The aerobic system is a complex collection of several different components sometimes collectively called oxidative metabolism. Because of its ability to use carbohydrates, fats, and proteins as sources of fuel, and because it produces only carbon dioxide and water as end products, the aerobic system has a virtually unlimited capacity for making ATP. Its complexity and its need for a constant supply of oxygen, however, limit the rate at which ATP is produced. The aerobic system supplies all of the energy for low-to moderate intensity exercise. It supplies energy for sleeping, resting, sitting, walking, and other forms of low-intensity, long duration physical activity. As the activity becomes more intense, to the point that it can only be sustained for a matter of a few minutes, the aerobic system can no longer provide energy at a sufficient rate. At this stage, ATP production is supplemented by the lactic acid and phosphagen systems.
The term “aerobic system” refers to a complex series of reactions that, for the purpose of description, can be divided into three components (Figure). The first component can actually be one of three pathways depending upon whether the source of fuel is carbohydrate, fat, or protein. When carbohydrate is used, the first component is glycolysis, which under these conditions operates slowly or aerobically. As a result, lactate is not formed and the end product is pyruvate. When fat is used, the first component is a process called fat oxidation, in which large molecules of fat are made into much simpler molecules that fuel subsequent reactions in the system. If the source of fuel is protein, the first component pathway is protein metabolism. While a minimal amount of ATP is formed directly in each of these pathways, the main purpose of the first component is to produce acetyl groups (small two-carbon compounds) and a supply of electrons for subsequent reactions. The second component is a cyclical process called the Krebs cycle and is common to all types of fuel. The main purpose of the Krebs cycle is to remove electrons and protons for subsequent reactions. The final component, also common to all types of food fuel, is the electron transport system (ETS). The electron transport system, because of a coupled process called oxidative phosphorylation, accounts for over 85% of the total ATP produced by the aerobic system. A good way to begin to understand the components of the aerobic system is to follow 1 mole of glucose from start to finish throughout the entire system.
An overview of the pathways involved in the aerobic system. Complex food molecules (e.g., glucose, free fatty acid, amino acid) are broken down into much simpler molecules of C02 and H20. In the process, energy is released that can be used to produce ATP.
ATP Production from Carbohydrates
The aerobic production of energy from carbohydrate begins with “slow” or “aerobic” glycolysis as seen in Figure. The enzymes and intermediate compounds of glycolysis are simply dissolved in the sarcoplasm (fluid portion) of the muscle cell. Although they are not physically arranged in any particular order, the compounds react in the specific sequence described previously. The muscle cell is also composed of subcellular (within the cell) structures called mitochondria. The mitochondria are oval-shaped structures, existing separately or possibly in “networks” that contain the enzymes associated with the Krebs cycle and electron transport system (ETS).
It is within the mitochondria that most of the ATP is produced aerobically. The difference between “fast” and “slow” glycolysis is the utilization of the electrons and pyruvates produced. If the activity of the mitochondria (which relies in part on the supply of oxygen and the rate at which energy is being produced) is sufficient, the electrons and pyruvates formed enter the mitochondria. The electrons flow directly to the ETS, while the pyruvates are oxidized (lose electrons) and decarboxylated (lose C02) forming acetyl groups that enter the Krebs cycle. Because of the entry of the pyruvates and electrons into the mitochondria, there is no lactate produced under these conditions.
Overview of the aerobic system using carbohydrates as a source of fuel. Glycolysis produces pyruvates that are converted into acetyl groups and enter the Krebs cycle. Electrons are sent from many sources to the electron transport system where most of the ATP is actually made.
The acetyl groups formed from the pyruvates enter into the Krebs cycle. The combination of the acetyl with other compounds results in the production of citric acid, the first intermediate in the Krebs cycle. Once citric acid is formed it goes through a series of reactions, including several oxidations in which more electrons are removed. These electrons are very important, because they are the driving force for the electron transport system (ETS). The electrons are actually shuttled into the ETS by something called coenzymes. A significant portion of the structure of the coenzymes consists of two B vitamins called niacin and riboflavin. Severe vitamin deficiencies could lead to reduced aerobic function.
Electron Transport System
The final sequence of reactions in the aerobic production of ATP is the electron transport system. This system consists of a number of reusable electron-carrying compounds that can exist in either oxidized or reduced form. These compounds are arranged into specific “complexes” and are physically located within the mitochondrial membrane. They are arranged so that an electrical gradient (difference) exists between the beginning and the end of the system. The gradient created by this arrangement allows the electrons to pass from one intermediate to the next, or in other words to “flow” through the system. This flow, through a very complicated chemical process beyond the scope of this explanation, supplies the energy necessary to make a tremendous amount of ATP. The entire aerobic breakdown of glucose can be summarized as seen in Equation 1.
ATP Production from Fat
Fats are stored in the body in adipose tissue and within skeletal muscle in the form of triglycerides. For fat stored in adipose tissue to be used for exercise, it must first be mobilized and transported to the muscle. The fats must then be converted into a form the muscle can use as fuel. This usable form of fat is called a free fatty acid. A fatty acid is a molecule much longer than glucose that can contain as many as 26 carbons in a long chain. Typical fatty acids used by humans for energy production include the saturated fats-stearic acid (18 carbons) and palmitic acid (16 carbons), and the unsaturated fats-oleic acid and linoleic acid (each possessing 18 carbons). The utilization of fat as a fuel begins with a cyclical process called the fat oxidation cycle, which occurs within the mitochondria. The fatty acid is first “activated” through a priming step involving the input of 1 mole of ATP. This priming step is not required for every revolution of the cycle, only for the initial entry of the fatty acid into the cycle. Three significant reactions occur during fatty acid oxidation. Two oxidations occur feeding electrons into the electron transport system, and the third involves the cleaving of an acetyl group from the carbon chain of the fatty acid.
The fatty acid (less two carbons) then revolves a second time through the cycle. This process will continue until only two carbons remain in the skeleton of the fatty acid. At this point, the two-carbon remnant (an acetyl group) enters the Krebs cycle leaving nothing of the fatty acid. It has been completely oxidized to carbon dioxide and water through the aerobic system with a considerable amount of ATP-energy produced in the process as seen in Figure. When stearic acid (an 18-carbon fatty acid) is used as a source of fuel, the combination of eight full revolutions of the fat oxidation cycle and the remaining acetyl group remnant results in the production of nearly 150 moles of ATP (or nearly 1,500 kcal of energy). A fatty acid with a longer carbon chain (> 18 carbons) results in greater energy production, while a shorter carbon chain ( < 18 carbons) produces less energy. The aerobic breakdown of stearic acid can be summarized as seen in Equation 2.
Overview of the aerobic system using free fatty acids as a source of fuel. One revolution of the fat oxidation cycle produces an acetyl group that enters the Krebs cycle. Electrons are sent from many sources to the electron transport system where most of the ATP is actually made.
ATP Production from Protein
Finally, a brief word about the use of protein as fuel is in order. Protein usually does not provide more than 10-15% of the total energy requirement of an activity. As such, protein does not play as significant a role as carbohydrate or fat as a fuel for exercise. The main source of stored protein in the body is muscle. It is obviously not advantageous to use this source for fuel during exercise. Some dietary protein (from animal or vegetable origin) is used for fuel. It must first be broken down into amino acids its simpler, more usable form. Typically, amino acids consumed through the diet include alanine, leucine, valine, and tryptophan. One mole of alanine, metabolized aerobically, produces one acetyl group and one pair of electrons, which result in the production of 15 ATP. In summary then, a mole of carbohydrate (glucose) produces 38 ATP, a mole of fat (stearic acid) produces 147 ATP, and a mole of protein (alanine) produces 15 ATP when combusted by the aerobic system.
The regulation of the aerobic system is more complex than that of the lactic acid system. This complexity is understandable given the vast number and nature of the reactions involved in aerobic metabolism. The discussion of the regulatory factors focuses on the control of the Krebs cycle, fat and carbohydrate metabolism, and the electron transport system. The rate at which the Krebs cycle operates depends primarily on the activity of its enzymes. All of these enzymes are stimulated by elevated concentrations of ADP, and inhibited by high concentrations of ATP. The enzyme that assumes the key regulatory role within the cycle is isocitrate dehydrogenase, which regulates the oxidation of isocitrate. Under resting conditions, the level of ATP in the mitochondria is high. To avoid the overproduction of ATP, which cannot be stored, the elevated mitochondrial ATP inhibits the regulatory enzyme and slows the Krebs cycle.
During low- to moderate-intensity aerobic exercise, the amount of ADP entering the mitochondria rises. This has a stimulating effect on isocitrate dehydrogenase which speeds up the Krebs cycle. Determining which gets burned-fat or carbohydrate-has been a subject of much concern to fitness instructors. The control of fat and carbohydrate entry into the aerobic system is intimately involved with its overall regulation. Under resting conditions, fatty acids are readily available and provide the primary source of fuel. The presence of high concentrations of fatty acid and citric acid inhibit glycolysis by inhibiting PFK. Therefore, under these resting conditions, fat metabolism flourishes while carbohydrate metabolism is inhibited. During prolonged, moderate-intensity exercise ( < 75% HRrnax), subtle changes occur in the level of secretion of several hormones. The secretion of epinephrine (adrenaline) from the adrenal glands rises and the secretion of insulin from the pancreas decreases.
These hormones influence the rate of fat and carbohydrate uptake by muscle in such a way that fat metabolism still predominates and is further enhanced naturally or endogenously during prolonged work. With higher intensity exercise (> 85% HRrnax), changes occur that begin to inhibit the use of fats. The most significant inhibitor is the lactic acid produced. It reduces the availability of fatty acids by slowing their release from triglycerides. As a result, fat metabolism is inhibited and carbohydrate becomes the preferred source of fuel, used by the aerobic system and the lactic acid system. The status of the electron transport system also influences the overall regulation of aerobic metabolism. Oxygen must be in constant supply for the proper functioning of the system. The increase in blood flow to the muscle during aerobic exercise ensures a sufficient oxygen supply and allows the aerobic system to increase its rate of energy production. The increased influx of ADP into the mitochondria during exercise also stimulates the enzymes associated with ETS, further enhancing its performance. The system is inhibited, on the other hand, by reduced blood flow resulting in reduced oxygen availability. A strong, isometric muscular contraction, caused by exerting pressure on blood vessels, causes a brief restriction of blood flow. This results in the temporary inhibition of the aerobic system so that the muscle relies more on the lactic acid and phosphagen systems. If all fuels are considered, including the total carbohydrate, fat, and protein stored in the body, the aerobic system has a virtually unlimited capacity for producing ATP energy. Its complexity and its need for oxygen, however, limit the maximal power at which the system can operate.
Capacity and Power
The only practical limit to the capacity of the system comes when analyzing prolonged, continuous aerobic exercise. The best example of this type of exercise is a marathon completed in competitive time (under 3 hours). A marathon run at this pace requires significant reliance on carbohydrate metabolism. If the competitor is not careful, the carbohydrate within the muscle can be depleted before the end of the race, resulting in premature fatigue or “hitting the wall.” The total amount of ATP that can be produced aerobically from stored muscle glycogen can be estimated. If the same assumptions are used regarding the level of training, body weights, and muscle weights, a male subject may store about 450 grams ( 1 lb) of glycogen. This much glycogen would theoretically produce nearly 100 moles of ATP, equivalent to 1,000 kcal of energy, or sufficient energy to walk or run about 10 miles.
A female subject, with a muscle mass of 20 kg and a similar glycogen concentration would have a capacity of approximately 65 moles of ATP upon glycogen depletion. The capacity of the aerobic system grows tremendously if stored fat is included as a potential source of energy. For example, a 70 kg male of above average body composition ( 15% body fat) possesses just over 10,000 grams of stored body fat. Since 1 gram of fat yields 9 kcal of energy, the amount of energy available from the complete combustion of stored fat would result in over 90,000 kcal of energy, or 9,000 moles of ATP. Theoretically, this would be enough energy to walk from New York City to Chicago (900 miles) without eating. A 100 kg subject with 31 % body fat would have enough stored energy to walk from New York City to Los Angeles (2,800 miles) without refueling. The power of the aerobic system depends on the maximal rate at which the body can transport and consume oxygen. The maximal rate of oxygen uptake (V02max) is determined by a graded exercise test to exhaustion. If done on a treadmill, the protocol usually consists of increasing the speed and grade of the treadmill every 2-3 minutes during the exercise test.
Oxygen uptake and other physiological variables are measured throughout the test. Testing continues until the subject can no longer maintain the speed of the treadmill belt and voluntarily stops due to exhaustion. An average value for maximal oxygen uptake in an untrained 70 kg person is about 3 liters/min. If it is assumed that for every liter of oxygen consumed, about 5 kcal of energy are expended, the V02max can be converted into a maximal energy expenditure of about 15 kcal/min. Finally, if 1 mole of ATP is required for each 10 kcal of energy expenditure, an estimated maximal rate of ATP production would be about 1.5 moles/min. Thus, when producing energy at a maximal rate (15 kcal/min), the aerobic system produces energy 75% as fast as the lactic acid system (20 kcal/min) and less than 40% as fast as the phosphagen system ( 40 kcal/min).
Types of Exercise
The aerobic system, because of its limited power, provides energy primarily for low- to moderate-intensity exercise. Virtually all of the energy necessary for resting activities, including sitting, reading, studying, watching television, surfing the Internet, and sleeping, comes by way of the aerobic system. With slightly higher intensity activity, like walking, leisurely bicycling, shopping, and office work, the aerobic system still supplies most of the energy. It is not until the intensity reaches a moderately high level (above 75-85% of maximum heart rate) that the limit of the aerobic system is reached and other energy systems need to be recruited to provide supplemental energy. Such activities would include aerobics, running, swimming, cycling, rowing, skating, and others that are performed above 75-85% of maximum intensity. This intensity is such that the activity could be sustained continuously for at least 5 minutes without fatigue, yet requires a significantly elevated heart rate to accomplish. The best examples of exercises relying primarily on the aerobic system for energy include 40-60 minutes of aerobics, distance running (> 5,000 meters), distance swimming (> 1,500 meters), distance cycling (> 10 kilometers), cross-country skiing (> 5,000 meters), and the triathlon. Any activity, providing it is sustained continuously for a minimum of 5 minutes, relies primarily on the aerobic system. This encompasses portions of several team or more complex individual sports, including soccer, field hockey, lacrosse, basketball, tennis, and squash, to name a few. All of these sports, however, also periodically require energy production from the lactic acid and phosphagen systems for more intense rallies and bursts of sprinting, jumping, and kicking.
“Aerobic system” is a term used by exercise physiologists to refer to a complex system of metabolic reactions. The system is capable of using any form of food, including carbohydrate, fat, or protein, as a source of fuel. The first component of the system, which is different depending on the source of fuel, begins with large food molecules and breaks them down so that pyruvates, acetyl groups, and/or electrons are produced. This provides the fuel for the Krebs cycle and for the subsequent reactions associated with electron transport and oxidative phosphorylation. With a sufficient supply of oxygen, the aerobic system can completely catabolize these food fuels into carbon dioxide and water, while saving much of the energy released through the formation of ATP. (On the assumption that someone could utilize most of the carbohydrate and fat stored in the body, the aerobic system has a virtually unlimited capacity.) However, because of its complexity and the need for oxygen, the power of the system is somewhat limited. These characteristics make it an ideal source of energy for prolonged activity of a low to moderate intensity ( < 65-75% HRmax)
Summary and Comparison of Energy Systems
A full appreciation of the energetics of exercise requires a fundamental understanding of the energy systems. Conclusions can be drawn as to the significance of each system with regard to many sports based on the system’s characteristics.
Summary of Energy Systems
Energy is produced within the body in response to demands placed upon it. The body attempts to produce energy in the most efficient manner possible and at the necessary rate. When conditions permit, the body produces energy aerobically because of the efficiency with which this process is completed. However, when exercise is performed at an intensity that exceeds the capability of the aerobic system, the energy requirement is met through anaerobic metabolism.
Capacity and Power
The aerobic system produces a virtually limitless supply of energy through the catabolism of carbohydrate, fat, and protein stored within the body. The combustion of these fuels occurs in such a way that the only remaining end products ( C02 and HzO) are easily removed by exhalation from the lungs. The lactic acid system is more limited in its energy-producing capacity due to the disruption of the normal acid-base balance that it creates. Hydrogen ions produced by dissociation from lactic acid quickly saturate the body’s buffer systems and fatigue ensues. The phosphagen system provides an immediate source of readily available energy. The amount of usable fuel is so limited, however, that the capacity is minimal and can be completely exhausted within a matter of seconds. The significance of the anaerobic energy systems lies more in their ability to produce energy at high speed. The phosphagen system, because of its simplicity, is able to provide energy immediately for forceful muscular contractions. It produces energy at least twice as fast as either of the other systems. Anaerobic glycolysis, the sequence of reactions constituting the lactic acid system, is similarly simple and, as such, provides a rapid source of energy as well. The complexities of the aerobic system and its reliance on oxygen as an acceptor of electrons limit its rate of energy production to below that of the anaerobic systems.
Types of Exercise
Any sport, exercise, or physical activity that can be accomplished with a level of exertion not exceeding 65-75% of one’s maximal capability (as indicated by heart rate for example) can be accomplished almost exclusively with aerobic metabolism. Frequently in sports, however, the athlete cannot exercise at a constant intensity, but instead must alternate between low ( < 65% HRmax), moderate (65-75% HRmax), high (80-90% HRmax), and very high (> 90% HRmax) intensities. When this is the case, the body frequently shifts between energy systems, taking advantage of the differing characteristics of each.
Comparison of Energy Systems
The characteristics of the systems are compared in Table. Based on these characteristics, conclusions are made as to the types of exercise for which each system is best suited. The basic facts contained within this chapter provide a foundation for developing a training program for any sport. Through proper training, the capacities and powers of each of the systems can be improved with the accompanying expectation of improved sport performance.
Summary of characterstics of three energy systems