Conditioning 101: Energy Systems
What does it mean to be “conditioned?”
Often times when people think of working out, their first thought is training a muscle. This isn’t always the case, though.
Many times, the purpose of a training is to improve your body’s energy systems. Building these up is what “conditioning” is all about.
So, the purpose of this article is to explain what these energy systems are, how to train them, and to do so without getting too in the weeds of their biochemistry. There are three different energy systems that are relevant, and we will break them all down together.
ATP: The molecule of energy
You may or may not have heard of this compound before. ATP is an acronym for adenosine triphosphate. It is a molecule of adenosine that is holding onto three phosphate groups (hence, triphosphate). When ATP releases one of these phosphate groups, it becomes adenosine diphosphate (ADP; “di” = 2). This chemical reaction of releasing a phosphate group releases the energy that we use to move.
The goal of the energy systems in our body is to continually replenish ATP as we deplete it during exercise. The energy systems will mainly differ in how much ATP they are able to produce and how quickly they are able to do so.
When we train or eat in ways to improve our energy systems, we are working to improve their ability to produce ATP. This will result in the energy system being able to provide energy for longer or allowing you to do more work in the same time constraint.
The Phosphagen System
As we break down these energy systems, we will go in order from high intensity exercise to low intensity exercise. Following this fashion, the first that we will discuss is the Phosphagen system.
This system is a simple, one-step reaction. A molecule of creatine phosphate donates its phosphate group to ADP to convert it back to ATP. As you may be able to guess, this is what supplementing with creatine helps with.
Due to the fact that this reaction is only one step, it is able to replenish energy at a very rapid rate. However, creatine stores are very limited compared to other energy substrates. As a result, this energy system is the main producer of energy for highly intense exercise lasting ~1-10 seconds. Examples include weight lifting in the 1-10 rep range, sprinting 10-100 yards, vertical jumps, etc. In fact, one of the biggest reasons why we are unable to sustain exercise of this intensity is because our creatine phosphate stores run out; which is why creatine supplementation is so powerful.
Training to improve this energy system is characterized by bouts of all-out intensity. Some examples include: sprinting, sprints on an air bike, heavy lifting, power training, etc. Duration makes an easy metric to make sure you are targeting the energy system that you want to. If you can sustain the movement for more than 10 seconds, you likely are not training with high enough intensity to specifically target this energy system.
Supplementing with creatine is another easy, practical way to improve this energy system. The best way to optimize this energy system is to saturate your muscles with creatine, and the supplement form of the molecule will do just that. For a longer breakdown, check out the article below:
Related: Creatine: Everything You Need to Know
Glycolysis
This energy system contributes to exercise of slightly less intensity, but still sufficiently intense exercise nonetheless. Glycolysis is comprised of more steps in the overall reaction but is able to produce more ATP than the Phosphagen system. As a result, it is the big contributor of exercise lasting ~1-4 minutes.
The substrate that fuels glycolysis is carbohydrate. Carbohydrates enter glycolysis and are broken down throughout many steps. Along the way, ATP is replenished at various points in the process. This is the main reason why having a sufficient carb intake is crucial for optimizing performance in exercise. Further, one of the main causes of true physical fatigue is a depletion in stored carbohydrates. This is because this energy system begins to fail without carbohydrates available.
The end-product of glycolysis can be either one of two things: lactate or pyruvate. Which one ends up being produced depends mainly on the intensity of the exercise being performed.
If the exercise is of a high enough intensity, then lactate ends up as the end-product. Lactate being produced proposes a trade-off to the system. First, it allows glycolysis to produce energy faster. However, with lactate comes an associated build-up of acidity in the muscle. If the acidity builds too much, then glycolysis becomes inhibited and limits energy production & contractility of the muscle. The end result ends up being less energy produced overall but at a faster rate.
Related: Lactate Threshold Explained
If the exercise is of low enough intensity, then pyruvate ends up being the end-product. Pyruvate will end up leaving this energy system and contributing to oxidative phosphorylation (discussed next). When pyruvate goes down this pathway, much more energy is able to be produced but at a slower rate.
Think of it this way: your body has to make a choice based on how intensely your exercising. If it is experiencing high intensity exercise, then it must produce lactate to produce the energy needed in the moment; sacrificing a greater magnitude of energy. If the intensity of the exercise is low enough, then it will prioritize pyruvate. With it not needing so much energy in the moment, it’s able to choose the pathway that produces more energy at a slower rate.
There are two ways you can ensure that this specific energy system is being trained. You can monitor duration of movement, or you can monitor heart rate.
As we previously said, this energy system provides the majority of energy during exercise sustained for 1-4 minutes. So, if you cannot physically continue 3:30 into the exercise, then you can rest assured that this energy system was trained.
If you have a heart rate monitor, such as a Garmin watch or polar chest strap, then this can also make an easy way to make sure you are training at the right intensity. The key is to make sure your heart rate is in either zone 3 (70-80% heart rate max) or zone 4 (80-90% heart rate max). An easy ball park to determine your maximum heart rate is subtracting your age from 220. It’s not always the most accurate, but it’s good enough for most people.
Examples of modalities that are helpful here include:
running
air bikes
ski erg
concept 2 rowers
steady state kettlebell swings (lasting 1-4 minutes)
etc.
There are also a few nutritional strategies you can utilize to improve glycolysis. The first of which being to eat enough carbohydrates every day (~6-10 g per kilogram body mass per day), especially before you train. Also, beta-alanine can be a helpful supplement. Its main benefit is buffering the build-up of acidity that can inhibit glycolysis. For more information, check out the article below:
Related: Beta-Alanine: What You Should Know
Krebs Cycle/Oxidative Phosphorylation
Let’s start this section with a fun fact. Do you remember in high school biology when your teacher told you that the mitochondria is the powerhouse of the cell? Well, that’s because it’s where the krebs cycle and oxidative phosphorylation occurs.
Really, these are two separate energy systems, although they are dependent on each other. So, we will consider them the same for the purpose of this article.
The pyruvate produced from glycolysis contributes to the energy produced here. Also, your body is able to break down fats in this process and utilize them for ATP production. Fats overall have much more stored energy than carbohydrates, which is why they have 9 calories per gram and carbohydrates have 4 calories per gram.
The process of producing energy here requires many more steps, so it is less able to contribute to exercise where energy is needed quickly. However, it is able to produce a much larger magnitude of energy than the previous two energy systems. This is the energy system that fuels long distance, low intensity movement. Examples include: long bike rides, long hikes, slow jogging, etc.
The krebs cycle and oxidative phosphorylation are crucial training points for endurance athletes of any kind. If you are a soccer player, basketball player, or similar athlete, then optimizing these energy systems will improve your recovery in between your higher intensity runs; as well as maintain stamina at the end of a match. Similarly, if you are a recreational lifter, then this can improve your recovery in between sets and workouts.
Similarly to glycolysis, duration of movement make excellent indicators for making sure this energy system is being trained. However, the parameters are different. They are as follows:
Duration of movement needs to be extensive to specifically target these energy systems. Ideally, somewhere between 20-60 minutes. You could either pick a time within this range and try to improve the work done in that time; or you can pick a pace and increase the time in which you sustain that pace.
Again, heart rate monitors make excellent tools to ensure you aren’t working too hard. Here, though, you want to keep your heart rate in a zone 2 range, which is 60-70% of your heart rate maximum. If you want to take your conditioning work to the next level and use heart rate based training, I recommend one of the two following tools:
Summary
In conclusion, the following are some good ending bullet points regarding these energy systems:
Phosphagen system: very high intensity; lasts ~1-10 seconds
Glycolysis: high/medium intensity; lasts ~1-4 minutes
Krebs cycle/oxidative phosphorylation: low intensity; contributes primarily ~4+ minutes
Phosphagen system: uses creatine phosphate as the substrate
Glycolysis: uses carbohydrates as the substrate
Krebs cycle/oxidative phosphorylation: can use carbs or fats
Phosphagen system: produces little energy very quickly
Glycolysis: produces moderate energy at a moderate pace
Krebs cycle/oxidative phosphorylation: produces a ton of energy very slowly
References
Haff, Gregory G., Triplett, Travis N., 2016, Essentials of Strength Training & Conditioning, 4th edition.
