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Energy Systems Part I

How to Properly Train Energy Systems.

This article is part 1 of 5 discussing energy systems and how to properly train them. The first part will be a dive into the physiology of the 3 energy systems, parts 2-4 will consist of methods to train the different energy systems and the 5th part with consist of how to periodize energy system training.

As strength and conditioning coaches, we have all taken exercise physiology and probably human physiology, in addition our certification exams required us to know some basic information about energy systems. If you are anything like me, you need an occasional refresher into how energy systems function at a physiological level. I feel as though this is important information to understand when it comes to programming energy system training. Understanding the details of how each energy system functions is imperative to ensure that when you program a specific modality to address 1 of the energy systems, that the method chosen does indeed address the adaptation you are trying to elicit. Furthermore, understanding how the 3 energy systems interact with one another is essential to train them optimally.

Adenosine Triphosphate (ATP) is the molecule which is used for energy in the body often referred to as “the energy currency of the body” (McArdle, 2007). ATP will donate a phosphate start a chemical reaction. For example, in muscle tissue, ATP binds with myosin, causing it to connect to actin. This action starts the process of contracting a muscle (Howley, 2009). There are several other things involved to continue the contraction but, that is not important for this article. After, myosin and actin disconnect the ATP has now lost a phosphate group and is now Adenosine Diphosphate (ADP). The goal of our energy systems is to replenish ATP through a variety of methods. Each of the three energy systems does this through different means, ranging from very quickly to slowly; and each does so at different speeds.

There are 3 energy systems in the human body, all of which have the same objective, replenish ATP stores. Each of the 3 energy systems can produce energy for a varying about of time at different rates. The Aerobic System can create the most total energy but does so at the expense of how fast it can do it. Next is the Lactic System which creates energy semi-quickly and can do so for about a minute. Last, is the Alactic System which can create energy very quickly but can only function at a high level for a very brief amount of time, about 10 seconds.

Aerobic System

The aerobic system requires the use of oxygen to replenish ATP. The aerobic system is unique from the other 2 energy systems in both, that it needs oxygen for ATP replenishment and that it can use multiple substrates in the replenishment of ATP; glycogen, fat, protein, and pyruvate for example. Due to the fact that the aerobic system can use multiple  substrates it takes far more steps to replenish ATP, but what it lacks in speed it makes up for in the total amount of ATP it can create over time. A single glucose molecule yields a net 36 ATP (McArdle, 2007) and a single 18 carbon chain triglyceride yields 460 ATP (McArdle, 2007)..

The primary driver of energy production in the aerobic system is the Kreb’s Cycle (Howley, 2009). The Kreb’s Cycle is also called the Citric Acid Cycle (McArdle, 2007), depending on which resource you reference. The Krebs Cycle starts with Acetyl CoA and Oxaloacetate and through a series of chemical reactions ATP is replenished and hydrogen ions are are created. The Hydrogen Ions are sent into the electron transport chain where ATP is created. Unlike other energy systems the metabolic by products at the end of energy creation in the aerobic system are Carbon Dioxide during the Kreb’s Cycle and water from the electron transport chain (McArdle, 2007).

The body prefers the use of glucose for energy, due to it needed the least amount of work for it to enter into the Kreb’s Cycle. Glucose is a 6 carbon ring simple sugar and is stored in long chains in muscle and the liver (Howley, 2009). The aerobic system can also use fat for energy, the body stores fat as triglycerides. Before fat can be used for energy it needs to undergo Beta Oxidation (McArdle, 2007).. This allows the triglycerides to be converted in Acteyl CoA, before undergoing the Kreb’s Cycle. Protein can also be used as an energy source in the Aerobic system. Before proteins can be used for energy, they are broken down into its constituent amino acids. Depending on the type of amino acid depends on the fate of the amino acid. Some are converted into pyruvate before becoming Acetyl CoA (McArdle, 2007).. Some are converted straight into Acetyl CoA before entering the Kreb’s Cycle (McArdle, 2007).. While others are used to create the substrates needed for the various chemical reactions in the Kreb’s Cycle (McArdle, 2007).

The last piece of the Aerobic system is the Cori Cycle. During Anaerobic glycolysis Pyruvate is created and depending on energy demands pyruvate becomes lactate. I will further discuss this process in the next section. When lactate enter the bloodstream it can go to; the brain, the heart, or the liver. In the heart and brain it is used for energy. If it enters the liver it is converted into glucose via the Cori Cycle (Howley, 2009). This only happens when energy demands are low.

The aerobic system can create energy from multiple sources for a long period of time at the expense of speed. In addition, the aerobic energy system is the Master System. It is the Master System because without it, the other energy systems can not be replenished. The importance of this system in sport is recovery between bouts of intense exercise.

 

 

The last piece of the Aerobic system is the Cori Cycle. During Anaerobic glycolysis Pyruvate is created and depending on energy demands pyruvate becomes lactate. I will further discuss this process in the next section. When lactate enter the bloodstream it can go to; the brain, the heart, or the liver. In the heart and brain it is used for energy. If it enters the liver it is converted into glucose via the Cori Cycle (Howley, 2009). This only happens when energy demands are low.

The aerobic system can create energy from multiple sources for a long period of time at the expense of speed. In addition, the aerobic energy system is the Master System. It is the Master System because without it, the other energy systems can not be replenished. The importance of this system in sport is recovery between bouts of intense exercise.

 

Lactic System

The lactic system is intermediate energy system. It can operate at a high level for about 1 minute. The process by which glucose in used to create ATP is called Anaerobic glycolysis. A singe glucose molecule yields 2 ATP following the process of anaerobic glycolysis. Anaerobic glycolysis also yields other metabolic by products, pyruvate and NADH to be specific (McArdle, 2007). The fates of the metabolic byproducts depend on the energy demands of the activity.

Pyruvate and NADH interact with each other in 1 of 2 ways. In times of high energy demands the Hydrogen ion picked up by NAD to create NADH is donated back to pyruvate to create lactate. In times of low energy demands the NADH can donate its hydrogen ion to the electron transport chain, part of the aerobic system (Howley, 2009). As for pyruvate if it become lactate it can also serve different purposes. In times of high energy demands of enough lactate is produced it will be shuttled into the blood where it undergoes different process. Discussed in the previous section. The lactate in the cell when energy demands drop low enough lactate will be converted back into pyruvate for use in the Kreb’s Cycle.

The anaerobic system is simple compared to the aerobic system. Therefore it can create energy quickly but does so for a brief period of time. The glucose used in anaerobic glycolysis come primarily from muscle glycogen stores, this will be further discussed in the article discussing how to train this system.

 

Alactic System

The Alactic System is the easiest system to understand. This system can create energy almost instantly but can only do so for 6 seconds. It does this through a molecule called Phosphocreatine (PCr). PCr, replenishes ATP by donating its phosphate group to ADP (McArdle, 2007). When energy demands are low enough and the aerobic system has replenished enough ATP, some of the ATP will donate its phosphate back to creatine, turning it back into PCr (McArdle, 2007).

 

Conclusion

            Now that I have bored you to death with over simplifying the biochemistry of the 3 energy systems, remember this was a refresher into how these systems operate. The aerobic system is the master system, needed to replenish the other energy systems so they can operate at a high level. The Lactic system is the intermediate system requiring glucose for ATP replenishment and produces metabolic byproducts. Lastly, the Alactic system used PCr for almost instant ATP creation, with no negative by products. Stay tuned for the next 3 parts which will discuss: testing and training specific adaptations of each of the 3 energy systems. This article series will conclude with how to periodize the training of energy systems.

 

References

Howley, P. &. (2009). Exercuse Physiolgy Theory and Application to Fitness and Performance 7th Ed. New York: McGraw Hill Higher Education.

McArdle, K. &. (2007). Exercise Physiology Energy Nutrition & Human Performance 6th ed. Baltimore: Lippincot, Wiliams & Williams.

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