Ever heard someone talk about any of the following things:

Blood Lactic?
Lactic threshold?

If so, where you left dumbfounded, did you feel like a rabbit caught in the headlights of bullshit? If so, don’t worry; it’s not your fault! It’s the Internet’s. However, I digress. With the ease with which people can put down their opinions on everything under the sun on the Internet, it becomes very hard to separate the wheat from the chaff. As such, I will be providing you with a basic, introduction to the various energy systems of the body, and all that this entails. I have taken all of this knowledge from various textbooks, so this should be up to date correct.

anaelacticphospho what now?!

Ok, so here we go! Right, so, the body is like a car, it requires fuel or its not going anywhere. This fuel is the food we eat, which in essence is broken down into the macronutrients of Carbohydrates, Proteins, and Fats. Each energy source is metabolized differently to generate useable energy. The only energy unit however, that is useable by the body is adenosine triphosphate (ATP). ATP is the bodies’ energy currency, and is used in one of the three bodily energy systems, which are the phosphocreatine system, the aerobic system, and the lactic system. We’ll get onto these later.

First things first, we get energy from foods, but all these foods are metabolized in different ways.
Carbs! Yes, fruit and veggies are carbs, get over it!

So, carbs have 4 kcal per gram. The NHS nutritional guidlines state that 60-65% of your caloric intake should be carbohydrate however, this prescription is based on an out dated understanding of how foods are metabolised and is slowly being stamped out in the minds of the public and our health officials thank God. All carbs are in essence sugar waiting to happen; besides the molecular structure the only thing that separates a gummy baby from a sweet potato is the time it takes the body to break down these foods into usable sugar, i.e. glucose.

Carbs are not easily stored within the body, with small amounts being stored in the muscles and liver as glycogen, which can be reverted into glucose when needed. Glycogen stored in the muscles can only be used in the muscles and cannot be released in the blood for use elsewhere. Carbs should be taken pre and post work out and limited the rest of the day, with the amounts of carbs varying depending on your goals… that’s right, I am saying you should limit carbs!

The reason for this being that Glycogen has a relatively large chemical structure, as it is made of many glucose molecules. When activity levels start to increase, or are anticipated to increase, adrenaline, and glucagon (a fuel mobilizing hormone) send messages for the enzymes within the muscles cells to work to break the glycogen apart, via a process called glycogenolysis.

Fats. Yummy yummy fats!

Fat provides the greatest amount of energy per gram at 9kcal per gram. Fat is stored in the body both underneath the skin and around the organs. The same hormones that stimulate the break down of glycogen into glucose stimulates that breakdown of fat in the adipose tissue into fatty acids to be used for energy production. This is known as lipolysis. Now, the funny thing about fats is this, not only are these vital for hormone production, testosterone levels, the fitter an individual, the more efficient they are at extracting energy from fat. Fat needs a great deal of oxygen to be metabolized effectively. The cardiovascular adaptations that occur with regular cardio vascular training improves the abilities of an individual to take up oxygen and deliver it to the working muscles making it easier for the body to use fat as fuel even at higher intensities. This is a useful survival mechanism, because which carbs run out quickly, there is an abundant supply of fat to fuel ongoing activity. A fit individual can spare the carbs until it is really needed and maintained activity, using fat as the main fuel. Some carbs are still needed to aid the metabolism of fat as it acts as a metabolic primer.

Meat is Murder! Tasty, tasty murder!

Protein, the building blocks of life and the key to you becoming the a more optimally functional human being. I am not going to do this section to death, as everyone seems to know a fair bit about protein. It provides 4kcal of energy per gram. Protein is stored in the body as muscle and is only used for energy production when carb stores are deleted. Proteins need to be broken down into amino acids and converted into glucose by the liver if they are to be used for energy production. This process is known as gluconeogeneis.

This is where magic happens!

Ok so, those are the fuel sources. Muscle glycogen is stored to fuel muscular activity, such as weightlifting, and muscle glycogen is stored for use by the brain. If there is no glucose remaining in blood and the liver glycogen store is running low, carbs must be ingested or it must start to be made internally. Amino acids are used for this through the break down or protein.

The demand of ATP synthesis varies depending on the intensity of the activity. The more ATP is required the quicker it needs to be synthesized. If it cannot be synthesized quick enough, then intensity of activity must lower, even at its fastest, it takes a minimum of 10 seconds to synthesize enough ATP. The body will use a mix of fat in the form of fatty acids and carbohydrates in the form of glucose to synthesis ATP. Fat can only be used in the presence of oxygen. Without oxygen, only carbohydrate can be used. As already mentioned, there are plenty of fat stores, but fat takes a relatively long time to metabolise.


The only source of energy that can be used directly by the body is ATP. It is made of one adenosine molecule and three phosphates, which are attached through high-energy bonds. Energy is produced when the bond between the second and third phosphate is broken. The by products of this are Adenosine diphosphate (ADP) and phosphate. ATP is really unstable and cannot be stored in the muscle. ADP and phosphate are stable. To maintain energy supplies ATP must be constantly re-synthesized, meaning energy is needed from somewhere else to reattach the phosphate that has broken away and allow the cycle to continue. There are three energy systems that perform this function.

The energy systems used to re-synthesized ATP will be depend on:

The intensity of the exercise / activity
Duration of the exercise
The type of exercise / activity

Phosphocreatine –

Muscle glycogen provides the initial fuel for movement when exercise starts. Straight away the body starts to metabolize fat and increase the flow of blood to the muscles being used, in order to provide the fat and oxygen required if activity is to continue. This process takes a minimum of 20 seconds, by which time the small amounts of glycogen held within the muscles has been completely used up in approximately 1-2 seconds.

Maximal intensity exercises have a huge glycogen demand

Within the muscle there is a store of creatine phosphate. Although this cannot be used directly, it can be converted very quickly when catalysed by the anaerobic enzyme creatine kinase. Only one chemical process is required to separate the phosophate from the creatine, so that phospate is added to the ADP to create more ATP.

Creatine phosphate is generated in the liver, but is also sourced from the meat that we eat. When muscle demands it, either more is synthesized in the liver, or more is taken from within the bloodstream. At best, a muscle can store no more than roughly 20 seconds worth of the stuff. The more the creatine phosphate is used, the greater the ability to sore it and the greater the amount of creatine kinase is made readily available. High intensity training involving repeated short bursts of explosive movements is likely to be the most effective way of achieving this.

Fast twitch muscle fiber’s (fast glycolytic) will use the phosphocreatine system for energy production. Their low aerobic ability means that they need to use an energy system that can provide energy without the use of oxygen (anaerobically). Their suitability to short bursts of intense activity also means that the best energy system for them to utilize is the phosphocreatine system.

The phosphocreatine system fuels short bursts of very high or maximal intensity activity. This should be reflected when one tries to improve the efficiency of the phosphocreatine system. By alternating maximal efforts with long recovery periods using interval training, this energy system can be effectively overloaded to bring about adaptation.

For example, a max effort squat would use all the muscular stores of creatine phosphate. A long recovery is then required to allow the body to refuel these stores, roughly 6 minutes. Therefore only allowing the body 3 minutes allows for a gradual depletion of the bodies creatine phosphate over a number of maximal efforts.

Lift heavy, often, with good technique.
The Lactic System

Once all available creatine phospate has been used, intensity of activity must lower, as no other system is able to re-synthesize ATP as quickly. If activity is continued then the next fastest system is the lactic system. The lactic system is also used if an activity is started at a less intense rate than required for the phosphocreatine system. The body continues to metabolize fat and increase the flow of blood to the muscles involved, to provide the fat and oxygen required if the activity is to continue. The lactic acid system is more efficient than the phosphocreatine system at generate ATP, producing a max of three ATP molecules per molecule of glycogen via a process called anaerobic glycolysis The process of anaerobic glycolysis takes longer than the phosphocreatine system, due to the more chemically complex nature of the glucose molecules, which causes a rapid depletion of the bodies glycogen stores.

Two variables limit the ability of the lactic system to continue working. Firstly that glucose stores run out pretty damn quick. The second is that the system produces more of its waste product, lactic acid… duh, than the body can handle. This leads to all those nasty cramps and vomiting that we sometimes experience when we feel the “burn”.

Why does it hurt so much to be this fast?!

The build up of lactic acid in fact prevents further ATP generation. The muscles continue to try to contract, but with each contraction less ATP is generated. Once intensity of exercise decreases, lactic acid is dispersed into the blood and taken to other muscles or the liver. The speed or recovery is related not to only the intensity of activity that is continued but also to each individuals personal abilities. Total recovery from an over load of the lactic system is approximately 30 minutes. An individual will first reach the aerobic threshold. This is the point at which energy production begins to shift in favor of anaerobic pathways, but lactic acid concentrations have not yet reached a level that will inhibit performance, which lets be honest, sucks.

The point at which lactic acid is being produced faster than it can be removed is known as anaerobic threshold. This threshold is also known as the point of onset blood lactate accumulation (OBLA). To improve the lactic system interval training is an effective method of improving the lactic acid system. By alternating work efforts that create a lactic build up with periods of recovery to remove the lactic acid by product we can begin to increase the anaerobic threshold and point of OBLA. Fartlek (speed play), cruise intervals, HIIT are all good methodologies to follow to improve the lactic acid system.

The Aerobic System

Once the lactic acid has prevented further generation of ATP, intensity of activity must be lowered. The aerobic system then takes over. Aerobic production of ATP is by far the most preferred method. As long as there is sufficient oxygen present, the body always selects the aerobic system. Not only does this utilize fat as well as carbs, leaving more carbs in the tank, but it also is able to generate more molecules of ATP. Aerobic (with oxygen) glycolysis generates up to 38 molecules of ATP for each molecule of glycogen.

Aerobic glycolysis takes place in specialized muscle cells called mitochondria. Mitochondria are essentially cell batteries and are found within muscle fibers. The point at which intensity of activity increases so much that the body is no longer able to get enough oxygen to get to the working muscles is known as the aerobic threshold. Again, to a certain extent this is genetically set. Appropriate training, in the form of interval training with peaks just about the aerobic threshold, can improve the aerobic threshold by enabling the body to transport and work with oxygen at a higher intensity.

When glucose is broken down in the presence of oxygen it is converted into pyruvate. This process is known as glycolysis. In the absence of oxygen the pyruvate becomes lactic acid and eventually inhibits further muscle contraction. As long as oxygen and carbohydrates are present the aerobic energy systems can last indefinably. The by products of aerobic production are CO2, water and heat. The body easily removes all these, so it presents no limiting factors. The aerobic energy system will at the time be using a combination of fats and carbs to produce ATP. Protein can be used when glycogen stores have been depleted. The proportion of fat and carbs used is determined by the intensity of the activity. As intensity increases a greater proportion of ATP synthesis will come from aerobic glycolysis until anaerobic threshold is reached. However, the maximal effective duration of the Aerobic system is only two hours.

So there we are boys and girls, a brief insight into the world of energy systems. I hope you enjoy.

Train Strong.

Live Strong.

Be Strong.


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