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Revisiting Energy Systems

Fitness professionals need to know the latest data to answer clients’ questions about metabolism and exercise.

While most fitness professionals are familiar with the basics of energy metabolism, it can be difficult to explain to clients the intricacies of how the body breaks down and uses nutrients to fuel physical activity. For example, can you explain why a greater percentage of fat is burned during low-intensity exercise, when the potential for losing weight is greater if exercise is performed at a higher intensity for an equivalent period of time? Or can you describe why power lifting requires longer rest intervals than circuit training? This article will provide the answers to these and other common questions regarding exercise training. It will also provide examples of how energy is utilized during different types of exercise, such as resistance training and indoor cycling.

Overview of Energy Systems

Performing any kind of exercise requires energy. The body maintains a continuous energy supply by extracting chemical energy from food (carbohydrate, fat and protein) during complex chemical reactions. Depending on the intensity and duration of the activity, these chemical reactions occur through one or a combination of all three of the body’s energy systems:

  • the phosphagen energy system
  • the glycolytic energy system
  • the mitochondrial respiration energy system

These energy systems are responsible for the production of adenosine triphosphate (ATP), the ultimate source of energy for muscle contraction. Because the energy liberated from ATP powers all forms of biologic work, ATP is often referred to as the cells’ “energy currency” (McArdle et al. 1996). For a comparison of how the different energy systems work, see “Energy Systems at a Glance” below.


The phosphagen energy system dictates our ability to initiate rapid, high-intensity activities, such as sprinting 100 meters, lifting a heavy weight or dashing up a flight of stairs. Because the phosphagen system does not require oxygen to operate, it is considered an anaerobic system. The phosphagen energy system is sometimes referred to as the “immediate energy system.”

When immediate energy is needed for rapid, high-intensity activities, the body breaks down the ATP that is readily available. The problem is that our cells can store only enough ATP to fuel about one second of explosive, all-out exercise. As a result, ATP must be continuously resynthesized and replenished to meet the body’s needs. One way the body immediately replenishes ATP is when a high-energy compound called creatine phosphate (CP) combines with adenosine diphosphate (ADP) in a reaction catalyzed by the enzyme creatine kinase. Because the body’s cells can store about four to six times more CP than ATP, CP is the predominant source of immediate energy (McArdle et al. 1999).

Another way the body immediately replenishes ATP is through a reaction involving the enzyme adenylate kinase (AK). The AK reaction generates one ATP molecule from two ADP molecules. However, compared to readily available ATP and CP, the energy available through this reaction is relatively insignificant (Brooks et al. 1995).

It is important to remember that the phosphagen energy system (including the ATP-CP and ATP-AK reactions) can provide only enough energy to sustain all-out exercise for about five to 10 seconds (Cotton 1997). Beyond this point, assistance from other sources of energy is required.


The glycolytic energy system, or simply glycolysis, is called into play when a series of chemical reactions breaks down glucose into a compound known as pyruvate. The glycolytic energy system is sometimes referred to as the “lactic acid system.”

Although glycolysis is an anaerobic process, in that it does not directly use oxygen, this should not be interpreted to mean that glycolysis occurs only in the absence of oxygen. In actuality, glycolysis occurs whether oxygen is present or not. However, the presence or absence of oxygen—along with the energy demands of the particular activity—do directly influence how pyruvate is used.

In general, when oxygen supply is sufficient to meet oxygen demands, as it is during prolonged exercise of light to moderate intensity, pyruvate enters the mitochondria of the cells (the primary source of cellular energy), where it undergoes a series of chemical reactions and helps in the aerobic formation of additional ATP. Under these types of conditions, glycolysis is the first step in the aerobic degradation of carbohydrates; thus, it is sometimes referred to as “aerobic” or “slow” glycolysis.

In contrast, “anaerobic” or “fast” glycolysis occurs when oxygen demands exceed oxygen supply, as occurs during strenuous exercise that lasts between a few seconds and a few minutes. The end result of this process is an accumulation of byproducts, such as lactate and hydrogen ions. Contrary to widespread belief, it is actually the accumulation of hydrogen ions, not lactate, that is primarily responsible for cellular acidosis and impaired performance (Brooks et al. 1995). In fact, lactate is now recognized as playing a positive role in metabolism by providing fuel for other active muscle groups.


Put simply, mitochondrial respiration is aerobic metabolism that occurs in the mitochondria of the cells when the oxygen
supply is sufficient to meet the oxygen demands of the activity. As exercise progresses beyond several minutes, as it does during distance running or cross-country skiing, the mitochondrial energy system becomes the main supplier of ATP. The mitochondrial energy system is sometimes referred to as the “aerobic energy system.”

In comparison to the phosphagen and “anaerobic” glycolytic energy systems, the “aerobic” glycolytic and mitochondrial respiration energy systems produce and activate energy much more slowly. As a result, these slower systems predominate when the activity is not as intense (Brooks et al. 1995; McArdle et al. 1999).

Mitochondrial respiration allows for the continued breakdown of pyruvate activated during “aerobic” glycolysis, as well as derivatives of lipid and amino acid metabolism. While it is typically understood that any prolonged exercise relies on the breakdown of fat, the role of carbohydrates in aerobic metabolism is often underestimated. Yet even during light to moderate exercise, carbohydrates supply approximately 40 to 60 percent of the total energy requirements through “aerobic” glycolysis and mitochondrial respiration (Brooks et al. 1995; McArdle et al. 1999).

Protein, on the other hand, is not a major source of energy during any form of exercise. Although protein is used to a greater degree than previously thought during both resistance training and endurance training, protein’s maximum contribution during high-intensity endurance training (assuming an adequate and well-balanced diet) is generally less than 10 percent of the total energy requirements (McArdle et al. 1999).

For an overview of the relative contributions of carbohydrate, fat and protein to energy metabolism during exercise at various intensities, see “How Different
Nutrients Fuel Exercise at Different Intensities” above.

Practical Applications

It is important to recognize that no one energy system acts in isolation. Energy transfer during exercise is best thought of as a continuum, with considerable overlap from one energy system to another. The relative contributions depend on the activity’s duration and intensity, as well as the client’s fitness level and ability to use oxygen efficiently. At one extreme, the total energy for short-duration, high-intensity exercise lasting less than 10 seconds is supplied almost entirely by the phosphagen energy system. At the other extreme, long-duration activities, such as marathon running and hiking, require a constant supply of energy derived from the “aerobic” glycolytic and mitochondrial respiration systems. When the intensity of a single workout fluctuates, the body will frequently shift between energy systems, in an effort to respond efficiently to the specific stimuli.

Regular heart rate monitoring or ratings of perceived exertion can help participants assess their level of intensity. It should be recognized, however, that an individual’s lactate threshold (i.e, the exercise intensity that increases the rate at which muscle lactic acid accumulates) can be measured accurately only through metabolic testing. Furthermore, lactate threshold can vary significantly, with some participants reaching their threshold at 70 percent of maximum heart rate (HRmax) and others reaching it at 85 percent. Therefore, participants should be instructed to watch for signs that indicate their threshold point is near. These signs include dramatic increases in heart rate, increases in breathing depth and frequency, and significant increases in muscle fatigue.

When it comes to helping clients achieve their fitness goals, training specificity is also an important consideration. For example, swimming will improve cardiovascular performance in swimming, but it will not increase running endurance. Similarly, prolonged cycling at 50 percent of HRmax will not improve cycling power. The principle of specificity also applies to resistance training, in that muscles should be stressed in a manner consistent with the way they are to perform. More specifically, if a client’s primary and long-term goal is to increase muscle endurance, then the client should perform high-repetition, low-intensity training as opposed to low-repetition, high-intensity training.

Energy Utilization During Different Forms of Exercise

Whether the information above was a review or completely new information, this knowledge is useful only if it can be applied to helping clients achieve their fitness goals. Therefore, in order to design the most efficient fitness programs, fitness professionals need to understand how the basic principles of energy metabolism apply to different forms of exercise training.

Power Lifting

Power lifting is characterized by maximal-effort, short-duration bouts, on the order of three to 10 seconds. The primary goal of this type of training is increased strength, and the predominant source of energy is the phosphagen system. If the phosphagen system’s stores of CP and ATP are depleted, the client should rest anywhere from two to five minutes between sets to completely replenish these energy stores.

Resistance Training

High-repetition resistance training (i.e., 8 to 20 repetitions per set) involves relatively less weight than power lifting and usually lasts longer than 10 seconds per set. For the greatest improvements in local muscular endurance, light loading (i.e., 12 to 20 repetitions at 70% of maximum) is recommended. During light loading, both the phosphagen and “anaerobic” glycolytic systems contribute submaximally. Therefore, complete exhaustion of the phosphagen system does not occur, nor is there significant lactate accumulation. As a result, short rest periods of 20 to 30 seconds allow for adequate recovery. For gains in muscle hypertrophy, moderate loading that causes significant muscle fatigue within 30 to 90 seconds (i.e., 8 to 12 repetitions at 70%-80% of maximum) is appropriate. Since this type of loading generally results in greater contributions from the phosphagen and “anaerobic” glycolytic systems than higher-repetition training using a lighter load, longer rest intervals of 30 to 120 seconds are required to sustain quality muscular activity.

Circuit Training

Circuit training is a hybrid between cardiovascular training and strength training in that circuit training can both improve muscle endurance and provide modest gains in aerobic capacity. A class format typically involves (1) high-repetition, low-resistance weight training (i.e., more than 20 repetitions at less than 70% of maximum), with the sets performed in quick succession or (2) high-repetition, low-resistance weight training interrupted by intervals of low-to-moderate-intensity aerobic exercise. In either case, the exercise is sustained and of a relatively low intensity, suggesting that the “aerobic” glycolytic and mitochondrial respiration energy systems are the major contributors to energy production.
As a result, short rest intervals of 15 seconds or less are usually adequate.

Traditional High-Low
& Step

Group exercise classes designed to promote cardiovascular fitness (e.g., traditional high-low or step) typically involve sustained exercise of low to moderate intensity lasting up to an hour or more. The aerobic breakdown of fats and carbohydrates via the mitochondrial respiration system provides the majority of the energy for this type of exercise. Relatively speaking, exercise at 25 percent of aerobic capacity is powered almost totally by fat combustion, whereas carbohydrate and fat contribute equally during moderate-intensity exercise (McArdle et al. 1999). During high-intensity cardiovascular aerobic exercise lasting one hour, glycogen stored in the liver is depleted by about 55 percent (McArdle et al. 1999). Only when exercise is performed at a relatively high intensity (i.e., approaching the lactate threshold) and extends beyond two hours does glycogen become significantly depleted and the contribution from fat increase substantially.

One common misconception is that because fat contributes a greater percentage of total calories burned during lower-intensity cardiovascular exercise than it does during higher-intensity cardiovascular exercise, the potential for weight loss is greater during lower-intensity exercise. In reality, while it is true that fat combustion contributes a greater percentage of the total energy during lower-intensity exercise, a larger total quantity of fat is used in higher-intensity aerobic exercise performed for an equivalent period of time. When determining the efficacy of any exercise program designed to promote weight loss, the total number of calories expended is more important than the percentage mixture of macronutrients used. It is also important to understand that recovery metabolism contributes minimally to total energy expenditure after low-to-moderate- intensity cardiovascular exercise. Recovery occurs relatively fast, and the body soon reverts back to its original metabolic state. However, after high-intensity training, metabolism is stimulated for several hours to days, and the potential for building muscle mass is much greater. And since lean tissue is metabolically more active than fat, when the percentage of lean tissue increases, so does resting metabolic rate.

Indoor Cycling

Although indoor cycling emphasizes cardiovascular endurance simply because of the class length, this form of exercise also offers an excellent opportunity to focus on improving both speed and power. According to Lucinda Christian, director of education for Mad Dogg Athletics Inc., the originators of Spinning®, the great thing about indoor cycling is that “participants can choose an intensity based on how they feel, whether that be a more intense or a more subdued workout.” Because the intensity can vary to such a large degree, indoor cycling provides an ideal opportunity for participants to improve their ability to transport and use oxygen and increase their lactate threshold. Indoor cycling classes incorporate both interval and sprint training to help clients achieve these goals. For an example of how the different energy systems are used during indoor cycling, see “General Energy Utilization During an Indoor Cycling Class” on page 50.

Interval Training

Interval training is one of the most
effective ways to burn a large number of
calories while conditioning both the cardiovascular and muscular systems. While essentially any mode of exercise (walking, jogging, running, hiking, swimming, etc.) can be used to perform intervals, indoor cycling provides an optimal setting to maximize the effectiveness of this type of training.

There are two main types of interval training: aerobic and anaerobic. As a general guideline, aerobic interval training involves exercise bouts lasting from two to 15 minutes, followed by rest intervals of the same duration, at intensities between 60 and 85 percent of maximum capacity. Participants can determine the intensity of their workout by using their age-determined HRmax (i.e., 220 beats per minute minus their age) as an estimate of functional capacity, or they can use ratings of perceived exertion. Participants should perform aerobic interval training at an intensity just below their lactate threshold by watching for signs of increased heart rate, breathing depth and frequency, and muscle fatigue. The primary goal of aerobic interval training is to improve the ability to consume and use oxygen, although a secondary goal may be to help elevate the lactate threshold.

During anaerobic interval training, the training stimulus is much shorter than it is during aerobic interval training—usually 30 seconds to three minutes, with recovery periods lasting approximately two to three times as long, at an intensity of 85 percent or more of HRmax or maximum capacity. In addition to improving speed, anaerobic interval training will improve lactate threshold and overall aerobic power. This form of training usually results in greater lactate accumulation than aerobic interval training, owing to the greater reliance on the “anaerobic” glycolysis energy system. During these high-intensity intervals, as much as 85 percent of the total energy requirement is derived from carbohydrate metabolism (Brooks et al. 1995; McArdle et al. 1999).

Sprint Training

True sprint training involves bouts that last from a few seconds to 30 seconds at an intensity at or near HRmax or maximum capacity. The energy for sprint training comes from the phosphagen and “anaerobic” glycolytic energy systems. Consequently, fats contribute little if any of the total energy requirement. Generally speaking, rest intervals should be three to six times as long as the sprint itself. However, to focus specifically on the phosphagen system, rest periods of two to five minutes are recommended. If performed correctly, sprint training will both enhance the ATP and CP energy transfer pathways and also improve lactate tolerance and clearance. When sprint training is used in indoor cycling, Christian stresses, it is important to focus on intensity rather than all-out speed. To achieve the desired intensity, participants should increase resistance rather than pedaling as fast as they can.

Energizing Your
Clients’ Workouts

Because no one energy system operates in a vacuum, fitness professionals need to think of energy transfer during exercise as a continuum. During any workout that fluctuates in intensity, frequent shifts will occur among the various energy systems to accommodate the body’s needs. However, when designing exercise programs, fitness professionals need to understand which systems are the primary contributors during different types of exercise and why. Only through proper program design will the exercises performed develop the targeted energy systems and muscle groups—and only then will clients achieve their goals.


Brooks, G.A., et al. 1995. Exercise Physiology: Human Bioenergetics and Its Applications (2nd ed.). California: Mayfield Publishing Company.
Cotton, R. 1997. Personal Trainer Manual: The Resource for Fitness Professionals. San Diego: American Council on Exercise.
McArdle, W.D., et al. 1996. Exercise Physiology, Energy, Nutrition and Human Performance (4th ed.). Baltimore: Williams & Wilkins.
McArdle, W.D., et al. 1999. Sports & Exercise Nutrition. New York: Lippincott Williams & Wilkins.


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