Key Takeaways

  1. An energy system is a physiological process that describes how the body transforms the energy from food into a form your cells can use to power various functions.
  2. Weightlifting relies on the phosphocreatine, anaerobic, and aerobic systems for energy, and improving these three energy systems can improve your weightlifting performance.
  3. Many weightlifters neglect one or more energy systems, but by training each one at least a little bit every week, you’ll likely look and perform better.

If you’ve read any books on weightlifting, endurance sports, or training in general, you’ve probably come across the term energy system before.

Unfortunately, the more you read about energy systems, the more confused you’re likely to become.

Most explanations of energy systems quickly devolve into a complex firehose of scientific jargon like Krebs cycle, mitochondrial respiration, anaerobic glycolysis, and other tongue twisters. 

Luckily, you don’t need a degree in exercise science to decipher energy systems. You don’t even need to know what most of these words mean.

At bottom, energy systems describe how your body produces energy during physical activity, from squatting a barbell to running a marathon to typing on a keyboard. 

As you’ll see in this article, gaining a better understanding of how energy systems work can help you improve your performance by optimizing your training and nutrition to support different energy systems, which in turn can improve your performance in different activities. 

For example, have you ever wondered why your muscles begin to burn and you lose your breath when doing high reps?

Or why you start to feel tired after running for more than a few minutes? 

Or why most people recommend you eat a lot of carbs to perform well as an athlete?

Energy systems hold the answers to all these questions.

What’s more, understanding energy systems gives you a much better idea of how to fuel your body for exercise, as what you eat can significantly impact the efficiency of different energy systems in your body.

In this article, you’ll learn: 

  • How energy systems work
  • Why your body uses different energy systems during different kinds of exercise
  • How to eat and train to improve the efficiency of different energy systems
  • How improving one energy system in particular may help you gain muscle and strength
  • And more

Let’s begin.


What Is an Energy System?

energy systems explained

An energy system is a physiological process that describes how the body uses energy from food to fuel various bodily functions. 

More specifically, an energy system is the process your body uses to transform carbs, protein, and fat into a form of energy that cells can use to power different activities like weightlifting, cycling, and running.

There’s a lot to unpack in that sentence, so let’s start at the beginning.

After you eat a meal, the body breaks down the protein, carbs, and fat into smaller molecules.

Protein is broken down into molecules called amino acids, carbs are broken down into molecules of a simple sugar called glucose, and fat is broken down into molecules called triglycerides.

Amino acids are primarily used as “building blocks” of cells and are rarely used for energy. The body will sometimes convert a small portion of amino acids into glucose when it has more than it needs or when it isn’t able to get enough glucose from other sources, but this process is very inefficient and only produces enough glucose for basic bodily functions. For all intents and purposes, we can ignore protein as a source of energy.

Glucose is either used immediately for energy or packaged into bundles of glucose molecules known as glycogen, which is packed away in the muscles and liver. Then, the body breaks down this glycogen when it needs more glucose in the future, such as during exercise.

Triglycerides are either used immediately for energy or, more often, stored in the form of body fat, which is then broken down and used for energy as needed.

This is where things get interesting. 

Before any of these molecules can be used for energy, they must be converted into another compound called adenosine triphosphate (ATP). 

You can think of ATP as a kind of cellular “currency.” 

Although you can get energy from a variety of different foods and macronutrients, in the end it’s all converted into ATP, which cells then “spend” on various functions. 

For a cell to use an ATP molecule for energy, it must first break it down into several smaller molecules. This process produces byproducts that are then “recycled” back into ATP to be used again.

Of course, some energy is always lost in this process of recycling ATP, which is why you need to provide your body with more energy (calories) for it to keep recycling ATP.

When you exercise, your cells gobble up ATP at an astonishing rate. For example, when walking at a slow pace, your body burns 200 percent more energy than when lying at rest, and when sprinting, your cells burn through ATP 1,000 times faster than when you’re sitting.

This brings us back to the topic of energy systems.

At bottom, your body’s energy systems are responsible for ensuring your cells have a steady supply of ATP no matter if you’re sleeping, working at your desk, or sprinting. In other words, no matter how much ATP your body spends on various activities, your energy systems ensure your cells’ coffers don’t run dry.

As you can imagine, keeping your cells well supplied with ATP is a difficult, complex job—too complex for a single energy system.

This is why the body uses three different energy systems to supply cells with ATP, depending on how quickly your body is burning through energy. Keep reading to learn what these are.

Summary: An energy system is a physiological process that describes how the body converts energy from food into ATP to fuel various activities.

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The Three Energy Systems

energy systems in sport

You know what an energy system is and how it works. 

You also know that your body’s energy needs can vary several-fold depending on your activity levels, and that your body uses several different energy systems to make sure ATP production keeps pace with consumption.

Now, let’s take a deeper look at each of these energy systems and how they impact your performance.

The three energy systems are called:

  1. The phosphocreatine system
  2. The anaerobic system
  3. The aerobic system

All three of these systems have the same job—to generate ATP for cells—but they each have different strengths and weaknesses that make them uniquely suited for particular demands.  

1. The phosphocreatine system

Phosphocreatine, also known as creatine phosphate, is a naturally occurring energy source found in muscle cells. The body creates phosphocreatine by chemically modifying creatine, which is found in animal products, especially red meat, and in creatine supplements.

The phosphocreatine system uses phosphocreatine to generate ATP.

The main advantage of the phosphocreatine system is that it can be used to generate a lot of ATP in the blink of an eye. This makes it perfect for activities that require a very brief, intense burst of effort, like sprinting or a heavy one-rep max attempt.

The downside of the phosphocreatine system is that phosphocreatine can only be stored in small amounts before it becomes exhausted—just enough to power about 10 to 20 seconds of activity. 

The body naturally regenerates phosphocreatine, but it takes about five minutes or longer for your muscles to fully “recharge.” (This is why taking creatine improves your performance—it improves the efficiency of your phosphocreatine system.)

You can think of the phosphocreatine system like a nitrous-fueled drag racing engine—it can produce a staggering amount of energy very quickly, after which it fizzles and abruptly stops.

To see this process in action, we can look at elite sprinters like Wayde van Niekerk, the current world record holder for the 200-, 300-, and 400-meter dash. 

The phosphocreatine system primarily powers his efforts up to about 200 meters, but as his phosphocreatine stores become depleted, his pace slows the further he runs.

Here’s the man in action: 


And here’s what his split times typically look like: 

  • 0 to 100 meters: ~10 seconds
  • 100 to 200 meters: ~20 seconds (no decrease in pace)
  • 200 to 300 meters: ~31 seconds (1-second decrease in pace)
  • 300 to 400 meters: ~43 seconds (3-second decrease in pace)

In other words, even the best sprinters in the world don’t hold a perfectly even pace throughout the entire race. Instead, they redline their phosphocreatine system as hard as they can, after which point they have to rely more and more on their anaerobic system to carry them to the finish line.

As esteemed exercise physiologist and researcher Ross Tucker has pointed out, this phenomenon isn’t unique to Niekerk or due to muscle fiber type or training methods, but is instead an innate aspect of human physiology. Although you can improve the efficiency of your phosphocreatine system by doing lots of intense, brief intervals interspersed with lots of rest (training like a sprinter), this will only take you so far. 

After 10 to 20 seconds of all-out effort, the anaerobic system begins to take over.

Summary: The phosphocreatine system generates ATP for brief, intense bursts of effort lasting around 10 to 20 seconds, but it quickly fades and takes much longer than the other energy systems to recharge. 

2. The anaerobic system

After around 10 to 20 seconds of effort, the anaerobic energy system starts producing the majority of ATP for your muscles.

This is called the anaerobic system because it regenerates ATP without the presence of oxygen. (An– meaning “without,” and aerobic meaning “associated with oxygen.”) This allows it to produce energy faster, but not as efficiently as the aerobic system.

This is also referred to as the glycolytic system because it gets the majority of its energy from the glycogen stored in your muscles and the glucose in your blood.

The main advantage of the anaerobic system is that it can produce a lot of energy for about two minutes before it starts to peter out. 

The main downside is that the anaerobic system produces a lot of metabolic byproducts that quickly build up in the blood and muscles, and after about two minutes these byproducts begin to cause fatigue.

In other words, you can think of the anaerobic system as a race car engine that runs “dirty.” You can go pedal to the metal for a couple of minutes, but any longer and the wheels start to fall off.

The anaerobic system is highly relevant to weightlifting because most of your sets in the 8-to-12-rep range will be powered by your anaerobic system. This is also true for most forms of HIIT cardio that last longer than 20 seconds.

The best way to improve your anaerobic system is to (surprise!) do a lot of anaerobic-type exercise:

  • High-rep sets in the gym
  • HIIT intervals of one to two minutes
  • Sports that involve a lot of sprinting, like soccer, football, and basketball

Summary: The anaerobic system generates ATP for intense exercise lasting more than 20 seconds and less than about two minutes, and it recharges relatively quickly compared to the phosphocreatine system.  

3. The aerobic system

The aerobic system, also called the oxidative or respiratory system, produces the majority of ATP for your muscles after about one to two minutes of exercise. 

The aerobic system is strange in that it takes several minutes to “warm up.” We don’t need to get into the nitty-gritty details of how this works, but suffice to say that it takes several minutes for the chemical processes of the aerobic system to begin producing substantial amounts of ATP.

The advantage of the aerobic system, though, is that once it gets going, it can produce a lot of ATP very efficiently for a long time. You can think of the aerobic system as a diesel engine—it can produce a lot of energy almost indefinitely, but it takes a few minutes to get cranking.

The aerobic system uses a combination of glucose, glycogen, and body fat to generate ATP, with the exact ratio depending on how intense the exercise is. 

As your workout intensity rises, your aerobic system uses more and more glucose and glycogen to produce ATP and less body fat. As your workout intensity lessens, your aerobic system uses more and more body fat and less and less glucose and glycogen.

This is why many people believe low-intensity cardio is better for fat loss than high-intensity cardio—it burns a greater percentage of fat. What these people are missing, though, is that higher-intensity cardio burns a lot more total calories, which is what really drives long-term fat loss.

Since the aerobic system can create substantial amounts of ATP from body fat, it can power ATP production for hours on end, even if you don’t eat any carbs. This is even true for lean people with little extra body fat.

For example, let’s say a guy is about 8 percent body fat and 170 pounds—“shredded” by most standards. About 3 percent of that body fat is in his brain, nerves, and organs, which only disappears when you’re close to death. This leaves him with about 5 percent body fat that’s available for use as energy. 

If we multiply his weight (170) by his body fat percentage (5 percent, or .05 as a decimal), we calculate he has 8.5 pounds of body fat that can be burned for energy. If we assume each pound of body fat has around 3,500 calories, that means this guy has 30,000 calories of potential energy stored as body fat.

And since the aerobic system is the only one that can tap into body fat for energy, that means it provides you with a nearly unlimited source of ATP.

This is why the aerobic system is the primary system responsible for powering all forms of endurance exercise, from running a mile to cycling the Tour de France.

Now, some low-carb advocates have misconstrued these facts to suggest that, since the aerobic system can use fat for energy, eating carbs doesn’t improve endurance performance. 

Not so much.

It’s true that the aerobic system does allow you to exercise for hours at a time without eating carbs, but there’s a caveat—this only works if you keep your intensity at about 60 percent of your maximum. 

That is, you can rely on your body fat stores up until your exercise intensity exceeds about a 6 on a scale from 1 to 10. After that point, your body needs glucose to power your workouts, which can only be obtained in large quantities by eating carbs.

If you aren’t familiar with endurance sports, a 6 out of 10 intensity is akin to a light jog or easy spin class. If you’re doing anything more intense than this, you’re going to benefit from eating carbs. 

Summary: The aerobic system generates most of the ATP during workouts lasting more than two minutes, and it relies primarily on fat and glucose for energy. 

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How to Use Energy Systems to Improve Your Weightlifting Performance

energy systems for dummies

All three of these energy systems are working all the time, but the contribution of each depends on the intensity and length of your workouts.

At rest, your aerobic system is producing almost all the energy for your body.

At moderate-intensity exercise, the aerobic system produces most of the energy, but you also get some from the anaerobic system.

At high-intensity exercise, the aerobic system produces some energy, but the anaerobic system produces most of it. 

And during an all-out, balls-to-the-wall effort, your phosphocreatine system produces most of the energy, the anaerobic system produces some, and the aerobic system produces almost none.

What does this have to do with weightlifting? 

Well, many people think weightlifting is powered mostly by the phosphocreatine system and the anaerobic system, and that’s sort of true.

During your sets, the body gets most of its energy from these two systems. 

Between your sets, though, as you catch your breath and your body attempts to regenerate ATP before your next set, your aerobic system does the lion’s share of the work.

The best way to improve the phosphocreatine and anaerobic systems when it comes to weightlifting is to just lift weights in a variety of rep ranges—as you do with a program like Bigger Leaner Stronger or Thinner Leaner Stronger

While weightlifting performance does depend somewhat on your aerobic system, it doesn’t improve it much, which means it’s likely that doing a little bit of aerobic training might benefit your weightlifting.

In other words, it’s fair to say that improving your aerobic system—doing some cardio—would help you recover faster between sets, which could translate into more muscle and strength gains over time. 

For example, let’s say you normally rest three minutes between sets. Let’s also say that improving your aerobic conditioning allows you to feel more refreshed after three minutes of rest, or just as refreshed after only two minutes of rest. If that helped you squeeze more hard sets into your workouts or lift heavier weights, it would almost certainly help you build more muscle and get stronger over time. 

Of course, there are a lot of “ifs” in this idea, and an easy counterargument is that there are plenty of big, strong guys waddling around gyms who do little or no cardio.

I’d argue, though, that there are few to no downsides to this approach, so it’s still worth doing.

For one, there’s irrefutable evidence that aerobic training provides myriad health benefits, reducing the risk of cardiovascular disease, diabetes, cancer, Alzheimer’s, and many other maladies

Second, moderate amounts of aerobic training, especially low-impact forms like rowing or cycling, don’t interfere with weightlifting performance or muscle gains. 

In other words, it makes sense to do at least some cardio given the many benefits and lack of downsides.

Summary: Your body relies heavily on the aerobic system to regenerate ATP between sets, and doing some aerobic training (cardio) will likely improve your weightlifting performance over time.

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The Bottom Line on Energy Systems

An energy system is a physiological process that describes how the body converts energy from food into ATP to fuel various activities.

The three energy systems of the body are the: 

  1. Phosphocreatine system
  2. Anaerobic system
  3. Aerobic system

The phosphocreatine and anaerobic systems primarily power weightlifting during your sets, but the aerobic system does most of the work between your sets as your body recovers.

Therefore, there’s a sound theoretical argument that improving your aerobic system would improve your weightlifting performance and the quality of your workouts, which could translate into greater muscle and strength gains over time. Maybe.

At the very least, there’s little reason not to do some cardio given the many health benefits it offers, and the fact that moderate amounts of low-impact activity don’t interfere with weightlifting performance.

If you want to experiment with this idea, here’s a good way to get started: 

  1. Do two to three low-intensity cardio workouts per week of around 30 minutes apiece, using low-impact forms of cardio like walking, cycling, or swimming.
  2. Do one or two HIIT cardio workouts per week of no more than 25 to 30 minutes apiece. If you’re bulking, feel free to do no HIIT cardio, and if you’re cutting, feel free to do two HIIT workouts, but it’s best not to do more than three.

And of course, make sure you’re following a well-designed strength-training plan like Bigger Leaner Stronger, Thinner Leaner Stronger, or Beyond Bigger Leaner Stronger.

What do you think of energy systems and weightlifting? Have anything else to add? Lemme know in the comments below!

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