Energy metabolism

Energy metabolism | The provision of energy in the muscle is often not that easy to understand for many students or trainees, especially if they have had little to do with biology up to now. In the article, I try to explain the energy supply simply. If you don’t yet know the basic structure of a muscle and what has to happen for muscles to contract, watch the Planet Schule video “How a Muscle Works” before reading this article. At one point or another, I refer to things that happen in the muscle, and I think that the video can help you quite a bit in further understanding the supply of energy in the muscle.

Energy metabolism, in general

Every cell in the body needs oxygen (O2), including muscles. Because oxygen is the prerequisite for the viability of our organism. Movement, or life in general, requires energy. The Planet Schule video showed that proteins contract to induce muscle contractions. If that didn’t happen, we couldn’t live. So that the proteins can hook into each other, etc., the body needs energy. The energy comes mainly from food, such as pasta, oil, meat, or high-energy drinks…if you feel like eating something, then look at the delicious dishes in the free nutrition plans here on Fitness Simply Explained 😅… Okay, now on to the energy supply. So, the energy mostly comes from food. Food contains the macronutrients carbohydrates, fats, and proteins. Food gives us the energy we need. However, the muscles cannot use carbohydrates directly as an energy source. The body must first convert these nutrients. During the conversion, the human body produces ATP as a universal energy carrier. The cells’ constant ATP production with oxygen is essential because they mainly use the energy stored in the ATP to maintain their structures and specific functions (Faller & Schünke, 2016; Hollmann & Strüder, 2009).


  • The main energy source for muscle contraction is ATP
  • Nutrients (especially carbohydrates, fats, but also proteins) are converted into ATP
Energy metabolism through indirect energy out of foods. Image by stevepb from Pixabay
Energy metabolism through indirect energy out of foods. Image by stevepb from Pixabay


Energy metabolism in general – Adenosine triphosphate

The universal energy carrier is called adenosine triphosphate, or ATP for short. ATP is made up of three chemical substances. These are the nitrogenous adenine, the sugar ribose, and three phosphate molecules. High-energy compounds link the phosphates. The name adenosine triphosphate results from the composition (Faller & Schünke, 2016; Hollmann & Strüder, 2009). 🙂


  • Adenosine triphosphate = adenine + ribose + 3 phosphates

  • Adenosine = adenine + ribose

Therefore, Each cell must constantly produce ATP, primarily a form of energy that can be used directly and biologically. As a result, it enables the following energy-demanding processes (Faller & Schünke, 2016; Hollmann & Strüder, 2009):

  • Mechanical work, i.e., the movement (contraction) of muscles
  • Transport of substances through the cell membrane
  • Synthesis of protein and other cell components
  • Phosphorylation of glucose and fructose-6-phosphate
  • Activation of free fatty acids

However, the article will focus exclusively on ATP and muscle contraction.

ATP is already readily available in the body. That’s cool! But humans consume about as much ATP as their body weight every day. However, the supply of ATP in the muscle cell is very limited and amounts to about 6 mmol/kg in the resting muscle, which is probably sufficient for one to three muscle contractions or 2 – 3 seconds of muscle work. Apparently, we cannot change this basic amount of ATP through training and nutrition either. To use the muscles for longer than just 2 – 3 seconds, e.g., the macronutrients are broken down (Hüter-Becker & Dölken, 2011). In general, various starting materials are used for the production of ATP. In addition to carbohydrates and fats, there are other substances, e.g., creatine phosphate, from which the body can produce ATP (Raschka & Nitsche, 2016). The protein content in energy production is 10 – 15%. Proteins can also provide energy, but their proportion is relatively small (Hüter-Becker & Dölken, 2011).

Which energy reserves the body burns and to what extent is primarily determined by the intensity of the load (Raschka & Nitsche, 2016). Other factors that can have an influence are the duration of stress, diet, and training status (Raschka & Nitsche, 2016):


Energy metabolismHow does ATP provide energy?

The ATP molecule has the 3 phosphate residues (P). Each phosphate has been glued to the adenosine through a complex process. To put it bluntly, the binding energy is the glue. And if you were to detach a phosphate from adenosine now, the binding energy would be released. The energy released could then be used for other purposes…and that’s precisely what happens in the muscles. The basis of energy generation is the splitting off of phosphate. If a phosphate is now split off, the remaining molecule is called adenosine diphosphate (ADP). There are only 2 phosphates left on the adenosine, hence the prefix “di” for “two” (Greek “dis” ≙ “twice”). When phosphate is split off, around 7 kcal per mole of ATP is released, necessary for cell performance and can be used (“mole” is the international unit of substance amount). In skeletal muscle, ATP is broken down by the enzyme myosin ATPase to ADP. In a separate chemical reaction, the breakdown of ATP to adenosine monophosphate (AMP, just a phosphate ” still sticks to the adenosine”) is possible. In addition to ATP, other energetically comparable compounds exist in the human body. These are guanosine triphosphate and uridinium triphosphate. However, as mentioned, ATP is mainly used (Hollmann & Strüder, 2009). For the reaction to continue, ATP has to be supplied repeatedly. The reaction has the following formula (Hollmann & Strüder, 2009):

ATP + H2O → ADP + P + Energy

The figure below is only intended to provide a striking illustration of what is happening. The red circle stands for adenosine and the blue pen means phosphate. Lightning represents energy, and water is not visible in the illustration.😁 Note: The actual structures do not simply correspond to a circle shape and a bar but are much more complicated. It probably doesn’t flash every time a phosphate residue is split off, either…😉

Energy metabolism
Energy metabolism – ATP → ADP + P + Energy



  • 1 phosphate is released
  • ADP is formed
  • Energy is released


Energy metabolismWhere is ATP formed?

Of course, ATP also has to be produced somewhere. A distinction is made between the production sites of ATP depending on whether oxygen is needed during formation. When oxygen is involved, ATP is produced in the mitochondria, the powerhouses of the cells. This is also referred to as “aerobic energy supply” (Biesalski et al., 2017; Faller & Schünke, 2016) (Greek “aēr” ≙ “air”). When oxygen is not used, ATP is formed outside the sarcoplasm’s mitochondria. This way of providing energy is also called “anaerobic” (Adeva-Andany et al., 2014; Lackner & Peetz, 2019) (Greek “aēr” ≙ “air”, with Alpha privativum α(ν)- a(n)- “without”).

If the load is low, these starting materials are primarily broken down with the help of oxygen (aerobic). You can imagine it quite strikingly in such a way that the muscles also have the corresponding time to produce ATP under low loads. Accordingly, they can wait for oxygen to be delivered from the lungs. With the participation of oxygen, the body uses fats (slowly) or carbohydrates (not as slowly as fat). However, if the intensity of exercise increases, the comparatively slow production of ATP using oxygen is no longer sufficient to cover the increasing need for ATP. Therefore, at higher intensity of exertion, energy supply pathways are required to deliver ATP quickly or quickly. This includes the oxygen-independent breakdown of carbohydrates (fast) or creatine phosphate (really fast). Suppose carbohydrates are broken down without the participation of oxygen, e.g., lactate (salt of lactic acid). In addition to hydrogen ions, produced during the anaerobic breakdown of carbohydrates, lactate accumulates in the muscle tissue and causes muscle fatigue. Because the environment becomes a little more acidic, the musculature slows down the anaerobic provision of energy using carbohydrates (Faller & Schünke, 2016). Just imagine you want to sprint 400 m. In a 400m sprint, carbohydrates are the leading supplier of ATP. For the first 100 m, you run your best time and try to keep the pace. Sometimes, on the way, after maybe 200 – 300 m, you can feel your thighs burning. But don’t give up now and keep sprinting. However, you find that even if you keep up your speed, you keep getting slower. The muscles burn more and more. That’s because of the lactate and the hydrogen ions. The milieu within the muscles becomes more acidic. This manifests in the burning sensation and hinders energy provision via carbohydrates.


  • The more lactate, the less well the anaerobic ATP production work


Energy metabolism How is ATP formed?

So ATP has to be replenished over and over again. So that the following reaction can take place again and again:

ATP + H2O ADP + P + Energy

I just mentioned that there are different ways to produce ATP. Now, let’s take a closer look at each way. A distinction is made (Hollmann & Strüder, 2009; Hüter-Becker & Dölken, 2011; Raschka & Nitsche, 2016):

  1. Anaerobic-alactic energy supply
  2. Anaerobic-lactic energy supply
  3. Aerobic energy supply

Energy metabolism – Anaerobic-alactacid energy supply

If the body needs energy really quickly because the intensity of the exercise is very high and ATP has to be supplied immediately, the anaerobic-alactacid energy supply is used. The anaerobic-alactic pathway probably dominates the energy supply for maximum exertion of up to 20 seconds (Schnabel, 2014).

As the name suggests, oxygen is not involved in this pathway. Lactate is also apparently not formed during the production of ATP. So fat (only aerobic) and carbohydrates (lactate formation, if anaerobic) fall out as energy carriers. Which energy source can it be?🤔 The depot for the anaerobic-alactic energy supply is creatine phosphate (PCr). The creatine phosphate is available in about three to four times the amount of ATP in the muscle cell. The breakdown of creatine is controlled by the enzyme creatine phosphokinase (CK), the activity of which depends on the respective ATP consumption. During dynamic work, creatine degradation increases linearly with muscular performance under static load with the tension developed (Hollmann & Strüder, 2009). Depending on how well the aerobic performance is developed, the KP stores can be almost completely refilled after about 3 minutes (Hüter-Becker & Dölken, 2011).


  • Creatine phosphate provides the fastest recovery of ATP

We already know that 1 phosphate is split off from ATP, so 1 ADP is present after the energy has been provided. Now, it fits quite well that there is creatine phosphate. Because the creatine phosphate could also provide 1 phosphate, and it does. To do this, the phosphate is split off from the creatine beforehand with the help of an enzyme. The freely available phosphate can now be glued to the ADP using energy. And there you have 1 ATP again, which can be split again to support muscle action. The chemical reaction formula for this is (Hollmann & Strüder, 2009):

ADP + PCr → ATP + Cr


Energy metabolism – Anaerobic-lactic energy supply

The creatine phosphate storage and, thus, the anaerobic-alactic metabolism completely cover the energy requirement for only a few seconds of a high-intensity exercise. However, the resynthesis of ATP always has to go on continuously. This is why the breakdown of more complex molecules plays a role after 4 to 6 seconds of exertion. Anaerobic-lactic energy production involves using carbohydrates to produce ATP without the involvement of oxygen but with the formation of lactate. Although the anaerobic-lactic pathway is slower than the anaerobic-alactic pathway, more ATP is produced during the anaerobic utilization of carbohydrates than during the creatine phosphate metabolism (Raschka & Nitsche, 2016). This metabolic pathway probably dominates the energy supply in 20 to 80 seconds (Schnabel, 2014).


  • The anaerobic-lactic energy supply takes place via carbohydrates without oxygen

  • Recovery is slower than via PCr, but still fast

  • Supplies a greater amount of ATP than PCr

Carbohydrates are used by glucose or glycogen (storage form of glucose in the muscle and liver cells) to provide energy. The degradation pathway of glucose is also called glycolysis and that of glycogen glycogenolysis (Raschka & Nitsche, 2016; Schnabel, 2014). A simple chemical reaction equation is presented below (Hollmann & Strüder, 2009). It should be noted that the underlying cascades are much more complex. But the simple formula is also correct in principle; it’s kept quite simple. 😉

Glucose + P + ADP → Lactate + ATP

As mentioned in the 400 m sprint example, the blood lactate concentration can also increase as an expression of the anaerobic energy supply with intensive exertion. As the concentration increases, this enters the blood from the muscle cells and is broken down again, especially in the heart, kidneys, and liver (Raschka & Nitsche, 2016).


Energy metabolism – Aerobic energy supply

In the case of aerobic energy supply, e.g., Carbohydrates or fats are used to produce ATP using oxygen. Carbohydrates are faster than fats. Carbohydrates are completely burned with the participation of oxygen and provide more ATP compared to anaerobic utilization. Although ATP restoration via fats takes more time than the aforementioned metabolic pathways, it also supplies a significantly larger amount of ATP (Hollmann & Strüder, 2009). Aerobic metabolism probably dominates ATP resynthesis above 2 minutes of exercise (Schnabel, 2014). In principle, the simple reaction formulas for the aerobic combustion of carbohydrates and the utilization of fats look similar (Hollmann & Strüder, 2009).

Glucose + P + ADP + O2 → ATP + H2O + CO2

Fat + P + ADP + O2 → ATP + H2O + CO2


Energy metabolism – Percentage of metabolic pathways in energy supply

The different metabolic pathways all take place at the same time. Only the percentage of the respective energy supply routes varies depending on the stress intensity and nutritional situation. The following table gives an overview of the percentage of aerobic and anaerobic metabolism during maximum exertion of different durations and full carbohydrate stores.

Percentage of anaerobic and aerobic metabolism at maximum exercise, different duration and full glycogen stores [Schnabel, 2014].
Metabolism10 seconds1 minute2 minutes10 minutes30 minutes
Anaerobic85 %65 - 70 %50 %10 - 15 %5 %
Aerobic15 %30 - 35 %50 %85 - 90 %95 %



Adeva-Andany, M., López-Ojén, M., Funcasta-Calderón, R., Ameneiros-Rodríguez, E., Donapetry-García, C., Vila-Altesor, M., & Rodríguez-Seijas, J. (2014). Comprehensive review on lactate metabolism in human health. Mitochondrion, 17, 76-100.

Biesalski, H. K., Pirlich, M., Bischoff, S. C., & Weimann, A. (Eds.). (2017). Ernährungsmedizin: Nach dem Curriculum Ernährungsmedizin der Bundesärztekammer. Georg Thieme Verlag.

Faller, A. & Schünke, M. (2016). Der Körper des Menschen: Einführung in Bau und Funktion. Geoerg Thieme Verlag

Hollmann, W., & Strüder, H. K. (2009). Sportmedizin: Grundlagen für körperliche Aktivität, Training und Präventivmedizin; mit 91 Tabellen. Schattauer Verlag

Hüter-Becker, A., & Dölken, M. (2011). Biomechanik, Bewegungslehre, Leistungsphysiologie, Trainingslehre. Georg Thieme Verlag.

Raschka, C. & Nitsche, L. (2016). Praktische Sportmedizin . Thieme

Schnabel, G. (Ed.). (2014). Trainingslehre-Trainingswissenschaft: Leistung-Training-Wettkampf. Meyer & Meyer Verlag.



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