Human skeletal muscle contains a variety of fuels and enzymes that allow it to provide energy in a number of ways. Exercise physiologists have categorized these fuels and enzymes into “energy systems” and have described their importance for different types of exercise. Changes within the muscle itself, dependent on the duration and intensity of the exercise, control which of the energy systems is most active at any one time. As a result, energy is produced automatically, in the most efficient manner possible, to meet the energetic needs of the exercising muscle. The study of how energy is released and transformed in the body is referred to as energetics.
Basic principles of energetics
The term energy is most simply defined as the ability to do work. Various forms of physical or biological work that require energy include contraction of skeletal muscle that allows us to move, walk, and exercise; the growth of new tissue in children or healing adults; and the conduction through our bodies of electrical impulses that control heart rate, release hormones, or constrict blood vessels. Ultimately, the source of energy for all of these bodily functions comes from the sun. It is hard to imagine, in the middle of a cardio kickboxing session, that the energy you are using to contract your muscles has actually originated in the sun, but it is true. You cannot simply exercise in the sunshine, however, and absorb the energy. The energy needs to be transformed from light energy into a form of chemical energy that your body can use.
Energy Flow to Humans
The transformation of light energy begins with its absorption by green plants through the process of photosynthesis. Plants begin with very simple forms of synthetic compounds, such as water and carbon dioxide, and in the presence of light, produce complex food molecules that contain a large supply of stored chemical energy. Plants can form and store various types of carbohydrates, fats, and proteins. Animals and humans can derive energy by ingesting these plants and using them as sources of fuel. Vegetarians derive all their energy from plant sources alone. Those humans and animals who consume meat, derive a portion of their energy by consuming the protein and fat stored in the meat of other animals.
During this energy flow from the sun, neither plants nor humans are creating the energy. It is being transformed by plants from light energy into a form of stored chemical energy. Humans, after ingesting the plants, then transform the energy again. At this point, it is used for biological work or it is stored, primarily in adipose tissue, skeletal muscle, and the liver, for later use. None of these transformations are particularly efficient. In fact, humans use or store less than half of the original energy that was available from the food. The unused or lost energy escapes in the form of heat. When large amounts of energy are released, as is the case during exercise, the energy lost as heat is enough to increase body temperature. Equation 1 expresses the relationship or balance between the energy flowing into the body, and that which is used, stored, and lost.
Energy consumed= Energy used + Energy stored + Energy lost
Significance of Adenosine Triphosphate
In most cases of biological work, the source of energy is specific. Energy must first be transferred into a compound called adenosine triphosphate (ATP) before it can be used. A molecule of ATP possesses a significant amount of stored energy. ATP possesses this energy largely because of its structure. The last phosphate group attaches to the remainder of the molecule by way of a “high-energy” bond. When the bond breaks, the phosphate group is released, and at the same time a substantial amount of energy is released. The end result is adenosine diphosphate (ADP) and phosphate (P), as seen in Figure.
ATP releases energy necessary for muscular contraction by releasing a phosphate (P). Energy is required to reform ATP from ADP and P
This breakdown of ATP provides the only source of energy for muscular contraction. Any energy stored in the body in the form of carbohydrate or fat, must first be converted into ATP before it can be used for exercise. As may also be observed, this reaction is reversible. That is, ATP can be replenished if there is a source of ADP, P and energy.
The actual amount of energy released from the breakdown of ATP can be estimated. When a specific amount of ATP, described as cc 1 mole of ATP, reacts in a test tube under standard conditions of temperature and acidity, it consistently releases about 7 kilocalories (kcal) of energy. In the body, however, the amount of energy released from 1 mole of ATP is about 10 kcal due to the increase in body temperature and acidity during exercise. To put this amount of energy in perspective, let us assume that walking 1 mile requires about 100 kcal of energy. One mole of ATP could supply enough energy to walk only about one-tenth of a mile.
Overview of Energy Production
The body possesses three separate systems for the production of energy. Each muscle cell in the body contains these energy systems. The systems differ considerably in their complexity, regulation, capacity, power, and the types of exercise for which they are the predominant supplier of energy. They are called upon to provide energy at a rate dependent upon the intensity and duration of the exercise performed. The three energy systems are (a) the phosphagen system, (b) the lactic acid system, and (c) the aerobic system. The goal of each system is to release energy from chemical or food sources, and transform that energy into ATP that can subsequently be used for muscular contraction and exercise (Figure).
The common goal of all three energy systems is to release the chemical energy from food that can be used to make ATP, which subsequently breaks down and supplies energy to the muscle.
The energy systems are discussed in this article beginning with the simplest and moving toward the most complex. The phosphagen system is a simple system of coupled reactions; the lactic acid system is more complex involving a sequence of reactions; and finally, the aerobic system is a complex and intricate combination of several pathways. In terms of their significance in everyday life, however, the order of discussion would be reversed. Most of our energetic needs throughout the day (and night) are met by the aerobic system alone. It is only activities that require a significant amount of muscular effort, such as moderate to intense exercise, heavy manual labor, climbing several flights of stairs, carrying a baby, or changing a tire, that require the recruitment of the lactic acid or phosphagen systems.
There are two energy systems in the body-the phosphagen system and the lactic acid system-that can operate in the absence of oxygen. Because of this, they are frequently referred to as anaerobic energy systems. It is probably more important, however, to identify them as systems that are capable of producing ATP energy at a high rate or fast. They are utilized when the rate of energy production demanded of the exercise exceeds that of the aerobic system alone. The main limitation of these systems is the relatively small amount of ATP that can be made before fatigue ensues.
Phosphagen System (ATP-CP System)
The phosphagen system supplies energy very rapidly. It relies entirely on a chemical source of fuel, however, and because of this its total capacity for producing energy is severely limited. It is the primary source of energy for very high-intensity exercise.
Biochemically, the phosphagen system is by far the simplest of the three systems. Energy for the production of ATP comes by way of a coupled reaction involving the breakdown of creatine phosphate. The compound creatine phosphate (CP), also referred to as phosphocreatine, is similar to ATP. Because of this similarity, CP and ATP are referred to collectively as “phosphagens.” The structure of CP consists of a creatine base molecule with one phosphate group attached by way of a “high-energy” bond. The splitting of CP into creatine (C) and phosphate (P) results in the release of enough energy to attach a phosphate onto an ADP molecule thereby producing ATP (Figure). During high-intensity exercise, at almost the same instant ATP is produced, its terminal phosphate group is lost. The energy is then transferred into the contractile mechanism of the muscle. This mechanism transforms the chemical energy now available into the mechanical energy necessary for rapid or forceful muscular contractions.
The energy released from the breakdown of creatine phosphate is coupled to the production of ATP, which subsequently breaks down and supplies energy to the muscle for exercise.
The regulation of the phosphagen system-and the other energy systems relies in large part on the activity of its specific regulatory enzyme(s). Enzymes are protein molecules that speed up a chemical reaction by lowering the amount of energy necessary for the reaction to initially occur. Every reaction in a biological system has an associated enzyme. An enzyme is considered regulatory if it possesses the ability to alter or regulate the rate at which an entire series of reactions occurs.
The enzyme most responsible for the rate at which the phosphagen system operates is creatine kinase (CK), also called creatine phosphokinase. Any condition that stimulates or speeds CK will increase the rate at which the phosphagen system produces energy. Conversely, any condition that inhibits or slows CK will reduce the maximal rate of energy production of the system. The most significant stimulatory factor is the rapid accumulation of ADP within the muscle cell. This is a signal to the muscle that ATP is being consumed rapidly. In an attempt to maintain the concentration of ATP, creatine kinase is activated, and creatine phosphate is rapidly broken down. The energy released from CP is used to replace the ATP being consumed.
Capacity and Power
The capacity of the phosphagen system can be estimated by measuring the amount of fuel available in the muscle. Heavy physical exercise can be sustained only while CP and ATP are available. Once the level of phosphagen is depleted, fatigue will rapidly ensue. To determine the capacity of the system requires the measurement of the total amount of phosphagen stored in skeletal muscle. For years, the only way of making such measurements was through the use of a needle muscle biopsy in which a small piece of muscle is removed from the body for analysis. More recently a method of quantifying CP, ATP, ADP, and P from outside the body has been developed. It is called nuclear magnetic resonance (NMR) spectroscopy.
Throughout this article, whenever estimates of the capacity or power of any of the energy systems are made, they are based on the following assumptions. The proposed subjects are a young man and woman who are healthy, active, and trained. The man is assumed to weigh 70 kg ( 154 lb) with 30 kg ( 66 lb) of muscle. The woman is assumed to weigh 57 kg (126 lb) with 20 kg (44 lb) of muscle. Any changes in body weight, muscle mass, or level of training will significantly affect the estimated capacities and powers.
It has been estimated, using NMR spectroscopy, that the average amount of phosphagen (combined CP and ATP) in a man with 30 kg of muscle is about 1 mole. Therefore, the capacity of the phosphagen system would be limited to 1 mole of ATP, or equivalent to about 10 kcal of energy. This is a very small amount of energy, barely enough to sprint 200 m in 20-30 seconds before it is exhausted. The capacity in a woman with 20 kg of muscle is less, about 0. 7 mole of ATP or 7 kcal, due to the smaller muscle mass. The difference in capacity is actually less a gender issue and more a muscle mass issue. Those people with more muscle mass will have a higher capacity, and those with less muscle mass will have a lower capacity.
The power of the system expresses its ability to produce energy at a particular rate, usually in moles of ATP per minute, or in kcal of energy expended per minute. Thus, a system that is characterized as possessing a high power is able to produce ATP very rapidly. To estimate the power of the phosphagen system another assumption must be made. Assume that with maximal exercise, the total phosphagen in the body (LO mole) would last for no longer than about 15 seconds. The power of the system then is equal to the total phosphagen used divided by the amount of time required to utilize the fuel as seen in Equation 2. Therefore, the power of the phosphagen system is about 4.0 moles/min of ATP production, or 40 kcal/min of energy expenditure. With no frame of reference, this value has little significance. However, as will soon be seen, the power of the phosphagen system is twice that of its nearest competitor, the lactic acid system.
Types of Exercise
Because of its ability to supply energy immediately, the phosphagen system is most important in exercise in which energy is required immediately. Such exercises would include sprinting, jumping, throwing, kicking, and lifting heavy weights. Sports that include these activities would rely at least in part on the phosphagen system. The common factor of analysis is the time involved. If the activity can be sustained for no more than 15-20 seconds, the phosphagen system is the primary source of energy (supplying over 50% of the energy).
Shorter exercise (1-5-second duration) that requires even higher energy production relies more heavily on the phosphagen system. Exercise sustained slightly longer (30-45-second duration) relies less on phosphagen metabolism. Good examples of specific events that rely on the phosphagen system for their primary source of energy include 100 and 200 meter running sprints, 50 meter swimming sprints, high jump and long jump, shot put and discus, and power lifting.
Many other activities and sports are more difficult to analyze due to their vari-able, intermittent nature. Even with these complicating factors, however, it can be concluded that the more intense sections of an aerobics routine; the sprinting and kicking in soccer; the jumping and spiking in volleyball; and the sprinting, jumping, and shooting in basketball, all rely heavily on the phosphagen system.
Lactic acid system (fast glycolysis)
The lactic acid system also provides a rapid source of energy. Its fuel source is glucose, the usable form of carbohydrate in the body. Because the supply of glucose exceeds that of muscle phosphagen, the lactic acid system produces more ATP than the phosphagen system. But still, its capacity is limited because of the production of its end product, lactic acid, which is not tolerated well by the body. The lactic acid system is the primary source of energy for sustained high-intensity exercise lasting no longer than a few minutes.
Glycolysis is a process that occurs in the sarcoplasm or fluid portion of the muscle cell. It involves a sequence of reactions that partially breaks down glucose into a simpler compound called pyruvate. Once pyruvate is formed, it can take one of two pathways, depending on the need for energy or the presence of oxygen in the muscle. If the level of oxygen in the muscle is sufficient and the demand for energy is low, glycolysis operates in such a way that pyruvate enters the mitochondria and is combusted aerobically. This is referred to as “aerobic” or “slow” glycolysis. But if the level of oxygen is insufficient, or the demand for energy is high, the pyruvate is transformed into lactate (lactic acid). Under these circumstances, the process is referred to as “anaerobic” or “fast” glycolysis, and is also described as the “lactic acid system.”
The foods that we typically eat can be separated into three categories: (a) carbohydrates, (b)fats, and (c) proteins. Carbohydrates are the only form of food that can be used as fuel in the lactic acid system. Furthermore, the only form of carbohydrate that can be used is glucose, a simple six-carbon sugar. The glucose used for fuel can come either from the blood glucose or from stored glycogen within the muscle. In either case, the glucose enters glycolysis initiating a sequence of nine or more reactions resulting in the production of lactate. A simplified version of glycolysis is presented in Figure. It is believed that this sequence developed into the most efficient way of rapidly transforming food energy (from glucose) into ATP.
As mentioned previously, the source of fuel for glycolysis is provided by blood glucose through a phosphorylation (addition of phosphate) made possible by the breakdown of ATP. This is considered a “priming step.” It is analogous to priming a pump. Energy must first be added before any work can be done resulting in the flow of water. In this case, before any ATP can be made, one phosphate group is removed from ATP and attaches to glucose-making glucose phosphate. This priming step is not necessary if glycogen is the source of fuel. (Keep in mind, however, that the glucose had to proceed through this step to be stored as glycogen in the first place).
A simplified version of the lactic acid system (fast glycolysis). Note that (a) glucose can come from blood glucose or muscle glycogen, (b) priming steps are required, (c) the glucose splits into two equal parts, (d) ATP is produced, and (e) lactate is made in the muscle and diffuses into the blood. The increase in blood lactate indicates that the system is active.
Next, a second priming step occurs, with another mole of ATP donating its phosphate group. The compound then splits into two equal parts. Therefore, from this point on, every reaction actually occurs twice for every one glucose that originally entered glycolysis. The next step is the first reaction in the sequence in which ATP is produced. There is sufficient energy released during this reaction (10 kcal) to combine an ADP and P to produce ATP. There are actually two ATP formed from the input of one glucose. Another reaction now occurs in which ATP is formed. Again, two ATP are made since this reaction occurs twice.
The final step in glycolysis is the conversion of pyruvate to lactate. Also indicated in Figure is the net production of ATP from blood glucose and muscle glycogen, which is summarized in Equations 3 and 4.
The regulation of the lactic acid system is considerably more complex than that of the phosphagen system. It still depends, however, on the activity of regulatory enzymes, of which there are several. The most important regulatory enzyme in glycolysis is phosphofructokinase (PFK). Its importance lies in the fact that it exists in the lowest concentration and possesses the lowest activity of any enzyme in the sequence. Because of these characteristics, the reaction that PFK catalyzes is considered the “rate-limiting step” of glycolysis. It is analogous to the strength of a chain being determined by its weakest link. PFK, and hence the lactic acid system, is stimulated by the rapid accumulation of ADP and by the rapid depletion of CP that occur during very high-intensity exercise.
The lactic acid system is inhibited under resting conditions due to an interaction effect with the aerobic system. Specific intermediates in aerobic metabolism that are in relatively high concentration in skeletal muscle at rest inhibit PFK and suppress the use of carbohydrate and anaerobic metabolism at rest.
Capacity and Power
The primary limiting factor in the capacity of the lactic acid system is not fuel depletion, but the accumulation of lactic acid. If used to its fullest extent, the lactic acid system would fatigue before using 1 mole of glucose. If it is assumed that the average person stores at least 2.0-2.5 moles of glucose, this means that as much as 50% of the fuel remains in the body at the time of fatigue. It does not seem likely, therefore, that the level of fuel limits the capacity of the system.
What is important to remember about this system is that while producing ATP, it simultaneously produces lactic acid. As the lactic acid is formed it rapidly loses a proton or hydrogen ion (H+) and becomes lactate. The problem is the neutralization or “buffering” of the excess hydrogen ions. The result is the muscle becomes too acidic to operate, many of the enzymes are inhibited and the actual mechanism of muscular contraction is affected.
The maximal capacity of a person’s lactic acid system is determined by his or her ability to neutralize and tolerate lactic acid. Research suggests that the highest level of lactic acid that can be tolerated by a trained 70 kg person is about 90 grams (or about 1 mole oflactic acid). Based on Equation 4, this means that at the same time the person makes 1 mole of lactic acid they have concurrently made 1.5 moles of ATP. So the capacity of the system, or the total amount of ATP that can be made, is about 1.5 moles, corresponding to a total energy expenditure of 15 kcal. This amount of energy would allow for only about 45-90 seconds of high-intensity exercise.
The maximal power of the system again depends on the time required to produce a given amount of ATP. If it is assumed that the lactic acid system can be exhausted in as little as 45 seconds of intense exercise, the theoretical rate at which ATP energy is produced through the lactic acid system can be determined as seen in Equation 5. The estimated power of the system in our trained person turns out to be around 2.0 moles/min or 20 kcal/min.
Types of Exercise
Because of the relative simplicity of glycolysis, and because oxygen is not needed, the lactic acid system produces ATP rapidly. It provides the primary supply of energy for physical activity that results in fatigue in 45-90 seconds. Shorter, more intense exercise would rely to some degree on the phosphagen system, while longer, less intense exercise would begin to require aerobic metabolism. The lactic acid system is very important in prolonged sprints (400-800 meters running, 100-200 meters swimming or 1,000-2,000 meters cycling). It also provides much of the energy for sustained, high-intensity rallies in soccer, field hockey, ice hockey, lacrosse, basketball, volleyball, tennis, badminton, and other sports. The floor routine in gymnastics relies in part on this system, with intermittent bursts of higher energy production from the phosphagen system. The common denominator in all of these activities is a sustained, high-intensity effort lasting from 1 to 2 minutes.