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, CrossFitter, 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.
What is 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, compared to storing money in a bank.
ATP is found in cells of all living things and is responsible for almost all cellular processes.
This chemical substance 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 causes a chemical reaction that provides a burst of energy.
ADP can be recharged back to ATP by adding a phosphate back on, which requires energy (we will get onto this soon!).
These molecules can then be recycled over and over to provide a constant stream of ATP available for all metabolic pathways in the cell.
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 all 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.
Introducing: The three energy systems!
Energy Systems in the Body
There are 3 different energy systems in the body that produce ATP through different pathways. Your body can use one or multiple simultaneously, which all depends on the activity you’re doing.
Anaerobic - ATP-PCr (phosphagen system)
The immediate source of energy for regenerating ATP, fueling the first 5-10 seconds of near-maximal activity. This is fueled by stores already in the muscles.
Examples of this include a short sprint, a tennis serve, or lifting a heavy load for 3 reps.
After this short duration, if exercise/movement continues, the body will switch to a different energy system to produce ATP.
Anaerobic - Glycolytic system
The breakdown of carbohydrate sources (glycolysis) to produce ATP, fueling the first 30 to 120 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 glycogen (stored glucose) to create ATP through a process called glycolysis. Similar to the ATP-PCr system, this system also does not require oxygen for the process of glycolysis.
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.
Aerobic (Oxidative pathway)
The aerobic production of energy from carbohydrate, lipid and sometimes protein to produce ATP.
This system is 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.
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.
Energy Systems: 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 a 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.
Though, 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 contributes1:
- Basketball - 60% ATP-PCr, 20% glycolytic, 20% oxidative
- Golf swing - 95% ATP-PCr, 5% glycolytic, 0% oxidative
- Gymnastics - 80% ATP-PCr, 15% glycolytic, 5% oxidative
- Hockey - 50% ATP-PCr, 20% glycolytic, 30% oxidative
- Long distance running - 10% ATP-PCr, 20% glycolytic, 70% oxidative
- Soccer - 50% ATP-PCr, 20% glycolytic, 30% oxidative
- Tennis - 70% ATP-PCr, 20% glycolytic, 10% oxidative
As you can see, the type of sport, as well as intensity and duration, largely determines which energy systems to tap into and when.
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.
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.
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).
However, major improvements in aerobic power are only really seen in untrained, sedentary individuals, not so much in already well-trained individuals and athletes.
Final Take-Home
Understanding bioenergetics can be a big challenge but getting your head around the basics can leave you feeling more informed on what goes on in your 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
- https://www.ptdirect.com/training-design/anatomy-and-physiology/the-energy-systems-2013-an-overview
