Since a specific amount of work requires a given amount of energy, the body must always have an appropriate level of energy available to meet the demands placed on it. The following descriptive timetable illustrates how energy is produced during the initial stages of exercise and during moderate intensity exercise:

* When exercise first starts, only a limited supply of energy is present in the muscles for immediate use. During this phase, oxygen is not required.

* Either glycogen stored in the muscle or glucose transported by the blood from the liver can be used without oxygen to provide a limited supply of energy. Lactic acid is the by-product of this anaerobic reaction.

* Most of the lactic acid formed during an anaerobic reaction is released into the blood and transported to the liver, where it is converted back to glycogen and stored.

* As additional oxygen becomes available, the aerobic system is used more and more. After a few minutes, the aerobic system is able to supply all the energy needed for relatively mild exercise.

* At this time, liver glycogen is converted to glucose and released into the blood to provide fuel for both systems (aerobic and anaerobic).

* Finally, adipocytes (fat cells) release more and more fat, the preferred fuel for the aerobic system.

If the exercise bout is relatively intense, other events take place to ensure that adequate amounts of energy are provided for the working skeletal muscles. The production of energy during exercise at relatively high levels of intensity occurs as follows:

* The speed of the aerobic reactions increases to provide more energy. More carbon dioxide is also produced.

* The faster anaerobic system supplies increasing amounts of energy as the exercise becomes more intense. The intensity of the muscle contractions causes a compression of the small arteries and, in effect, prevents oxygen, glucose or fat from entering the muscle cell. Thus, the majority of the carbohydrate needed comes from that which is already stored within the muscle itself.

* Eventually, more lactic acid is formed, and increased amounts of lactic acid are released into the bloodstream. As lactic acid levels within the muscles increase, the efficiency of the aerobic chemical reactions are inhibited. When this occurs, inadequate amounts of energy can be produced aerobically. Accordingly, an individual has to either decrease the intensity level of the exercise bout (thereby reducing the amount of energy needed) or rely more heavily on the anaerobic system.

* Only a small percentage of lactic acid is transformed back into glycogen in the liver; the majority remains in the blood and the muscles. An individual's body can accumulate and tolerate a limited amount of lactic acid. In all likelihood, it is the presence of lactic acid that causes excessive breathing while exercising, and causes feelings of fatigue and heaviness in the muscles, eventually forcing an individual to stop exercising.

To better comprehend how the body works during different types of exercising, an individual should understand the relative importance of the anaerobic and aerobic systems for energy production. Figure 1 provides an approximate idea of the maximal amount of energy a well-trained individual can produce over time, and how that energy is produced. For comparison purposes, the energy required at rest is given a value of one.

Although the stored energy can be used to perform a lot of work very rapidly, these stockpiles are essentially exhausted after 10 to 20 seconds. This factor partially explains why individuals cannot run 400 meters as fast as they can run 100 meters, or why weightlifters can lift more in one lift than they can in three lifts without a pause.

The production of energy anaerobically is relatively high (peaking in approximately 40 to 50 seconds), but it doesn't last long because individuals are limited by the body's relative intolerance of lactic acid. After 10 minutes, the amount of energy produced is very small.

After five to six minutes of continuous exercise, the majority of energy the body requires has to be produced aerobically. The longer the duration of exercise, the greater the importance of the aerobic system. Anything over 10 minutes has to be performed aerobically, except for the occasional and brief increases in work output.

Maximal oxygen uptake. If an individual increases the intensity level of exercise, a number of things will happen. Increases occur in heart rate, respiration and oxygen intake, as well as in the activity levels of other parts of the aerobic systems. A point occurs, however, beyond which oxygen intake cannot increase even though more work is being performed. At this point, the individual has reached a level that is commonly referred to as maximal oxygen uptake (VO2 max). This measure is considered to be the best single indicator of aerobic fitness, since it involves the optimal ability of three major systems of the body (pulmonary, cardiovascular and muscular) to take in, transport and utilize oxygen. Thus, the higher an individual's level of maximal oxygen uptake, the greater the level of physical work that can be performed.

Energy production and exercise intensity. If the amount of work being performed is progressively increased along the continuum to levels of maximum capacity, the ability to produce energy aerobically will not be able to completely match the energy demands. For most sedentary individuals, this point occurs at a work output requiring approximately half of their VO2 max. In other words, below 50 percent of VO2 max, the "slower" aerobic system can provide all the energy a person needs. Of course, the human body does not switch over to the anaerobic system all at once, but gradually shifts gears to produce energy at a faster rate. A level between 50 and 70 percent of VO2 max represents a transition phase for most people. Above 70 percent of VO2 max, the aerobic system does not produce energy fast enough, thereby causing individuals to rely more and more on their anaerobic systems.

Another important factor that must be considered when examining the relationship between the production of energy and exercise intensity is lactic acid. Figure 2 presents a schematic diagram of the level of lactic acid in the blood relative to the intensity of exercise. The level of lactic acid is a rough indictor of the degree to which the anaerobic mechanism is being used. As Figure 2 illustrates, lactic acid will begin to rise slowly around 50 percent of VO2 max. Because the increase is not too great, an individual's body can compensate up to 70 percent of maximum with little trouble. Beyond 70 percent of VO2 max, however, as the buildup of lactic acid becomes progressively more dramatic, an individual will start to get "winded." This explains why a person can run at a certain pace (50 to 60 percent of VO2 max) with no problem, but will become exhausted relatively quickly after trying to run faster (80 to 90 percent of VO2 max).

Depending on the intensity and duration levels of the activity, it is important to note that many physical activities require both aerobic and anaerobic production of energy. For example, soccer players who often are required to run for extended periods (i.e., 20 to 30 minutes nonstop) perform aerobic exercise. Obviously, if the activity did not depend on the aerobic system for energy, they would not be able to run for nearly as long. However, when a game situation requires players to sprint after the ball (high-intensity intervals which exceed 70 percent of VO2 max), these athletes are forced to draw upon their emergency (anaerobic) sources. In other words, whether an activity involves the body's aerobic system or its anaerobic system depends on the demands placed on it by the activity at a particular point in time. Anaerobic chemical reactions are primarily used in high-intensity exercise of relatively brief duration (e.g., sprinting short distances or heavy weight lifting), while aerobic chemical reactions are primarily involved in low-intensity, long-duration exercise activities such as walking, cycling, stair climbing, etc.

Physiological adjustments to exercise

The aerobic metabolism of fats and carbohydrates is the preferred and more efficient mode of energy production. This method, however, is limited by the body's ability to transport and deliver oxygen to, and the utilization of oxygen by, the working muscles. Several physiological adjustments are made during exercise. The primary objective of these adjustments is to provide an exercising muscle with oxygenated blood that can be used for the production of energy. An individual's endurance capabilities will be greatly influenced by the magnitude and direction of these changes.

Cardiac output. The amount of blood pumped per minute by the heart is explained by the term cardiac output (Q). This measure is indicative of the rate of oxygen delivery to the peripheral tissues (e.g., exercising skeletal muscles). Cardiac output, which is the product of heart rate (HR) and stroke volume (SV), increases linearly as a function of work rate. At rest, Q is roughly four to five liters per minute, but can rise to 20 to 25 liters per minute during exercise in young, healthy adults. This exercise-induced increase of Q is due to alterations in both HR and SV.

Heart rate. Heart rate, one of the two primary determinants of Q, also rises linearly with work rate. The gradual withdrawal of vagal (parasympathetic nervous system) influences and the progressive increases in sympathetic nerve activity which occur during exercise, are largely responsible for the observed increases in HR. At or near VO2 max, HR begins to level off and is referred to as maximal heart rate. The equation "220 minus your age" (expressed in whole years) provides a rough estimate of an individual's maximal heart rate (with a standard deviation of 10 to 12 beats per minute). As the equation implies, an individual's maximal heart rate declines with age.

Stroke volume. Stroke volume (SV) is the other primary determinant of Q, and represents the amount of blood ejected from the heart during each beat. Unlike HR, SV does not increase linearly with work rate. SV increases progressively until a work rate equivalent to approximately 50 to 75 percent VO2 max is reached. Thereafter, continued increases in work rate cause little or no increase in SV. Exercise-induced increases in SV are believed to be the result of factors that are both intrinsic and extrinsic to the heart. According to the Frank-Starling law, a greater stretch is placed on the muscle fibers of the heart (due to a greater venous return of blood to the heart during physical activity or exercise), resulting in a more forceful contraction of those fibers and, consequently, a greater SV. Extrinsic factors such as increased nervous (sympathetic) or endocrine (release of adrenal hormones epinephrine and norepinephrine) stimulation to the myocardium can also contribute to the increased SV that occurs during exercise.

Blood pressure. Systolic blood pressure (SBP) represents the force developed by the heart during ventricular contraction. SBP increases linearly with work rate. In healthy adults, SBP tends not to exceed 220 mm Hg at maximal exercise levels. Diastolic blood pressure (DBP), on the other hand, is indicative of the pressure in the arterial system during ventricular relaxation and reflects peripheral resistance to blood flow. DBP changes little from rest to maximal levels of exercise. Therefore, an individual's pulse pressure (the algebraic difference between SBP and DBP) increases in direct proportion to the intensity of exercise. Pulse pressure is important because it reflects the driving force for blood flow in the arteries.

Total peripheral resistance. The sum of all the forces that oppose blood flow in the systemic circulation is expressed by the term total peripheral resistance (TPR). Numerous factors can affect TPR, including blood viscosity, vessel length, hydrostatic pressure and vessel diameter. Vessel diameter is by far the most important of these factors, since TPR is inversely proportional to the fourth power of the radius of the vessel. If one vessel has half the radius of another, and if all other factors are equal, the larger vessel would have 16 times (24) less resistance than the smaller vessel. As a result, 16 times more blood would flow through the larger vessel at the same pressure. This factor has important implications for exercise, since certain organs require more blood flow than others during physical activity.

During exercise, resistance in the vessels supplying the muscles and skin is decreased. As a result, blood flow to these parts of the body is enhanced. On the other hand, resistance in those vessels that supply the visceral organs of the body (e.g., the liver, gastrointestinal tract, kidneys) is increased, thereby reducing the level of blood flow to those areas of the body. These changes are almost entirely due to intrinsic factors (i.e., the increased metabolic demands of the muscles and the requirement of blood flow to the skin to facilitate heat dissipation). The TPR tends to decrease during progressive dynamic exercise because vasodilation occurring in the muscles and skin seem to override the vasoconstriction that is occurring in the visceral organs.

Arteriovenous oxygen difference. The arteriovenous oxygen difference is the difference between the oxygen contents of the arterial blood and mixed venous blood. It is a reflection of the amount of oxygen extracted from the blood by the muscles. Obviously, the more oxygen that is actually extracted from the blood, the more oxygen there is for aerobic energy production. The oxygen content of venous blood can be reduced to one-half to one-third of an individual's resting levels by the exercising muscles. Such a reduction indicates that the muscles are extracting a much higher proportion of the oxygen delivered to them in the arterial blood. (It should be noted that approximately 85 percent of the oxygen in arterial blood can be extracted during maximal exercise.) Aerobic conditioning results in alterations (e.g., increased capillary density, increased mitochodrial size and density, increased myoglobin content and enhanced activity of oxidative enzymes) that can enhance the ability of skeletal muscle to extract oxygen.

A matter of chemistry

By definition, all physical activity involves movement. In turn, sound exercise is often defined as purposeful movement. Because exercise involves the body's muscular system, energy is required. The source of this energy for muscular actions is a series of chemical reactions which involve varying amounts of oxygen and foodstuffs.

Knowing how the body responds to the energy demands attendant to exercise -- particularly how the cardiovascular and muscular systems adjust to supply oxygen to exercising muscles -- can affect exercisers in a variety of positive ways. For example, such an understanding can enable an individual to be better prepared to select an appropriate exercise mode or to manipulate a particular exercise prescription to achieve a specific personal goal. At the least, such knowledge will provide a clearer insight into why a given exercise bout may vary (i.e., more challenging, less challenging, etc.) from another. Ultimately, a potential byproduct of such insight will be to provide a better exercise environment for individuals.

REFERENCES

Astrand, P.O., & K. Rodahl. Textbook of Work Physiology, 3rd ed. New York, NY: McGraw-Hill, 1986.
Brooks, G.A., & T.D. Fahey. Exercise Physiology: Human Bioenergetics and Its Applications. New York, NY: John Wiley & Sons, 1984.
Fox, E.L., R.D. Bowers & M.L. Foss. The Physiological Basis of Physical Education and Athletics, 4th ed. Philadelphia, Penn.: W. B. Saunders Company, 1988.
Kenneth W. L., R.H. Humphrey & C.X. Bryant. ACSM's Guidelines for Exercise Testing and Prescription, 5th ed. Philadelphia, Penn.: Williams & Wilkins, 1995.
Lamb, D.R. Physiology of Exercise: Responses & Adaptations, 2nd ed. New York, NY: Macmillan, 1984.
Peterson, J.A., & C.X. Bryant. The Fitness Handbook, 2nd ed. Champaign, Ill.: Sagamore Publishing Co. Inc., 1995.
Skinner, J.S. Functional effects of physical activity. In Zeigler, E.F. (Ed.), Physical Education and Sport: An Introduction. Philadelphia, Penn.: Lea & Febiger, 1982.
Skinner, J.S., S.P. Noeldner & J.S. O'Connor. The development and maintenance of physical fitness. In Ryan, A.J., & F.D. Allman (Eds.), Sports Medicine, 2nd ed. New York, NY: Academic Press 1989.
Wilmore, J.H., & D.L. Costill. Training for Sport and Activity, 3rd ed. Dubuque, IA: Wm. C. Brown Publishers, 1988.

Figure 1. Relative importance of various energy-producing systems over time.
Used with permission from Body Energy by James S. Skinner, 1981 (Anderson World Inc., Mountain View, Calif.).

Figure 2. Blood lactic acid related to exercise intensity.
Used with permission from Body Energy by James S. Skinner,
1981 (Anderson World Inc., Mountain View, Calif.).

James Peterson, Ph.D., FACSM, is a sports medicine consultant, fellow of the American College of Sports Medicine, a former faculty member at the United States Military Academy and a former director of sports medicine for StairMaster Sports/Medical Products Inc.