By Andrew Richardson
In this article we will be looking at all the energy systems used in the human body, looking at how energy is created and stored, what energy systems are used in certain sports, how lactic acid can be used as an energy system.
The cellular respiration process that converts food energy into adenosine triphosphate (a form of energy) is largely dependent on the availability of oxygen.
Adenosine Triphosphate (ATP) is the usable form of chemical energy for muscular activity. It is stored in most cells, particularly in muscle cells. Other forms of chemical energy, such as those available from food, must be transferred into ATP form before they can be utilized by the muscle cells.[1] During exercise, the supply and demand of oxygen available to muscle cells is affected by the duration and intensity of the exercise and by the individual's cardiorespiratory fitness level. This can be based on if the sport/activity at hand is short and maximal intensity or it is long duration with submaximal effort.
There are three exercise energy systems that can be selectively recruited, depending on the amount of oxygen available, as part of the cellular respiration process to generate the ATP energy for the muscles. They are adenosine triphosphate (ATP-Pc system), the anaerobic system and the aerobic system.
Think of ATP as petrol and the human body as a car. The energy systems are 3 different speeds we can choose. ATP is broken down using the following equation (I will use two examples one scientific the other simplified);
Since energy is released when ATP is broken down, energy is required to rebuild or resynthesize ATP. The building blocks of ATP synthesis are the by-products of its breakdown; adenosine diphosphate (ADP) and inorganic phosphate (Pi). The energy for ATP resynthesize comes from three different series of chemical reactions that take place within the body. Two of the three depend upon the food we eat, whereas the other depends upon a chemical compound called phosphocreatine. The energy released from any of these three series of hi reactions is coupled with the energy needs of the reaction that resynthesizes ATP. The separate reactions are functionally linked together in such a way that the energy released by the one is always used by the other.[2]
There are 3 ways to resynthesize ATP;
- ATP–CP system/Phosphagen System: This is the primary system behind very short, powerful movements like a golf swing, a 100 m sprint, or powerlifting.
- Anaerobic system/Anaerobic Glycolysis: An example of an activity of the intensity and duration that this system works under would be a 400 m sprint.
- Aerobic system: In a 1 km run, this system is already providing approximately half the energy; in a marathon run it provides 98% or more.[3]
The next picture shows the duration of each system when in use (Aerobic and anaerobic systems usually work concurrently. When describing activity it is not which energy system is working but which predominates.[7]) As you can tell from the graph ATP-Pc system (phosphogen system) gives the most energy but for the least amount of time and the oxidative (aerobic system) gives the lowest amount of energy at the start but keeps going till it plateaus and stays consistent maintain that submaximal intensity for a long period of time. In between the two is the glycolytic system (glycolysis system) which gives nearly as much energy in a short space of time like the ATP-Pc system but can’t sustain it due to the build-up of lactic acid over a period of time.
What is Phospho-Creatine and how it is used by the body?
Michel Eugène Chevreul discovered it along with margarine. It is a legal supplement also known as an ergogenic aid. Creatine is found in the muscles, brain and testicles in stores called phosphocreatine stores/creatine phosphate stores. Creatine phosphate is readily available to the cells and rapidly produces ATP. It also exists in limited concentrations and it is estimated that there is only about 100g of ATP and about 120g of creatine phosphate stored in the body, mostly within the muscles. Together ATP and creatine phosphate are called the high-energy phosphogens
ATP and creatine phosphate (also called phosphocreatine or PCr for short) make up the ATP-PCr system. PCr is broken down releasing a phosphate and energy, which is then used to rebuild ATP. Recall, that ATP is rebuilt by adding a phosphate to ADP in a process called phosphorylation. The enzyme that controls the breakdown of PCr is called creatine kinase.
The ATP-PCr energy system can operate with or without oxygen but because it doesn’t rely on the presence of oxygen it said to be anaerobic. During the first 5 seconds of exercise regardless of intensity, the ATP-PCr is relied on almost exclusively. ATP concentrations last only a few seconds with PCr buffering the drop in ATP for another 5-8 seconds or so. Combined, the ATP-PCr system can sustain all-out exercise for 3-15 seconds and it is during this time that the potential rate for power output is at its greatest. If activity continues beyond this immediate period, the body must rely on another energy system to produce ATP. However, the usefulness of the ATP-CP system lies in the rapid availability of energy rather than quantity. This is extremely important with respect to the kinds of physical activities that humans are capable of performing.[5]
Anaerobic Glycolysis
“Glycolysis” refers to the breakdown of sugar. In this system, the breakdown of sugar supplies the necessary energy from which ATP is manufactured. When sugar is metabolized anaerobically, it is only partially broken down and one of the by-products is lactic acid. This process creates enough energy to couple with the energy requirements to resynthesize ATP.
When H+ ions accumulate in the muscles causing the blood pH level to reach very low levels, temporary muscular fatigue results. Another limitation of the lactic acid system that relates to its anaerobic quality is that only a few moles of ATP can be resynthesized from the breakdown of sugar as compared to the yield possible when oxygen is present. This system cannot be relied on for extended periods of time.
The lactic acid system, like the ATP-CP system, is extremely important, primarily because it also provides a rapid supply of ATP energy. For example, exercises that are performed at maximum rates for between 1 and 3 minutes depend heavily upon the lactic acid system for ATP energy. In activities such as running 1500 meters or a mile, the lactic acid system is used predominately for the “kick” at the end of a race.[6]
Depending on a number of factors such as age, gender, training history and injury everyone will have a different level of energy systems. What I mean by this some people are more efficient at one energy system compared to another person.
Anaerobic exercise is an exercise intense enough to trigger lactate formation. It is used by athletes in non-endurance sports to promote strength, speed and power and by body builders to build muscle mass. Muscle energy systems trained using anaerobic exercise develop differently compared to aerobic exercise, leading to greater performance in short duration, high intensity activities, which last from mere seconds to up to about 2 minutes. [7]
Anaerobic metabolism, or anaerobic energy expenditure, is a natural part of whole-body metabolic energy expenditure.[8] Fast twitch muscle (as compared to slow twitch muscle) operates using anaerobic metabolic systems, such that any recruitment of fast twitch muscle fibers leads to increased anaerobic energy expenditure. Intense exercise lasting upwards of about four minutes (e.g., a mile race) may still have a considerable anaerobic energy expenditure component. High-intensity interval training, although based on aerobic exercises like running, cycling and rowing, effectively become anaerobic when performed in excess of 90% maximum heart rate. Anaerobic energy expenditure is difficult to accurately quantify, although several reasonable methods to estimate the anaerobic component to exercise are available.[9][10][11]
In contrast, aerobic exercise includes lower intensity activities performed for longer periods of time. Activities such as walking, long slow runs, rowing, and cycling require a great deal of oxygen to generate the energy needed for prolonged exercise (i.e., aerobic energy expenditure). In sports which require repeated short bursts of exercise however, the anaerobic system enables muscles to recover for the next burst. Therefore training for many sports demands that both energy producing systems be developed.
The two types of anaerobic energy systems are: 1) high energy phosphates, ATP adenosine triphosphate and CP creatine phosphate; and 2) anaerobic glycolysis. High energy phosphates are stored in limited quantities within muscle cells. Anaerobic glycolysis exclusively uses glucose (and glycogen) as a fuel in the absence of oxygen, or more specifically when ATP is needed at rates that exceed those provided by aerobic metabolism. The consequence of such rapid glucose breakdown is the formation of lactic acid (or more appropriately, its conjugate base lactate at biological pH levels). Physical activities that last up to about thirty seconds rely primarily on the former, ATP-CP phosphagen system. Beyond this time both aerobic and anaerobic glycolysis-based metabolic systems begin to predominate.
The by-product of anaerobic glycolysis, lactate, has traditionally been thought to be detrimental to muscle function. However, this appears likely only when lactate levels are very high. Elevated lactate levels are only one of many changes that occur within and around muscle cells during intense exercise that can lead to fatigue. Fatigue, that is muscle failure, is a complex subject. Elevated muscle and blood lactate concentrations are a natural consequence of any physical exertion. The effectiveness of anaerobic activity can be improved through training.[12]
Before we go into the aerobic system, there is usually another energy pathway that is forgotten about. That is the Cori Cycle.
Cori Cycle
The Cori cycle (also known as the Lactic acid cycle), named after its discoverers, Carl Ferdinand Cori and Gerty Cori,[13] refers to the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is metabolized back to lactate.[14]
The cycle's importance is based on the prevention of lactic acidosis in the muscle under anaerobic conditions. However, normally before this happens the lactic acid is moved out of the muscles and into the liver.[15]
The cycle is also important in producing ATP, an energy source, during muscle activity. The Cori cycle functions more efficiently when muscle activity has ceased. This allows the oxygen debt to be repaid such that the Krebs cycle and electron transport chain can produce energy at peak efficiency.[16]
Scientific Photo
As the picture below shows a detailed pathway of how energy is used a transported from an anaerobic route to an aerobic route. 
To finish the cycle we must look at the aerobic pathway.
Aerobic Pathway
Krebs cycle (citric acid cycle) – is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats and proteins into carbon dioxide and chemical energy in the form of adenosine triphosphate (ATP)
Aerobic capacity refers to the maximum amount of oxygen consumed by the body during intense exercises, in a given time frame.[18] It is a function both of cardiorespiratory performance and the maximum ability to remove and utilize oxygen from circulating blood. To measure maximal aerobic capacity, an exercise physiologist or physician will perform a VO2 max test, in which a subject will undergo progressively more strenuous exercise on a treadmill, from an easy walk through to exhaustion. The individual is typically connected to a respirometer to measure oxygen consumption, and the speed is increased incrementally over a fixed duration of time. The higher the measured cardiorespiratory endurance level, the more oxygen has been transported to and used by exercising muscles, and the higher the level of intensity at which the individual can exercise. More simply put, the higher the aerobic capacity, the higher the level of aerobic fitness. The Cooper and multi-stage fitness tests can also be used to assess functional aerobic capacity for particular jobs or activities.
The degree to which aerobic capacity can be improved by exercise varies very widely in the human population: while the average response to training is an approximately 17% increase in VO2max, in any population there are "high responders" who may as much as double their capacity, and "low responders" who will see little or no benefit from training.[17] Studies indicate that approximately 10% of otherwise healthy individuals cannot improve their aerobic capacity with exercise at all.[19] The degree of an individual's responsiveness is highly heritable, suggesting that this trait is genetically determined.[17]
Different Sports & Their Energy Systems
Depending on the sport it will ultimately affect what sort of energy system it will use. Here are a few pictures which will help you understand better what each energy system does at specific time points. Remember all energy systems work together some are more dominant than others during specified intensities.
Here are a list of sports which show which energy system's they rely on.
Journals on all Energy Systems
Here are some of my favourite journals on each of the energy systems;
Aerobic
- Gastin, P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports medicine, 31(10), 725-741.
- MacVICAR, M. G., Winningham, M. L., & NICKEL, J. L. (1989). Effects of aerobic interval training on cancer patients' functional capacity. Nursing research, 38(6), 348-353.
- Lee, C. P., & Ernster, L. (1968). Studies of the Energy‐Transfer System of Submitochondrial Particles. European Journal of Biochemistry, 3(4), 385-390.
Anaerobic (ATP-Pc)
- Spriet, L. L., Soderlund, K. A. R. I. N., Bergstrom, M. A. T. S., & Hultman, E. R. I. C. (1987). Anaerobic energy release in skeletal muscle during electrical stimulation in men. Journal of Applied Physiology, 62(2), 611-615.
- Bangsbo, J. (1996). Oxygen deficit: a measure of the anaerobic energy production during intense exercise?. Canadian Journal of Applied Physiology, 21(5), 350-363.
- Bangsbo, J., Gollnick, P. D., Graham, T. E., Juel, C., Kiens, B., Mizuno, M., & Saltin, B. (1990). Anaerobic energy production and O2 deficit‐debt relationship during exhaustive exercise in humans. The Journal of Physiology, 422(1), 539-559.
Glycolysis
- Hill, D. W. (1999). Energy system contributions in middle-distance running events. Journal of sports sciences, 17(6), 477-483.
- Connett, R. J., Honig, C. R., Gayeski, T. E. J., & Brooks, G. A. (1990). Defining hypoxia: a systems view of VO2, glycolysis, energetics, and intracellular PO2. Journal of Applied Physiology, 68(3), 833-842.
- Duffield, R., Dawson, B., & Goodman, C. (2005). Energy system contribution to 400-metre and 800-metre track running. Journal of Sports Sciences, 23(3), 299-307.
Cori Cycle
- Vandewalle, H., Péerès, G., & Monod, H. (1987). Standard anaerobic exercise tests. Sports Medicine, 4(4), 268-289.
- Brooks, G. A. (1991). Current concepts in lactate exchange. Med Sci Sports Exerc, 23(8), 895-906.
- Billat, V. L., Sirvent, P., Py, G., Koralsztein, J. P., & Mercier, J. (2003). The concept of maximal lactate steady state. Sports Medicine, 33(6), 407-426.
I hope you found this helpful in explaining what the energy systems are
Regards
Andrew
References
1. Fox, Edward (1979). Sports Physiology. United States of America: Saunders College Publishing. pp. 7–8.
2. Fox, Edward (1979). Sports Physiology. United States of America: Saunders College Publishing. pp. 8–9.
4. Fox, Edward (1979). Sports Physiology. United States of America: Saunders College Publishing. p. 9.
5. Fox, Edward (1979). Sports Physiology. United States of America: Saunders College Publishing. pp. 9–11.
6. Fox, Edward (1979). Sports Physiology. United States of America: Saunders College Publishing. pp. 11–12.
7. ^ a b Medbo, JI; Mohn, AC; Tabata, I; Bahr, R; Vaage, O; Sejersted, OM (January 1988). "Anaerobic capacity determined by maximal accumulated O2 deficit". Journal of Applied Physiology 64 (1): 50–60. Retrieved 14 May 2011.
8. ^ Scott, Christopher B (June 2005). "Contribution of anaerobic energy expenditure to whole body thermogenesis". Nutrition & Metabolism. 14 2. doi:10.1186/1743-7075-2-14. Retrieved 14 May 2011.
9. ^ Di Prompero, PE; G. Ferretti (Dec 1, 1999). "The energetics of anaerobic muscle metabolism" (PDF). Respiration Physiology 118 (2-3): 103–115. doi:10.1016/s0034-5687(99)00083-3.
10. ^ Scott, Christopher B (2008). A Primer for the Exercise and Nutrition Sciences: Thermodynamics, Bioenergetics, Metabolism. Humana Press. p. 166. ISBN 978-1-60327-382-4.
11. ^ McMahon, Thomas A (1984). Muscles, Reflexes, and Locomotion. Princeton University Press. pp. 37–51. ISBN 0-691-02376-X.
- acs.org/content/acs/en/education/whatischemistry/landmarks/carbohydratemetabolism.html
- Nelson, David L., & Cox, Michael M.(2005) Lehninger Principles of Biochemistry Fourth Edition. New York: W.H. Freeman and Company, p. 543.
15. ^ a b Medbo, JI; Mohn, AC; Tabata, I; Bahr, R; Vaage, O; Sejersted, OM (January 1988). "Anaerobic capacity determined by maximal accumulated O2 deficit". Journal of Applied Physiology 64 (1): 50–60. Retrieved 14 May 2011.
16. ^ Scott, Christopher B (June 2005). "Contribution of anaerobic energy expenditure to whole body thermogenesis". Nutrition & Metabolism. 14 2. doi:10.1186/1743-7075-2-14. Retrieved 14 May 2011.
17. ^ Di Prompero, PE; G. Ferretti (Dec 1, 1999). "The energetics of anaerobic muscle metabolism" (PDF). Respiration Physiology 118 (2-3): 103–115. doi:10.1016/s0034-5687(99)00083-3.
18. ^ Scott, Christopher B (2008). A Primer for the Exercise and Nutrition Sciences: Thermodynamics, Bioenergetics, Metabolism. Humana Press. p. 166. ISBN 978-1-60327-382-4.
19. ^ McMahon, Thomas A (1984). Muscles, Reflexes, and Locomotion. Princeton University Press. pp. 37–51. ISBN 0-691-02376-X.
About the Author
Andrew has competed for Ireland in the WDFPF federation and is representing them on August 1st at the 4 Nations in Glasgow. He is training for the IPF Worlds in 2016 which are being held in Texas (USA). He has set his eyes on medalling in the Squat and Deadlift for the U23 U105kg Class. He coaches the Teesside Barbarians at Teesside University where he is studying Applied Sport Science. Andrew is keen for more students to get involved in the sport, so much so, he is the current Student Development Officer for the Y.N.E.P.F (Yorkshire and North East England) and the I.P.F (Irish Powerlifting Federation).
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