There are three systems of energy production within the body; this includes the ATP-PC energy system, the lactic acid energy system and the aerobic energy system. These energy systems are responsible for converting the carbohydrates, fats and proteins we consume into ATP (adenosine triphospate). ATP is the body’s energy currency and is used to fuel every movement we make along with driving various cellular processes that keep us alive. Having a basic understanding of how these energy systems work will allow you to tailor your training to best suit your goals.
How does ATP Work?
ATP is an energy molecule that consists of one adenosine and three (tri) phosphate groups. ATP molecules are broken down in a process called ATP hydrolysis. This process is triggered by an action potential which releases the energy stored in its energy dense phosphate bonds. During hydrolysis the last phosphate group splits from the ATP molecule to provide energy for the required action. After ATP molecules are broken down you are left with ADP (adenosine diphosphate) which is the remaining adenosine and two phosphate groups as well as one single phosphate. Our body only stores small amounts of ATP and therefore must continuously synthesis new ATP in order to keep us moving. In a process called phosphorylation new phosphate groups are added back to ADP to form new ATP molecules. The food we consume provides the fuel which allows this synthesis to occur. If phosphorylation occurs in the presence of oxygen it is referred to as aerobic metabolism, if this process occurs without the presence of oxygen it is referred to as anaerobic metabolism. Understanding how your body synthesis ATP will allow you to understand each energy system and how to fuel your body sufficiently.
ATP-PC System
This system is comprised of ATP and phosphocreatine which is stored in short supply within the muscle. Our body relies predominantly on the ATP-PC system for short, high intensity burst of activity (ie. throwing a ball or jumping). In the immediate seconds of action the stored ATP is used for fuel, once ATP is broken down into ADP it reacts with the phosphocreatine to produce new ATP molecules. This form of anaerobic energy production happens quickly but a limited fuel supply means that the ATP-PC system becomes depleted after only 8-12 seconds depending on the intensity of the activity. Once phosphocreatine stores have been depleted it can take up to 5 minutes to restore them back to a resting level. Energy production via the ATP-PC system happens without the production of any byproducts.
Lactic Acid System
After phosphocreatine has been exhausted the body requires glucose in order to continue. The lactic acid system breaks down carbohydrates or stored glycogen in a process called anaerobic glycolysis. No oxygen (anaerobic) is required for the breakdown of a glucose molecule which means the process of glycolysis can happen quickly, for this reason the lactic acid system is well suited to support high intensity efforts about 10 seconds and up to 3min (ie. 400m run or 100m swim). The body can store around 500 grams or approximately 2000 calorie of glycogen in the tissues of the liver and therefore fatigue does not typically occur as a result of inadequate fuel, rather it is related to the process of anaerobic metabolism. During the process of glycolysis a by product called pyruvic acid builds up in the muscle. This byproduct is converted to lactate and transported from the muscle to the liver so it can be converted to glucose. When high intensity activity continues lactate cannot be removed fast enough, this results in a build up of pyruvic acid. This causes the muscle to become acidic and this is the reason for the burning sensation you experience during high intensity efforts.
Aerobic System
This is the only energy system that uses oxygen to breakdown carbohydrates, fats and if necessary protein for the production of ATP. This complicated system has three components; aerobic glycolysis, the Krebs cycle and the electron transport chain. Together these three components can produce ATP continuously for hours provided there is an adequate source of fuel. Once glucose stores are depleted which is typically around 60-90 minutes fat becomes the dominant fuel source, fat requires more oxygen for breakdown and if these oxygen requirements aren’t met intensity will decline. If our body runs out of fatty acids then it must breakdown protein into amino acids to help with ATP production, this is a last resort. The aerobic energy system produces ATP very efficiently (38 ATP molecules per glucose compared to 1 ATP per PC) however production is slow meaning it simply cannot cope with high levels of intensity. The aerobic system is better suited to long duration activities (ie. marathons) but is also the system that keeps us functioning on a day to day basis. Water and carbon dioxide are byproducts of aerobic metabolism; water can build up in the muscle and cause swelling or stiffness but is generally transferred into the bloodstream and lost through sweat. Carbon dioxide is transferred from the muscle into the lungs where it is expired. For the aerobic system to recover you must restore fuel back to pre-exercise levels. This requires the ingestion, digestion and transportation of fuel which can take anywhere from 12-48 hours. The lactic acid system also relies on the aerobic energy system to help with the removal of waste products to allow for better recovery.
Whilst each energy system may be dominant through particular phases of energy production it is important to remember that no one system ever functions in isolation. Instead all three energy systems work in conjunction with each other to produce ATP in the most efficient way possible. With appropriate conditioning and sport specific programming the efficiency of these energy systems can be improved in order to help you achieve your desired results.
xx Tara
Which energy system do you think you utilise more when you train? Let me know in the comments section below.
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