You may hear a lot about ATP and energy systems in the body, particularly if you’re an athlete or gym goer. However, human bioenergetics is an interesting yet complex topic that isn’t easily understood for many.
Exercise physiology is no easy subject to wrap your head around, though learning more about how your body works during exercise and in general can be very enlightening.
Of course, you don’t need this knowledge to get by in life, however knowing the basics about how we generate energy can be helpful in understanding how we fatigue during exercise and what strategies you can implement to minimize this.
So, if you’re a weightlifter, Cross-Fitter, runner, cyclist, rower, or anything else that involves movement - this one is for you!
In this short guide, you will find out all the basics you need to know about ATP, energy systems in the body, and how this applies to exercise.
Key Takeaways
- Three systems fuel training: phosphagen (max power), glycolytic (anaerobic), and oxidative (aerobic).
- They overlap—your set length and intensity determine the dominant system.
- Programming work:rest, volume, and tempo targets the adaptation you want.
- Mitochondrial and nutrient support improve output, recovery, and repeatability.
What is Adenosine Triphosphate (ATP)?
Firstly, it’s important to establish what ATP is and how it plays a vital role in the body.
Adenosine triphosphate is a chemical substance better known as the energy currency of the cell - you can compare it to storing money in a bank.
ATP, found in the cells of all living things, is the main carrier of chemical energy in cells, and that energy is used to power various cellular processes.
An ATP molecule is made up of adenosine and 3 phosphate molecules. To release energy, one of these phosphates breaks off, which then makes ATP become ADP (adenosine diphosphate).
This process is called ATP hydrolysis, where the removal of a phosphate group from ATP results in the energy released as free energy. The hydrolysis of ATP into ADP and inorganic phosphate releases this free energy that can be used for cellular processes.
ADP can be recharged back to ATP by adding a phosphate back on, which requires energy (we will get onto this soon!). This process is known as ATP synthesis and occurs in the cell through pathways such as oxidative phosphorylation.
These molecules can then be recycled over and over to provide a constant stream of ATP available for all metabolic pathways in the cell.
ATP is also involved in signal transduction by serving as a substrate for kinases, which transfer phosphate groups to proteins.
Additionally, ATP is one of four monomers required for RNA synthesis and is also involved in DNA synthesis.
It’s key to remember that any muscle contraction or force exertion is all down to ATP.
So, how is ATP produced? What happens when we run out?
How ATP is produced depends on the type of activity you’re doing, as there are multiple ways to produce ATP depending on the intensity, amount of force required, and length of the activity. (1, 2)
Introducing: The three energy systems!
The Different Energy Systems in the Body that Create ATP
There are 3 different energy systems in the body that produce ATP through different pathways, also known as energy pathways. Your body can use one or multiple energy pathways simultaneously, which all depends on the activity you’re doing.
The three energy systems are the creatine phosphate system (ATP-PCr), the anaerobic glycolytic system, and the aerobic oxidative system.
Understanding these energy pathways is crucial for designing effective training programs tailored to different athletic goals. Let's examine each in turn...
Anaerobic - ATP-PCr (phosphagen system), also known as ATP CP System
The immediate source of energy for regenerating ATP is the ATP-PCr system (which stands for Adenosine Triphosphate-Phosphocreatine). It's also known as the ATP CP system (Adenosine Triphosphate-Creatine Phosphate) or phosphagen system.
This system is fueled by stores already in the muscles, specifically through creatine phosphate.
THE ATP-PCr SYSTEM IS USEFUL FOR...
The ATP-PC system is primarily used for short and intense, explosive movements lasting less than 10 seconds (though some say up to 15 seconds), such as a short sprint, a tennis serve, or lifting a heavy load for 3 reps.
These explosive movements rely on the rapid availability of ATP to support maximal power output.
After this short duration, if exercise or movement continues, the body will switch to a different energy system to produce ATP. Rest periods are crucial for replenishing creatine phosphate stores and restoring power output for repeated efforts. (3 - 5)
Anaerobic System - Glycolytic system
The breakdown of carbohydrate sources (glycolysis) to produce ATP is a key function of the anaerobic system, which produces energy without the need for oxygen.
THE ANAEROBIC GLYCOLYTIC SYSTEM IS USEFUL FOR...
This anaerobic glycolytic system is predominant during high-intensity exercise lasting from 10 seconds to a few minutes, peaking at around 60-90 seconds of near-maximal activity.
This energy system would be next in line to produce ATP once the ATP-PCr system has run its course.
This energy system relies on dietary carbohydrates to supply glucose and muscle glycogen (stored glucose in muscles) to create ATP through a process called glycolysis.
The anaerobic lactate system produces energy from muscle glycogen, which can lead to lactic acid accumulation and muscle fatigue. Similar to the ATP-PCr system, this system also does not require oxygen for the process of glycolysis.
The anaerobic threshold is defined as the intensity at which the blood concentration of lactic acid begins to increase exponentially. (6)
Though, there are two ways this energy system can be used.
Once the ATP-PCr stores are depleted but the exercise intensity continues for a further 30 seconds, we use “fast glycolysis” - which continues to fuel this maximal effort.
When this happens, lactic acid accumulates, and muscle fatigue sets in as a result of by-products of fast glycolysis.
Slow glycolysis is when the glycolytic system is being used but at a lower intensity, meaning less power is being generated. This means that no by-products are being made to cause fatigue, and instead the by-products of slow glycolysis are actually being used to create more ATP energy.
So, extreme fatigue can be avoided during high-intensity exercise if a less intense effort is expressed, thus enabling us to use slow glycolysis, which is a more efficient energy system than fast glycolysis. (7)
Aerobic Energy System (Oxidative pathway)
The aerobic energy system functions to generate ATP through the aerobic production of energy from carbohydrate, lipid, and sometimes protein.
THE AEROBIC SYSTEM IS USEFUL FOR...
The oxidative system uses oxygen to break down carbohydrates and fats for long-duration, lower-intensity activities lasting longer than 3 minutes.
The aerobic oxidative system generates ATP through the breakdown of carbohydrates, fats, and sometimes proteins in the presence of oxygen. This system is the most complex of the three energy systems and is primarily used for prolonged, low-intensity activities.
It relies heavily on the circulatory system to supply oxygen and provides the foundation for all activity and most of the body's ATP production. Continuous exercise at an intensity just below your lactate threshold is a great way to train the aerobic system.
This energy system works to continuously used to fuel our daily activity and very low intensity, long duration efforts.
Going back to the beginning, your maximal efforts were fueled by the ATP-PCr system. Once your performance declined, your body then switched up to using the glycolytic system, either fast or slow glycolysis, which further results in a decline in performance after a short while.
Now we enter the aerobic (with oxygen) energy pathway. The demand for energy is low, so the oxidative system takes its time producing ATP via three ways:
1. Krebs cycle (citric acid cycle)
The krebs cycle is a sequence of chemical reactions that use up glucose and the by-products of glycolysis to create more ATP.
The citric acid cycle, also known as the Krebs cycle, is at the heart of the aerobic energy system. This series of chemical reactions takes place deep within the mitochondrial matrix, where it plays a crucial role in cellular respiration and the production of ATP to fuel everything from muscle contraction to nerve impulses.
When you engage in sustained, lower-intensity exercise like distance running or cycling, your body relies heavily on the citric acid cycle to provide energy.
The citric acid cycle is essential for aerobic metabolism, providing a steady supply of ATP for prolonged activities. It’s also tightly regulated - when your cells need more energy, the cycle speeds up to produce more ATP; when energy demand drops, it slows down. This balance ensures your muscles have enough energy for both intense exercise and everyday movement.
But the citric acid cycle does more than just produce ATP. It’s a central hub for other metabolic pathways, helping to break down fatty acids and synthesize amino acids as needed.
It also connects with the other energy systems - the phosphagen system (ATP-PC system) and the glycolytic system (lactic acid system) - to ensure your body can quickly adapt to changes in exercise intensity.
In summary, the citric acid cycle is a powerhouse of energy production, supporting the aerobic system and providing the ATP needed for sustained physical activity.
Its integration with the electron transport chain (more on that below) and other energy systems makes it a vital player in keeping your body moving, whether you’re powering through a marathon or simply going about your day. (8, 9)
2. Electron transport chain
When our by-products of glycolysis go through the krebs cycle, this then produces more by-products which can cause the muscles to become too acidic, leading to fatigue.
To alleviate this, the body throws these by-products into the electron transport chain to make use of them to create more ATP
So, this not only prevents acid buildup, but also creates more energy!
3. Beta oxidation
Beta oxidation is the process of breaking down and converting fatty acids into other substances that are suitable to be used in the krebs cycle.
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Energy Systems and ATP Production: Key Take-Homes
ATP is needed in the muscles for them to contract. ATP can be produced via the ATP-PCr system, the glycolytic system, or the oxidative system. When depleted, it must be replenished for muscle contractions to continue.
When you perform a high-intensity, explosive movement such as a plyometric box jump, you exert maximal effort, yet will not become fatigued through doing this single movement.
However, if you were to repeatedly box jump, you will eventually become fatigued. This is your ATP stores depleting, which then signals the glycolytic system to begin the process of producing ATP.
Once this system runs its course, our performance declines, efforts slow down, and the oxidative system is called upon to fuel the low-intensity activity.
However, it’s not as simple as the body just switching energy systems like a car switches gear. The body can actually use multiple energy systems, but always is using one predominately.
For example, we may be using the glycolytic system predominately, but the oxidative system is working slowly in the background at the same time.
This largely depends on the type of activity you’re doing, and your genetics - which interestingly can also play a role in how your body utilizes your energy systems.
Can Protein Be Used for Energy?
In some cases, protein can be used as a last resort for energy production. Though this is rare, as it would mean someone having completely depleted carbohydrate stores and minimal fat stores.
When this does happen, amino acids (the building blocks of protein) are converted into glucose via a process called gluconeogenesis or converted to other sources that can be used in the krebs cycle.
Protein is not an efficient energy source as it’s made at such a slow rate. This is why it’s recommended to fuel before and after your training sessions with carbohydrates so you can avoid using amino acids as energy.
Energy Systems Used in Sport
A good way to really understand energy systems is how we use them in sport. As mentioned previously, our body isn’t always using just one energy system; it actually more often uses all three, but one is being used more predominately.
Here are some examples of sports and the approximate percentages of how much each energy system contributes:
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Basketball - 60% ATP-PCr, 20% glycolytic, 20% oxidative
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Golf swing - 95% ATP-PCr, 5% glycolytic, 0% oxidative
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Gymnastics - 80% ATP-PCr, 15% glycolytic, 5% oxidative
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Hockey - 50% ATP-PCr, 20% glycolytic, 30% oxidative
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Long distance running - 10% ATP-PCr, 20% glycolytic, 70% oxidative
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Soccer - 50% ATP-PCr, 20% glycolytic, 30% oxidative
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Tennis - 70% ATP-PCr, 20% glycolytic, 10% oxidative. (10)
As you can see, the type of sport, as well as intensity and duration, largely determines which energy systems to tap into and when. Optimizing power output is especially important in sports that rely on explosive movements, as higher power output can enhance performance during short, intense efforts.
For example, tennis relies mostly on the ATP-PCr system - think explosive serves, short rounds, and short sprints which require maximal, quick bursts of energy. Similarly, sports like gymnastics involve explosive movements such as vaults and tumbling passes that depend heavily on the ATP-PCr system.
Then take long distance running, a very long and low-intensity activity which doesn’t require quick bursts of energy. This is where the oxidative system becomes the most suitable energy system to use.
Training programs can be designed to target specific energy systems, helping athletes improve their performance in different sports by focusing on the unique demands of each activity.
Can You Improve Your Energy Systems?
The capacity to generate power of each of the three energy systems can vary with training and genetics.
It is more difficult to train to improve the ATP-PCr and glycolytic systems, which may only improve marginally. The oxidative system appears to be a more trainable energy system (think aerobic/cardiovascular training).
Structured training programs are designed to target and enhance the capacity of different energy systems, tailoring workouts to the specific needs of athletes.
Incorporating appropriate rest periods during and after high-intensity sessions is crucial for recovery and for replenishing ATP and phosphocreatine stores, which helps maintain optimal performance and prevents fatigue. Additionally, training at faster speeds for longer durations can improve the lactate threshold and overall performance.
However, major improvements in aerobic power are only really seen in untrained, sedentary individuals, not so much in already well-trained individuals and athletes.
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Final Take-Home
Understanding bioenergetics can be a big challenge but getting your head around the basics can leave you feeling more informed about what goes on in the human body during sport and exercise.
With this information and better understanding, you can be more aware of the causes of fatigue and how best to minimize it, such as adequately fueling before and after training.
Supplements can also play a role in ATP production and fatigue delay, such as creatine, which increases stores of ATP, and beta-alanine which helps to buffer acidity in the muscles, thus delaying fatigue.
Having a better understanding about bioenergetics thus provides you with a better understanding of nutrition and training strategies, as well as a better understanding of how certain supplements work in the body.
References
- Dunn J, Grider MH. Physiology, Adenosine Triphosphate. StatPearls Publishing; updated February 13, 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK553175/
- Adenosine Triphosphate (ATP). Physiopedia; December 2021. Available from: https://www.physio-pedia.com/index.php?title=Adenosine_triphosphate_(ATP)&oldid=288381
- Wells GD, Selvadurai H, Tein I. Bioenergetic provision of energy for muscular activity. Paediatric Respiratory Reviews. 2009;10(3):83–90.
- Phosphagen System (ATP-CP System). PressBooks. Available from: https://pressbooks.calstate.edu/nutritionandfitness/chapter/8-2-phosphagen-system-atp-cp-system/
- McMahon S, Jenkins D. Factors affecting the rate of phosphocreatine resynthesis following intense exercise. Sports Medicine. 2002;32(12):761–784.
- Baker JS, McCormick MC, Robergs RA. Interaction among skeletal muscle metabolic energy systems during intense exercise. Journal of Nutrition and Metabolism. 2010;2010:905612. doi: 10.1155/2010/905612
- Baker JS, McCormick MC, Robergs RA. Interaction among skeletal muscle metabolic energy systems during intense exercise. Journal of Nutrition and Metabolism. 2010;2010:905612. doi: 10.1155/2010/905612
- Alabduladhem TO, Bordoni B. Physiology, Krebs Cycle. StatPearls Publishing; updated November 23, 2022. Available from: https://www.ncbi.nlm.nih.gov/books/NBK556032/
- Alghannam AF, Ghaith MM, Alhussain MH. Regulation of energy substrate metabolism in endurance exercise. International Journal of Environmental Research and Public Health. 2021;18(9):4963. doi: 10.3390/ijerph18094963
- The Energy Systems (2013): An Overview. PT Direct. Available from: https://www.ptdirect.com/training-design/anatomy-and-physiology/the-energy-systems-2013-an-overview
