carbohydrate is used as the substrate (8 NADH at 3 ATP per molecule, and 2 FADH2 at 2 ATP per molecule). One of the first enzymes present in the ETC is one called NADH dehydrogenase. The purpose of a dehydrogenase enzyme is to remove a hydrogen ion from a molecule. NADH dehydrogenase cleaves the hydrogen away from NADH, which oxidizes the molecule and returns it to its state as NAD. When NADH is oxidized, the hydrogen ion is then shuttled outward from the inner mitochondrial membrane. To help understand this process, see the following picture:


In examining this picture, let’s start at the left. You see NADH being converted to NAD. This is taking place due to the activity of NADH dehydrogenase. You see the hydrogen ion being sent upwards into the space between the inner and outer mitochondrial membranes. Let’s skip over the activity in the middle of the graph to simplify this process. The hydrogen ion that was removed from NADH moves from the left to the right of the picture until it reaches the final enzyme on the right hand side. The most rightward enzyme is ATP synthase. As you can see in the picture, hydrogen moves downward through ATP synthase. The kinetic energy of hydrogen moving through the ATP synthase enzyme is what powers the enzyme to rephosphorylate ATP. ATP synthase is the location where all of the ATP rephosphorylation takes place inside the ETC. From an ATP rephosphorylation standpoint, let’s say that ATP synthase is the show. While giving the credit to ATP synthase for the product that we’re looking for, let’s not forget that it is hydrogen that powers this enzyme’s activity. As I said before, in the internal universe of the human, it is hydrogen that drives life.

While hydrogen drives life inside the human, unchecked, overabundant hydrogen will also kill you very quickly. The hydrogen that powered ATP synthase must be accounted for once it has given this enzyme its motive force for ATP rephosphorylation purposes. Have you ever wondered why the oxidative energy system is named as such? The answer is simple. Oxygen must be present for the system to run. The location of oxygen in this process is inside the inner mitochondrial matrix, specifically right below ATP synthase. When the hydrogen passes through the ATP synthase enzyme, oxygen is sitting there ready to receive it. If I combine two hydrogen with an oxygen, I get water. Synthesizing water is the most effective and least harmful strategy that organisms have adopted for dealing with the potential threat of hydrogen. When your body is able to power its behaviors via an electron transport strategy, the organism is operating in the least costly, most highly efficient manner possible, with the least amount of threat presented. When oxygen supply inside the mitochondria is not sufficient to deal with the amount of hydrogen present inside the mitochondria, or the shuttling of NAD/NADH to and from the Krebs Cycle/ETC is not robust enough or fast enough to move hydrogen through the oxidative pathways, the body is forced to go to a checkdown option and deal with hydrogen another way. This other way is via the creation of lactate.


Lactate is created when pyruvate binds to two hydrogen ions. Pyruvate is the product of glycolysis. To see pyruvate, let’s revisit our glycolysis diagram.


When it comes to glycolysis, things can be summarized into the following statement: one glucose enters, two pyruvates leave. There is no aerobic or anaerobic glycolysis. There is only glycolysis where a glucose comes and two pyruvate leave through ten enzymatic steps. The fate of pyruvate is what determines whether we operate with an oxidative or non-oxidative strategy. The hydrogen load inside the cell determines the fate of pyruvate. If the Krebs/ETC processes can handle the hydrogen load, things run smoothly. If Krebs and ETC are unable to handle the hydrogen coming from a specific rate of ATP hydrolysis, then we must call on the backup system, which is the synthesis of lactate. Lactate equals pyruvate plus two hydrogen. It is as simple as that. View the following image to appreciate this concept:


In viewing the above image, focus on the bottom. Pyruvate is on the left, lactate is on the right. Look at the molecular makeup of the two substances. The only difference between pyruvate and lactate is that a singular bond attaches one hydrogen ion on the left side of the structure, and another hydrogen is bound to oxygen on the right side of the structure. Lactate is a fantastic method of removing two hydrogen ions from existing in a free state. The purpose of the lactate system is to act as an alternative strategy for dealing with hydrogen load during times of extreme behaviors. Lactate is your checkdown receiver on a hot read.

Closing Thoughts

As the great Mike Cantrell likes to say at PRI courses, it’s cool that the aspirin works, but it’s cooler to know how it works. It’s cool to know that the program design approaches of Joel Jameison work. It’s cooler to know what’s happening inside the system that drives the reasons behind why they work. If you do not know why things work, you do not have a good BS detector. You will fall for stupid training concepts and you will be a garbage strength coach. If you want to be a beast in the majority of American sports, you need quality energy system development coached in the proper sequence of development. This may not be the fastest road to success, but it will be the road to the highest success with the least amount of detrimental stress put on your organism’s homeostatic control systems. We live in an age of information and accountability. If you are stupid, you are easily replaceable. Be an intellectual savage who does not accept ordinary, mundane, or low level things in your life. As you were.

about the author


pat davidson

-Director of Training Methodology and Continuing Education at Peak Performance, NYC.

-Assistant Professor at Brooklyn College, 2009-2011

-Assistant Professor, Springfield College 2011-2014

-Head Coach Springfield College Team Ironsports 2011-2013

-175 pound Strongman competitor. Two time qualifier for world championships at Arnold Classic

-Renaissance Meat Head