6. Energy Systems
How the body powers all movement
This article provides an overview of the energy systems of the body, describing how all movement is powered.
For the sake of this post, we are going to assume that all “energy production” is for the purpose of producing movement by contracting muscles.
There are 3 key points you should take away from this post:
The role of ATP
The 3 energy systems
How the energy systems interact
ATP - the currency of energy
We eat food to give us energy. To bridge the gap between digesting the food we eat and producing movement, there is a molecule that acts as the currency of energy in the body. That molecule is called ATP (adenosine triphosphate). This molecule
When food is digested, it is broken down into smaller and smaller parts:
Carbohydrates → Glucose
Fat → Free Fatty Acids
Protein → Amino Acids
Only in this form can it be used by the energy systems which are described in the next section. The purpose of these energy systems is to produce ATP. The energy systems can be thought of as power plants. Each one differs because of the fuel they use, how fast they can produce ATP, how long they can sustain this production and what waste products are produced along the way. The three systems work together to ensure the body has energy available no matter the situation [sprinting up a flight of stairs vs running a marathon].
Once the ATP is produced, it can be used by the body for a number of tasks. We are focusing on its use by the muscle fibers, where it causes them to contract. The currency analogy is useful here. More contractions or faster contractions cost more ATP. As the demand goes up, such as in sport, the power plants [energy systems] are required to work harder. Luckily, the function of these power plants can be improved with training.
ATP is made up of 1 Adenosine molecule with 3 Phosphate molecules attached, hence Triphosphate.
ATP acts as a currency by moving energy around the body in the strong bonds between the Phosphate molecules. Therefore to power the muscles, a Phosphate molecule is broken off of the chain, transferring the energy in the process. This leaves a free Phosphate molecule and Adenosine Diphosphate [Di = 2, Tri = 3]. The energy systems then release the energy in the food to return ADP to ATP once again.
The Energy Systems - The Body’s Power Plants
As we’ve stated before the body has 3 energy systems. These differ in 4 key ways:
The fuel used
How fast they can produce ATP
How long they can produce ATP for
Waste products made along the process
We’ll focus on these 4 traits when explaining each of the systems. The term “Pathway” is sometimes used because it alludes to the fact that each system is really a series of chemical steps. It is not important to learn these steps at the moment, however take note that the more steps in a pathway, the longer it takes to turn a fuel source into ATP.
Single Effort Pathway
[Scientific Name: ATP-PCr/Phosphagen]
The first pathway is known as the ATP-PCr system, which stands for Adenosine Triphosphate – Phosphocreatine.
The name provides a clue, as the fuel used for this system is Phosphocreatine (PCr). PCr is stored in small, readily available quantities in muscle, where it interacts with ADP to quickly turn it back into ATP.
It is useful to call this the Single Effort Pathway as it can only provide ATP to the muscles for a short period of time, about 10 seconds, before the stores in the muscle are exhausted.
This reaction produces free Creatine. However, this is not a waste product as it is turned back into PCr when the body is at rest.
Repeated Effort Pathway
[Scientific Name: Glycolytic/Anaerobic Glycolysis]
The next pathway is known as the Glycolytic system.
If you are familiar with biology, you may recognise that Glyco- refers to sugar (glucose, carbohydrates etc.). Glucose is the simplest form of sugar and it ultimately is what is used as fuel. An important point about this system is that it does not require oxygen to produce energy.
The glycolytic system can provide energy fairly quickly. For one, the chemical pathway to turn Glucose into ATP is relatively short. Also, moderate amounts of a molecule similar to glucose can be stored in the muscle. This molecule is called Glycogen and it only needs 1 extra chemical reaction to be turned into glucose.
It is useful to think of this system as the Repeated Effort Pathway as it can be sustained for about 2-3 minutes. It is called upon when sub-maximal, yet still high movement demands are required such as repeated sprints, a round of boxing/martial arts, an intense period of team sport gameplay featuring combinations of intense efforts (run-tackle-cut-sprint-shoot).
There is a price to pay for the speed gained by not using any oxygen. By-products are produced as a result of the process, in particular lactic acid. If too much lactic acid builds up, it impairs the ability of the muscles to work.
The above two pathways fall under the category of ANAEROBIC, meaning they do NOT require oxygen at any stage of the process.
Endurance Pathway
[Scientific Name: Aerobic/Oxidative]
The final pathway is known as the Aerobic system.
This system can use two different fuel sources, carbohydrates and fat. Crucially, this system requires the presence of Oxygen to work.
We can refer to this system as the Endurance Pathway as it is relatively slow to produce ATP, therefore it is best suited for long, slow, continuous efforts such as endurance sports.
As you might imagine, the Endurance pathway can sustain it’s output for significant lengths of time, especially if fat is the fuel source being primarily used. While the exact yield depends on which type of fat is being burned, theoretically you can get 3-4 times as much ATP from fat versus glucose.
As a result of this system using oxygen, it is the “cleanest” energy pathway. The only byproducts of this system are Water and Carbon Dioxide, which are removed during breathing.
Energy System Interactions
While the energy systems are commonly presented as 3 independent options, in reality they are constantly working in sync to meet the demands of the body. A graph like the one below shows how much each system is contributing depending on how long the effort has been maintained.
In a sport such as marathon running, an athlete need only focus on developing the Endurance Pathway. For an athlete such as an Olympic Weightlifter, who needs to produce a lot of energy very quickly, the Single Effort Pathway is their go to system. An 800m runner is going to rely heavily on their Repeated Effort Pathway throughout their race. In these mono-structural sports, the task demands are well defined and unchanging, an 800m race is always 800m long. Therefore, they are relatively straight forward to prepare for as limiting factors to performance can be determined and measured. This is not the case however when it comes to team sports.
Team Sport athletes are unique because they need to be able to respond to anything that happens with high energy bursts over the course of a 60-90 minute game. This poses a unique challenge as these athletes must develop all 3 systems rather than focusing on 1 dominant one.
What’s next
This article provides an introduction to the key components of the energy systems, while touching lightly on practical implications. I’m sure questions are starting to bubble up as you digest this information. As your knowledge about the energy systems increases, you will begin to understand how different training methods can be used to improve limiting factors of our athletes energy systems.
Next up we’re doing to look at the Endocrine System, which is our hormones. We’ll see what how this system works and influences the body. This will be the last article focused on a scientific topic before we move on to practical applications!