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HOW YOUR BODY WORK?
In this video, you’ll learn:
Let’s dive in, starting with the first step: How do we produce energy?
- How your cells produce energy.
- What the energy systems are.
- How they are interconnected.
- Which organs are responsible for which energy systems.
- How different muscle types correlate with these systems.
- How our body shifts gears between energy systems.
- How to identify which system you're using.
- And finally, which type of training works best for each energy system.
Let’s dive in, starting with the first step: How do we produce energy?
How do our cells get energy?
Our body consists of trillions of cells, and each one generates energy in the same way: by using ATP (Adenosine Triphosphate). ATP is a molecule with three phosphate groups. The bonds between these groups store significant energy. When one phosphate group is ripped off, it releases energy for the cell to use.
And this is exactly what each of the cells of our body do. Someone provides them with ATP, they rip off one phosphate group and get a decent amount of energy.
And this is exactly what each of the cells of our body do. Someone provides them with ATP, they rip off one phosphate group and get a decent amount of energy.
Where do our cells get ATP?
The question is - who provides ATP to the cells?
The body has over 20 different ways to create ATP, but the four main pathways are:
The body has over 20 different ways to create ATP, but the four main pathways are:
- Creatine Phosphate System
- Glycolysis
- Aerobic Respiration
- Fatty Acid Oxidation
Creatine Phosphate System
Simplest one is the Creatine Phosphate: Phosphocreatine donates a phosphate to ADP, producing 1 ATP. It happens instantly, making it the fastest source of energy, but the downside is that the body only has a small amount of Phosphocreatine, so it depletes within 15 seconds.
Simplest one is the Creatine Phosphate: Phosphocreatine donates a phosphate to ADP, producing 1 ATP. It happens instantly, making it the fastest source of energy, but the downside is that the body only has a small amount of Phosphocreatine, so it depletes within 15 seconds.
Fatty Acid Oxidation
Second simple to explain is Fatty Acid Oxidation.
Second simple to explain is Fatty Acid Oxidation.
- Fats, which we have plenty in our body, come through many circles of Beta-oxidation. This will require a bit of ATP, but as a result we get a lot of Acetyl CoA and special Fuel.
- Acetyl-CoA then enters the Krebs Cycle, generating more special Fuel and ATP.
- All collected Special fuel goes to the Electron Transport Chain, where under heavy fire of protons phosphate is connected to ADP, producing ATP. Overall, one fat molecule generates about 106 ATP.
Glycolysis and Aerobic Respiration
Next two are complicated and connected. In fact, they almost could be considered as one, because the outcome of glycolysis is the starting position of aerobic respiration.
So the Aerobic method aerobic respiration can not exist without anaerobic method glycolysis.
However, glycolysis can exist on its own, so we need to keep 2 lines.
First, lets cover full cycle
Next two are complicated and connected. In fact, they almost could be considered as one, because the outcome of glycolysis is the starting position of aerobic respiration.
So the Aerobic method aerobic respiration can not exist without anaerobic method glycolysis.
However, glycolysis can exist on its own, so we need to keep 2 lines.
First, lets cover full cycle
- Glucose is converted into pyruvate, which produces 2 ATP, water, some special fuel and protons.
- Next, pyruvate is converted into Acetyl-CoA, again producing some special fuel and some CO₂.
- The Krebs Cycle then takes Acetyl-CoA, producing more special fuel, a little ATP, and more CO₂.
- Finally, the special fuel enters the Electron Transport Chain, where they donate electrons and protons, as a result we get 28 ATP, water, and heat.
Glycolysis Sideeffect
However, sometimes our body struggles to deliver oxygen, which blocks these processes. In that case, the way the body can get ATP is through Glycolysis.
Even though Glycolysis produces only 2 ATP, it is a faster energy method than aerobic methods. And a valid question would be, why would we even need aerobic methods?
Well, at some point you run out of NAD+ and you need NADH to be converted back to it, to proceed with Glycolysis. This is where your body developed a mechanism. To get NAD+, Pyruvate is converted to Lactate. And please don’t worry about Lactate; it’s a helpful byproduct that can be converted back to pyruvate anytime with almost zero energy input.
Issue of Glycolysis, that on top of Pyruvate, it produces a lot of protons. Protons make the muscles more acidic, leading to the “burning” sensation often felt during intense anaerobic exercise. And the more we go through the circle of Pyruvate production, the more Protons we generate, the more blocked our muscles are.
Why then do athletes measure Lactate instead of acidity to study how bad the situation in their body is? Because there is no convenient way to measure acidity, while lactate level is highly correlated with acidity. And this is where the myth that Lactate blocks muscle is born.
However, sometimes our body struggles to deliver oxygen, which blocks these processes. In that case, the way the body can get ATP is through Glycolysis.
Even though Glycolysis produces only 2 ATP, it is a faster energy method than aerobic methods. And a valid question would be, why would we even need aerobic methods?
Well, at some point you run out of NAD+ and you need NADH to be converted back to it, to proceed with Glycolysis. This is where your body developed a mechanism. To get NAD+, Pyruvate is converted to Lactate. And please don’t worry about Lactate; it’s a helpful byproduct that can be converted back to pyruvate anytime with almost zero energy input.
Issue of Glycolysis, that on top of Pyruvate, it produces a lot of protons. Protons make the muscles more acidic, leading to the “burning” sensation often felt during intense anaerobic exercise. And the more we go through the circle of Pyruvate production, the more Protons we generate, the more blocked our muscles are.
Why then do athletes measure Lactate instead of acidity to study how bad the situation in their body is? Because there is no convenient way to measure acidity, while lactate level is highly correlated with acidity. And this is where the myth that Lactate blocks muscle is born.
Comparisons
Anyhow, we finally got to the interesting part of the video. Let's do the comparison.
Limitations
We have four sources of ATP. The main limitation for two of them is oxygen — without it, they won’t function. The limitation of the Creatine Phosphate System is that there is only a small amount of Creatine Phosphate available in the body, so it doesn’t last long. As for Glycolysis, its limitation is it produces a large number of protons, which make the environment more acidic and therefore, it also can not last long if we use it on a maximum capacity.
ATP Production Comparison per one cycle:
However, it is more important how much we produce. Per one cycle we get:
ATP Production Comparison per hour (no limits):
The second thing we need to compare - speed of production. I need to note that in this example we will neglect all limitations of fuel shortage and other restrictions. Let's take an imaginary world, where it is true.
ATP Production Comparison per hour (realistic):
But what about some realistic scenarios? We do not have training that would last 15 seconds, right? What about how many ATPs can we realistically produce in an hour:
Limitations
We have four sources of ATP. The main limitation for two of them is oxygen — without it, they won’t function. The limitation of the Creatine Phosphate System is that there is only a small amount of Creatine Phosphate available in the body, so it doesn’t last long. As for Glycolysis, its limitation is it produces a large number of protons, which make the environment more acidic and therefore, it also can not last long if we use it on a maximum capacity.
ATP Production Comparison per one cycle:
However, it is more important how much we produce. Per one cycle we get:
- 1 ATP with Creatine Phosphate System
- 2 ATP with Glycolysis
- Roughly 30 ATP with Aerobic respiration
- Roughly 106 ATP with Fatty Acid Oxidation
ATP Production Comparison per hour (no limits):
The second thing we need to compare - speed of production. I need to note that in this example we will neglect all limitations of fuel shortage and other restrictions. Let's take an imaginary world, where it is true.
- Creatine Phosphate System can produce ~12000 ATP in an hour
- Glycolysis can produce ~7200 ATP in an hour
- Aerobic respiration can produce ~4500 ATP in an hour
- Fatty Acid Oxidation can produce ~2500 ATP in an hour
ATP Production Comparison per hour (realistic):
But what about some realistic scenarios? We do not have training that would last 15 seconds, right? What about how many ATPs can we realistically produce in an hour:
- Creatine Phosphate System can produce ~350 ATP in an hour
- Glycolysis can produce ~1200 ATP in an hour
- Aerobic respiration can produce ~4500 ATP in an hour
- Fatty Acid Oxidation can produce ~2500 ATP in an hour
Power From Energy Production:
Basically, each atp production method has its strengths: Some are fast,Some are more efficient for longer efforts. All methods are valid, but I believe you might not fully understand what to do with all of this information. How do You use these methods in real life?
In order to understand that we need to draw a graph of the possible atp production over time with each method. For example, for Creatine Phosphate. We said it only works for 15 seconds, then this is the Creatine Phosphate over time graph. It takes a bit of time to turn on this method to its fullest, that is why we have an uptrend in the beginning. Same will be for all methods.
One note, that all previous calculations were made for one cell. At this point we need to switch for the whole muscle. That means that we can not really use previous numbers and rather operate on the same scale.
Also, to make it a bit more accurate, let's rename ATP volume to Power. Because for us it is important how much power we can produce. Also it is something we can physically measure for each individual.
So, The graph shows the relationship between the power we can achieve during an exercise and the duration of the exercise. We already discussed Creatine Phosphate, which could produce a lot, but only in a short period of time.
Glycolysis can produce a bit less, but for longer. It is limited due to excessive protons and lactate production.
Aerobic respiration takes a bit more time to start working, however, after about 3 min mark, it is the biggest source of ATP production, since other sources have limitations.
To be fair, we only have about 2 hours of glucose storage in our body. Yes, we can eat during exercise and restore around 100g per hour, but let's neglect it for the moment.
Our Fats capacity however, is almost unlimited. But we only can produce a small amount of power from it.
Now, you see the pattern. Let's draw the graph of the maximum possible power output over time. It looks something like this.
In order to understand that we need to draw a graph of the possible atp production over time with each method. For example, for Creatine Phosphate. We said it only works for 15 seconds, then this is the Creatine Phosphate over time graph. It takes a bit of time to turn on this method to its fullest, that is why we have an uptrend in the beginning. Same will be for all methods.
One note, that all previous calculations were made for one cell. At this point we need to switch for the whole muscle. That means that we can not really use previous numbers and rather operate on the same scale.
Also, to make it a bit more accurate, let's rename ATP volume to Power. Because for us it is important how much power we can produce. Also it is something we can physically measure for each individual.
So, The graph shows the relationship between the power we can achieve during an exercise and the duration of the exercise. We already discussed Creatine Phosphate, which could produce a lot, but only in a short period of time.
Glycolysis can produce a bit less, but for longer. It is limited due to excessive protons and lactate production.
Aerobic respiration takes a bit more time to start working, however, after about 3 min mark, it is the biggest source of ATP production, since other sources have limitations.
To be fair, we only have about 2 hours of glucose storage in our body. Yes, we can eat during exercise and restore around 100g per hour, but let's neglect it for the moment.
Our Fats capacity however, is almost unlimited. But we only can produce a small amount of power from it.
Now, you see the pattern. Let's draw the graph of the maximum possible power output over time. It looks something like this.
For each individual it varies : some are sprinters who can generate high power for short durations, while others are endurance-oriented, maintaining a steady output over longer periods. Why are they like this? Because they have different combinations of ATP production sources developed. Sprinters have developed Glucolisis, while Stayers have developed Fatty Acid Oxidation.
Worth noting, that this graph is only about maximum effort. For sure, we could make less effort on our exercise and produce only, lets say, this much amount of power.
Do we have some examples? Yes. During short races you can run much faster than during long races. That is because you can produce more power over a short period of time.
Good examples for this are world records for different running distances. If we compare the speed of running, which actually represents power, we see that the longer the distance, the slower the speed. So, this would be your across all elite athletes maximum possible power-over-time graph.
Worth noting, that this graph is only about maximum effort. For sure, we could make less effort on our exercise and produce only, lets say, this much amount of power.
Do we have some examples? Yes. During short races you can run much faster than during long races. That is because you can produce more power over a short period of time.
Good examples for this are world records for different running distances. If we compare the speed of running, which actually represents power, we see that the longer the distance, the slower the speed. So, this would be your across all elite athletes maximum possible power-over-time graph.
Can we train our energy systems?
Now you know methods of energy production and which are better for what. One can give you burst in short races, other in long races. And here is some useful knowledge:
First, we can actually train different ATP production methods. This means, for example, if we want to increase endurance, we can enhance our Aerobic Respiration. On the other hand, if we want to become sprinters, we can improve the efficiency of Glycolysis, leading to a higher power output in shorter exercises.
Second, the more we increase Glycolysis, the more we limit Aerobic Respiration, and vice versa, the more we increase Aerobic Respiration, the more we limit Glycolysis. This will be explained in the muscle types subsection a bit further. Keep in mind, I said "limit." Most likely, this video is being watched by beginners, whose levels of Aerobic Respiration and Glycolysis are still relatively low, so they can easily develop both.
First, we can actually train different ATP production methods. This means, for example, if we want to increase endurance, we can enhance our Aerobic Respiration. On the other hand, if we want to become sprinters, we can improve the efficiency of Glycolysis, leading to a higher power output in shorter exercises.
Second, the more we increase Glycolysis, the more we limit Aerobic Respiration, and vice versa, the more we increase Aerobic Respiration, the more we limit Glycolysis. This will be explained in the muscle types subsection a bit further. Keep in mind, I said "limit." Most likely, this video is being watched by beginners, whose levels of Aerobic Respiration and Glycolysis are still relatively low, so they can easily develop both.
Which organs and structures are involved in each energy system?
By now, you likely feel confident in understanding energy production methods. However, to effectively develop these systems, it’s essential to understand where they occur and which organs and structures are involved. Let’s break this down.
Creatine Phosphate System
In muscle cells, the creatine phosphate system operates in the sarcoplasm. To enhance this system's efficiency, you need to increase your creatine phosphate reserves. This will provide more immediate energy for short bursts of high-intensity activity.
Glycolysis
Glycolysis also takes place in the sarcoplasm. To improve its efficiency, you should focus on building larger glycogen reserves.
On average, the human body stores about 400g of glycogen in muscles—approximately 15g per kilogram of muscle mass—and an additional 100g in the liver, which serves as an emergency reserve. Interestingly, this is why predators often target the liver first—it’s the most energy-rich organ in the body.
Another way to improve glycolysis is by developing tolerance to increased acidity in muscles, though this is easier said than done.
Some suggest focusing on improving the lactate threshold—the point where lactate is utilized faster than it is produced. However, utilizing lactate relies on aerobic respiration. So, when you aim to increase your lactate threshold ,you actually mean to increase efficiency of Aerobic respiration, which actually decreases efficiency of Glucolisis. I will follow up on it a bit later.
Aerobic Respiration and Fatty Acid Oxidation
Next is Aerobic Respiration and Fatty Acid Oxidation. These two are a bit more complicated. First of all, both of them occur in a special place within a muscle cell called Mitochondria. And the more you have them, the more ATP you can produce. Glycogen and Fats reserves are not the limiting factor. Instead, these systems are heavily reliant on oxygen.
Let’s explore how oxygen is delivered and consumed:
Oxygen Intake
Oxygen enters the body through the lungs during breathing. You would expect that we tell you that you need efficient lungs to consume more oxygen. But that is not the truth. Usually lungs are not a limitation. They produce enough oxygen, however further steps of delivering oxygen to the muscles are lacking capacities. This is why devices like “lung training masks” are largely ineffective—they address a step, which is not a limitation for most people.
Blood Quality
Next step is your blood quality — specifically How much blood you have and how much oxygen it can carry. Amount of blood is referred to the number of red blood cells (erythropoiesis) in it, while the hemoglobin amount refers to how much blood cells can carry oxygen. And please be aware that too much hemoglobin is not good. Excessive hemoglobin thickens the blood, slowing circulation and raising the risk of blood clots. Also, it is really hard for such blood to reach well developed small capillaries.
Usually blood is a limitation.For example, your lungs might supply 100 units of oxygen, but if your blood can only transport 80 units, oxygen delivery is limited. Yes, you feel that you need more oxygen. You're suffocating. But it is not because you are under breathing, it is because your blood is not delivering.
Heart Efficiency
Next component of oxygen delivery is the heart, which is the pump for our blood. A stronger heart pumps more blood, ensuring that oxygen reaches muscles effectively. The heart is arguably the most critical muscle in the body, but I will speak about the muscles just a bit later, so it makes sense to come back after that.
Capillary Network
Next is the number of small capillaries surrounding your muscles. It determines how efficiently oxygen can be delivered. A dense capillary network allows for better oxygen supply but may reduce available space for muscle fibers. Balancing capillary growth and muscle development is crucial for optimal performance.
Muscle Oxygen Utilization
Next our muscles consume the oxygen. First step is Myoglobin - which acts as a warehouse inside sarcoplasm. It stores oxygen for periods of high demand. Increasing myoglobin levels enhances the muscle’s capacity to absorb oxygen.
Mitochondria
Finally, mitochondria serve as ATP factories. All processes involving aerobic respiration and fatty acid oxidation occur here. The more Mitochondria you have within your Muscle cells, the more you can use aerobic respiration and fatty acid oxidation.
Creatine Phosphate System
In muscle cells, the creatine phosphate system operates in the sarcoplasm. To enhance this system's efficiency, you need to increase your creatine phosphate reserves. This will provide more immediate energy for short bursts of high-intensity activity.
Glycolysis
Glycolysis also takes place in the sarcoplasm. To improve its efficiency, you should focus on building larger glycogen reserves.
On average, the human body stores about 400g of glycogen in muscles—approximately 15g per kilogram of muscle mass—and an additional 100g in the liver, which serves as an emergency reserve. Interestingly, this is why predators often target the liver first—it’s the most energy-rich organ in the body.
Another way to improve glycolysis is by developing tolerance to increased acidity in muscles, though this is easier said than done.
Some suggest focusing on improving the lactate threshold—the point where lactate is utilized faster than it is produced. However, utilizing lactate relies on aerobic respiration. So, when you aim to increase your lactate threshold ,you actually mean to increase efficiency of Aerobic respiration, which actually decreases efficiency of Glucolisis. I will follow up on it a bit later.
Aerobic Respiration and Fatty Acid Oxidation
Next is Aerobic Respiration and Fatty Acid Oxidation. These two are a bit more complicated. First of all, both of them occur in a special place within a muscle cell called Mitochondria. And the more you have them, the more ATP you can produce. Glycogen and Fats reserves are not the limiting factor. Instead, these systems are heavily reliant on oxygen.
Let’s explore how oxygen is delivered and consumed:
Oxygen Intake
Oxygen enters the body through the lungs during breathing. You would expect that we tell you that you need efficient lungs to consume more oxygen. But that is not the truth. Usually lungs are not a limitation. They produce enough oxygen, however further steps of delivering oxygen to the muscles are lacking capacities. This is why devices like “lung training masks” are largely ineffective—they address a step, which is not a limitation for most people.
Blood Quality
Next step is your blood quality — specifically How much blood you have and how much oxygen it can carry. Amount of blood is referred to the number of red blood cells (erythropoiesis) in it, while the hemoglobin amount refers to how much blood cells can carry oxygen. And please be aware that too much hemoglobin is not good. Excessive hemoglobin thickens the blood, slowing circulation and raising the risk of blood clots. Also, it is really hard for such blood to reach well developed small capillaries.
Usually blood is a limitation.For example, your lungs might supply 100 units of oxygen, but if your blood can only transport 80 units, oxygen delivery is limited. Yes, you feel that you need more oxygen. You're suffocating. But it is not because you are under breathing, it is because your blood is not delivering.
Heart Efficiency
Next component of oxygen delivery is the heart, which is the pump for our blood. A stronger heart pumps more blood, ensuring that oxygen reaches muscles effectively. The heart is arguably the most critical muscle in the body, but I will speak about the muscles just a bit later, so it makes sense to come back after that.
Capillary Network
Next is the number of small capillaries surrounding your muscles. It determines how efficiently oxygen can be delivered. A dense capillary network allows for better oxygen supply but may reduce available space for muscle fibers. Balancing capillary growth and muscle development is crucial for optimal performance.
Muscle Oxygen Utilization
Next our muscles consume the oxygen. First step is Myoglobin - which acts as a warehouse inside sarcoplasm. It stores oxygen for periods of high demand. Increasing myoglobin levels enhances the muscle’s capacity to absorb oxygen.
Mitochondria
Finally, mitochondria serve as ATP factories. All processes involving aerobic respiration and fatty acid oxidation occur here. The more Mitochondria you have within your Muscle cells, the more you can use aerobic respiration and fatty acid oxidation.
What Types of Muscles Do We Have?
So these are all Organs and structures involved in energy production. As you can see, there are quite a few. But we forgot about one extra player - the one, who will be consuming all the atp - myofibrils located inside Muscles cells. They will be doing the work, while the rest we mentioned are only warehouses and factories of fuel.
And now let's do a couple of imaginary muscles.
Scenario 1: The Overloaded Factory
Imagine your muscles have large reserves of creatine phosphate, vast glycogen stores, Huge amounts of Myoglobin and Mitochondria. It sounds perfect, doesn’t it? But there’s a problem. Despite the robust ATP production systems, you don’t have enough myofibrils to perform the work. This setup is inefficient in real life because myofibrils must occupy at least 50% of the space to ensure effective muscle performance.
Scenario 2: The Powerhouse Sprinter
Now imagine a muscle with only a few mitochondria and minimal myoglobin but great glycogen reserves. The rest of the space is packed with myofibrils. This muscle type mainly relies on glycolysis for energy. Since glycolysis is fast, such muscles provide quick bursts of energy and increased strength due to the abundance of myofibrils. This setup is ideal for sprinters—it offers speed and power. However, the lack of mitochondria means the muscle fatigues quickly, and recovery takes longer. Still, this is a legitimate configuration for strength and speed.
And now let's do a couple of imaginary muscles.
Scenario 1: The Overloaded Factory
Imagine your muscles have large reserves of creatine phosphate, vast glycogen stores, Huge amounts of Myoglobin and Mitochondria. It sounds perfect, doesn’t it? But there’s a problem. Despite the robust ATP production systems, you don’t have enough myofibrils to perform the work. This setup is inefficient in real life because myofibrils must occupy at least 50% of the space to ensure effective muscle performance.
Scenario 2: The Powerhouse Sprinter
Now imagine a muscle with only a few mitochondria and minimal myoglobin but great glycogen reserves. The rest of the space is packed with myofibrils. This muscle type mainly relies on glycolysis for energy. Since glycolysis is fast, such muscles provide quick bursts of energy and increased strength due to the abundance of myofibrils. This setup is ideal for sprinters—it offers speed and power. However, the lack of mitochondria means the muscle fatigues quickly, and recovery takes longer. Still, this is a legitimate configuration for strength and speed.
Muscle Fiber Types in Humans
These examples illustrate how muscle type depends on its internal composition. And let me jump straight to the point, we as humans have 3 options of muscle fibers:
- Slow-Twitch Fibers
- Fast-Twitch Fibers
- Intermediate Fibers
Can We Change Our Muscle Fibers?
Next thing you need to learn is that you are born with a unique combination of slow, fast, and intermediate muscle fibers. Some people naturally have more fast-twitch fibers and tend to excel in sprinting, while others have more slow-twitch fibers and perform well in endurance events.
Can you with your combination muscle fibers change something? Well, first of all, you can train. And if you put into work Fast twitch fibers, they will grow. Same with slow, if you mostly will do Aerobic Jogs, they will grow.
If we take identical twins and train them differently, one might develop as a sprinter, while the other excels as an endurance athlete.
But can you change one type of muscle fiber into another? It’s incredibly difficult. It will take a lot of trainings just to change a small amount, however we can change:
Can you with your combination muscle fibers change something? Well, first of all, you can train. And if you put into work Fast twitch fibers, they will grow. Same with slow, if you mostly will do Aerobic Jogs, they will grow.
If we take identical twins and train them differently, one might develop as a sprinter, while the other excels as an endurance athlete.
But can you change one type of muscle fiber into another? It’s incredibly difficult. It will take a lot of trainings just to change a small amount, however we can change:
- Intermediate → Fast (Intermediate to Fast)
- Intermediate → Slow (Intermediate to Slow)
- Fast → Intermediate (Fast to Intermediate)
The Special Case of the Heart
Lastly, let’s revisit the heart. The heart muscle is composed of unique fibers called cardiomyocytes, which function similarly to slow-twitch fibers. The heart primarily relies on aerobic respiration, as its structural design supports this energy pathway.
How can you make your heart bigger and stronger? Aerobic training is key. However, a larger heart isn’t always necessary—it depends on your goals and lifestyle.
How can you make your heart bigger and stronger? Aerobic training is key. However, a larger heart isn’t always necessary—it depends on your goals and lifestyle.
How do all energy systems work together? Step by step
This knowledge is the base. However, there are a few more things you need to know.
Let's Make a timeline, what is happening with your body, when you do harder and harder exercises. So the line would be Exercise intensity.
All Energy Systems Are Active
First of all, At any exercise intensity, all energy systems contribute. However, depending on the intensity, your body prioritizes certain systems.
Low-Intensity Exercise
During low-intensity exercise, your body primarily burns fat, the most energy-rich resource. As long as the intensity is low enough, this fuel source is sufficient to meet your energy needs.
First Lactate Threshold
When fat burning alone cannot provide enough ATP, the body activates glycolysis and aerobic respiration. This stage is known as the First Lactate Threshold. At this point, glycolysis begins producing lactate, but aerobic respiration efficiently consumes it, preventing lactate and excessive acidity buildup.
Second Lactate Threshold
As intensity increases, lactate production via glycolysis surpasses the capacity of aerobic respiration to utilize it. This is the Second Lactate Threshold. Here, muscle acidity rises rapidly, limiting how long you can sustain this intensity.
Why Aerobic respiration is not maxed straight after Second Lactate Threshold
You may notice that Aerobic respiration is still not maxed out here. In this area, lactate accumulates faster than it can be utilized—not because aerobic respiration is maxed out, but because it is too slow to meet the immediate demand for ATP. For example, if your body needs 8,000 ATP molecules instantly, aerobic respiration might only provide 5,000. This delay forces your body to rely on faster, less efficient energy systems.
Maxed-Out Aerobic Respiration (VO2 Max)
The next stage is when aerobic respiration reaches its absolute limit. It is the point where we hit the limit of our oxygen supply. Also called VO2max ( “O” is a letter, not Number). Point where we max out capacities of all organs involved in oxygen production.
Maximum Intensity
Beyond VO2 Max, all energy systems operate at their maximum capacity. This is the upper limit of physical exertion.
Now you understand the breaking points, but you might still wonder how to apply this knowledge or identify where you are during exercise. This is where heart rate comes into play.
Heart rate correlates directly with exercise intensity—the higher the intensity, the higher your heart rate. Heart rate zones, which you may already be familiar with, represent different combinations of energy systems at work.
To determine the precise heart rate at your breaking points, a lab test is necessary. However, most smartwatches provide fairly accurate estimates, so you can rely on them for practical purposes.
Lastly, athletes include an additional zone in their training: the Recovery Zone. This zone is intentionally set at an intensity level where none of the energy systems are maxed out or approaching a breaking point. It’s not aimed at improving energy systems but instead focuses on relaxation, muscle tuning, and recovery.
Let's Make a timeline, what is happening with your body, when you do harder and harder exercises. So the line would be Exercise intensity.
All Energy Systems Are Active
First of all, At any exercise intensity, all energy systems contribute. However, depending on the intensity, your body prioritizes certain systems.
Low-Intensity Exercise
During low-intensity exercise, your body primarily burns fat, the most energy-rich resource. As long as the intensity is low enough, this fuel source is sufficient to meet your energy needs.
First Lactate Threshold
When fat burning alone cannot provide enough ATP, the body activates glycolysis and aerobic respiration. This stage is known as the First Lactate Threshold. At this point, glycolysis begins producing lactate, but aerobic respiration efficiently consumes it, preventing lactate and excessive acidity buildup.
Second Lactate Threshold
As intensity increases, lactate production via glycolysis surpasses the capacity of aerobic respiration to utilize it. This is the Second Lactate Threshold. Here, muscle acidity rises rapidly, limiting how long you can sustain this intensity.
Why Aerobic respiration is not maxed straight after Second Lactate Threshold
You may notice that Aerobic respiration is still not maxed out here. In this area, lactate accumulates faster than it can be utilized—not because aerobic respiration is maxed out, but because it is too slow to meet the immediate demand for ATP. For example, if your body needs 8,000 ATP molecules instantly, aerobic respiration might only provide 5,000. This delay forces your body to rely on faster, less efficient energy systems.
Maxed-Out Aerobic Respiration (VO2 Max)
The next stage is when aerobic respiration reaches its absolute limit. It is the point where we hit the limit of our oxygen supply. Also called VO2max ( “O” is a letter, not Number). Point where we max out capacities of all organs involved in oxygen production.
Maximum Intensity
Beyond VO2 Max, all energy systems operate at their maximum capacity. This is the upper limit of physical exertion.
Now you understand the breaking points, but you might still wonder how to apply this knowledge or identify where you are during exercise. This is where heart rate comes into play.
Heart rate correlates directly with exercise intensity—the higher the intensity, the higher your heart rate. Heart rate zones, which you may already be familiar with, represent different combinations of energy systems at work.
To determine the precise heart rate at your breaking points, a lab test is necessary. However, most smartwatches provide fairly accurate estimates, so you can rely on them for practical purposes.
Lastly, athletes include an additional zone in their training: the Recovery Zone. This zone is intentionally set at an intensity level where none of the energy systems are maxed out or approaching a breaking point. It’s not aimed at improving energy systems but instead focuses on relaxation, muscle tuning, and recovery.
How Do I Improve Specific Energy Systems?
The key to improving a particular energy system lies in fine-tuning your training intensity. To answer the question, “How do I improve my specific energy system?”, you need to train at the maximum capacity of that system. This sends a clear signal to your body that it must adapt and grow stronger.
- For fat metabolism, the optimal intensity is at the First Lactate Threshold.
- For aerobic respiration, it's at the Second Lactate Threshold.
- For glycolysis, you need to train at VO₂max intensity.
VO2max vs. Lactate Threshold: Key Insights
Before diving into specific training types tailored to each system, let’s focus on the relationship between VO₂max and the Second Lactate Threshold.
Many endurance athletes obsess over VO2max, striving to increase this number. However, training excessively in this zone primarily develops glycolysis, which can limit aerobic respiration. It means you are limiting your endurance. Opposite of what endurance athletes are looking for.
Second example would be, you are really focused on developing aerobic respiration. You are increasing and increasing your Second Lactate threshold, but at some point it hits your Vo2max, which is low, because you never tried to develop it. Then you are also not really well developed for endurance.
But first, you need to understand what is limiting you. Is it not a well developed oxygen delivery system, which stands for Vo2Max or Oxygen consumption system, which stands for Second Lactate threshold.
Many endurance athletes obsess over VO2max, striving to increase this number. However, training excessively in this zone primarily develops glycolysis, which can limit aerobic respiration. It means you are limiting your endurance. Opposite of what endurance athletes are looking for.
Second example would be, you are really focused on developing aerobic respiration. You are increasing and increasing your Second Lactate threshold, but at some point it hits your Vo2max, which is low, because you never tried to develop it. Then you are also not really well developed for endurance.
- The Second Lactate Threshold reflects the efficiency of your oxygen consumption system, which involves mitochondria and myoglobin inside the cells.
- VO₂max, on the other hand, measures the capacity of your oxygen delivery system, including your heart, blood, and capillaries.
But first, you need to understand what is limiting you. Is it not a well developed oxygen delivery system, which stands for Vo2Max or Oxygen consumption system, which stands for Second Lactate threshold.
How to burn weight?
One more pretty important point is about weight loss. Many of you might be thinking, “To lose weight, I need to train in the fat-burning zone.” Unfortunately, this is a common misconception. The truth is: weight loss happens when you consume fewer calories than you burn, no matter how and where you train.
When you train in the so-called "fat-burning zone," all you’re doing is teaching your body to improve its fat-burning efficiency - during exercise. I repeat - During exercise.
Let’s look at an example:
When you train in the so-called "fat-burning zone," all you’re doing is teaching your body to improve its fat-burning efficiency - during exercise. I repeat - During exercise.
Let’s look at an example:
- You can train in Zone 4 for 10 minutes and then to recover, your body will burn an incredible amount of fat.
- Or you can train in Zone 2 for 10 minutes and burn some fats, however your recovery will not be that big.
Which training is made to develop what?
Now, let’s talk about training types and how they help develop different energy systems. I have an older video on my channel that explains this in more detail, but in general, we have four common types of training:
A recovery run doesn't directly impact any energy system, but it’s still valuable. Outside of energy systems, there are other areas we need to develop. However, just doing recovery runs won't make you faster or improve your endurance.
A basic steady run is focused on developing your fat-burning system. This is why endurance athletes spend so much time in this zone. As you remember, fats are virtually unlimited, while glucose lasts only around two hours. That means most people run marathons using fat for fuel, and I’m not even talking about ultra-marathons here. Also, increasing the number of mitochondria benefits not just fat burning, but also aerobic respiration.
A fast tempo run focuses on aerobic respiration. It helps you balance glucose consumption through glycolysis and mitochondrial activity.
Intervals, on the other hand, push the limits of your body, where aerobic methods aren’t enough. This develops your glycolysis, but it also develops your VO₂max, which is still very necessary for aerobic systems.
- Recovery run
- Easy Tempo run
- Fast Tempo run
- Intervals (or fartlek or hill climbs)
A recovery run doesn't directly impact any energy system, but it’s still valuable. Outside of energy systems, there are other areas we need to develop. However, just doing recovery runs won't make you faster or improve your endurance.
A basic steady run is focused on developing your fat-burning system. This is why endurance athletes spend so much time in this zone. As you remember, fats are virtually unlimited, while glucose lasts only around two hours. That means most people run marathons using fat for fuel, and I’m not even talking about ultra-marathons here. Also, increasing the number of mitochondria benefits not just fat burning, but also aerobic respiration.
A fast tempo run focuses on aerobic respiration. It helps you balance glucose consumption through glycolysis and mitochondrial activity.
Intervals, on the other hand, push the limits of your body, where aerobic methods aren’t enough. This develops your glycolysis, but it also develops your VO₂max, which is still very necessary for aerobic systems.
My examples are simple, but even intervals can be broken down into three or more types, each targeting different aspects of your body. But that’s a bit much for today. This is exactly what we do for our athletes in the Run FAQ Running School. If you really want to learn more, the best way is to experience it firsthand by training with us.