Where Do We Get Our Energy?

Everything in our lives requires energy. Whether it is getting out of bed or running a marathon, it requires energy. However, our body does not rely on the same type of energy for both of these tasks. Our body relies on three different energy systems to complete tasks. The ATP-PCr or Phosphagen system, Glycolytic system, or the Aerobic/Oxidative system. The body is never completely shutting down these energy systems, it will just use one of the energy systems predominantly based on the task that is being performed.

The body is reliant upon the breakdown of Adenosine Triphosphate (ATP) to provide energy for physical bouts. These three energy systems regenerate ATP, but at different rates. These systems are responsible for the very energy our body requires to complete physical tasks (1). Throughout this article we will discuss what these 3 energy systems are responsible for. As well as how they regenerate ATP for the body to use.

Figure 1: Graph of energy system contribution over time of an exercise bout (2).

ATP-PCr/Phosphagen System

This is the energy system responsible for short explosive bouts of exercise.This system is an anaerobic system which means it can work independently of the presence of oxygen. Since this system works without oxygen it can only provide energy for bouts less than 10 seconds in duration (MB throw, jump, short sprint, max effort lift, etc.). After about 30 seconds of rest 70% of the system recovers, if full recovery is the goal then expect to wait 3-5 minutes for full recovery (2). Regardless of whatever the event is that is being completed this energy system is always the first one to be used. 

We talked about what this system is responsible for, now let’s discuss how this system produces ATP. This system resynthesizes ATP the fastest, but it also produces the least amount of ATP. Creatine Phosphate (CP) is a compound that is stored primarily in the muscles of the body. CP donates 1 phosphate to Adenosine Diphosphate (ADP) to allow for ATP to be resynthesized (3). The body can only produce about 1 gram of creatine a day, with there being such a limited supply of creatine stored in the muscles this is the reason that the muscles fatigue so rapidly when this is the primary system being used (4).

Glycolytic System

This system is responsible for work 15s-3min in duration. The Glycolytic system can be either anaerobic or aerobic. A great example of this energy source at work would be a 200-400m sprint, however this system is most dominant in high effort bouts done without full recovery. Aerobic Glycolysis occurs during long bouts of exercise done with a medium intensity. The Glycolytic system takes a considerable amount of time to recover. For full recovery it can take anywhere from 24-48 hours for muscle glycogen to replenish, this time can vary based on diet, exercise intensity, etc. When aiming for 1.0-1.2g/kg/bw of carbs after exercise it may take closer to that 24 hour mark compared to 48 hours (5).

Figure 2: Picture showing how glucose is used in both forms of glycolysis (6)

The Glycolytic System is a tricky one when it comes to resynthesizing ATP. I mentioned before that it is an aerobic and an anaerobic system which means the system can resynthesize ATP in multiple ways. This process is called glycolysis, which is just saying the breakdown of glycogen. Carbohydrates in the form of blood glucose or muscle glycogen are broken down to form pyruvate which is just a three carbon molecule. From there two pathways can occur, if the need for ATP exceeds the amount of oxygen present then Anaerobic Glycolysis will occur. The anaerobic pathway takes that pyruvate and turns it into a lactate which will allow for the Glycolysis to continue making ATP with low oxygen. The downside is that lactate carries hydrogen atoms which are “waste” in this process. When you are feeling that “burn” during a lift it is actually the hydrogen ion building up in the muscle is what causes that feeling, not lactic acid. If the need for ATP does not exceed oxygen present then Aerobic Glycolysis occurs. This would happen as we get closer to that 1:30-3 minute mark. Instead of turning pyruvate into lactate, the glucose will be turned into Acetyl-CoA where it will undergo the Krebs Cycle. The Krebs Cycle produces 1 ATP molecule per cycle as well as FADH₂ and NADH which will aid in the production of ATP further on in the electron transport chain (6).

Oxidative/Aerobic System

The Oxidative System is an aerobic pathway and is the slowest moving out of the three, but it produces the most ATP and is responsible for most of the processes that occur in the body, breathing, eating, blinking, are all taken care of by the Oxidative system. In addition to those seemingly tiny tasks this pathway is also responsible for long distance running and can also become a huge factor in continuous team sports. Anything that is taking place for longer than 3 minutes is the responsibility of the Oxidative System and it primarily functions on fat and glucose. During low power and long duration events this system is the one doing most of the work.

This system can create ATP in three different ways: The Krebs Cycle, Electron Transport Chain (ETC), or Beta Oxidation. Remember the Krebs Cycle from Glycolysis? Well it’s back. When Acetyl-CoA goes through the Krebs Cycle it also creates hydrogen which causes the “burn”. But in this sense those hydrogen ions combine with FAD and NAD and are sent down the ETC. A bunch of chemical reactions occur within the ETC, hydrogen will combine with oxygen which produces water and will prevent the “burn” from the hydrogen buildup. The Krebs Cycle and ETC also turn carbs and fat into ATP. The triglycerides aka the fat are broken down through a process called lipolysis which leaves us with free fatty acids and glycerol. The free fatty acids go through a process called Beta Oxidation which turns them into hydrogen and Acetyl-CoA which then will go through the Krebs Cycle just like carbs did. In low intensity or long bouts of exercise where carbs are not enough fuel the body will turn to fat for energy which is where this pathway makes its money (7). 

Conclusion

The body is a crazy and complex, but also amazing thing. Every single thing we do in life requires energy. But as we just talked about, all ways of producing energy are not equal. Depending what we need to accomplish the body will find the most efficient and proper way to produce that energy. Understanding how each system is used and how each one plays a huge impact in sports performance is vital for success when talking about training. As coaches we need to be able to prescribe the proper training to athletes based on what they need in order to be successful. It takes energy on our end to make sure our athletes have enough energy to succeed.

Citations

1. Baker JS, McCormick MC, Robergs RA. (2010).  Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise. J Nutr Metab. 2010;2010:905612. doi: 10.1155/2010/905612. Epub 2010 Dec 6. PMID: 21188163; PMCID: PMC3005844.

2. Gastin, Paul. (2001). Energy System Interaction and Relative Contribution During Maximal Exercise. Sports medicine (Auckland, N.Z.). 31. 725-41. 10.2165/00007256-200131100-00003. 

3. Karp, Jason. (2021). The Three Metabolic Energy Systems. ideafit.com

4. Kreider RB, Stout JR. Creatine in Health and Disease. Nutrients. 2021 Jan 29;13(2):447. doi: 10.3390/nu13020447. PMID: 33572884; PMCID: PMC7910963.

5. Alghannam AF, Gonzalez JT, Betts JA. (2018). Restoration of Muscle Glycogen and Functional Capacity: Role of Post-Exercise Carbohydrate and Protein Co-Ingestion. Nutrients. 2018 Feb 23;10(2):253. doi: 10.3390/nu10020253. PMID: 29473893; PMCID: PMC5852829.

6. Rabinowitz JD, Enerbäck S. (2020). Lactate: the ugly duckling of energy metabolism. Nat Metab. 2020 Jul;2(7):566-571. doi: 10.1038/s42255-020-0243-4. Epub 2020 Jul 20. PMID: 32694798; PMCID: PMC7983055.

7. Kelso, Tom (2021). Understanding Energy Systems: ATP-PC, Glycolytic and Oxidative – Oh My!. Breakingmuscle.com