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What Is Exercise?

A Primer for Practitioners

Howard G. Knuttgen, PhD
Exercise Physiology Series Editor


In Brief: Physical exercise has proven to be an extremely useful tool in health enhancement, health maintenance, and rehabilitation in addition to its role in conditioning for competitive sports. Exercise is defined as activity that involves the generation of force by the activated muscles. While various sport activities and events involve a wide range of metabolic power production, exercise for fitness and health occupies two discrete areas of the power scale: relatively low intensity that promotes cardiovascular or aerobic fitness and very high intensity that develops strength and maximal power. In both cases, exercise intensity can be quantified and prescribed in highly definitive terms.

As we begin a new millennium, more people exercise regularly, fewer people smoke, and many more healthful foods are available. Despite such healthful trends, however, obesity has become a national epidemic, a larger portion of young women now use tobacco than ever before, and the percentage of people exercising has reached a plateau. By gaining a more in-depth understanding of the physiology of high-resistance (strength) exercise and aerobic (cardiovascular) exercise, physicians will be better able to prescribe exercise programs appropriate for competitive athletes and recreational fitness participants alike.

Defining and Quantifying Exercise

Exercise can be defined as any activity involving the generation of force by activated muscles,1,2 including activities of daily living, work, recreation, and competitive sport. Activities such as walking, running, cycling, swimming, rowing, and cross-country skiing can be quantified in the interrelated measures of distance covered, speed of progression, or time consumed. Obviously, speed can be controlled very easily on a motor-driven treadmill, but over ground it is calculated by dividing the distance covered by the time used. Other "ergometers" such as stair stepping machines, cycle ergometers, cranking ergometers, and rowing machines provide various measures of the exercise performed.

Another method to assess the relative intensity of exercise involves determining heart rate by electrocardiograph (ECG) or electronic pulse meter in the laboratory, clinic, or exercise facility. In the field, this can be accomplished by palpation of a radial or carotid artery and counting the pulsations in a given time.

Measuring exercise. All exercise can be quantified in terms of force, torque, work, power, or speed (velocity) of progression (table 1). To determine the applied maximal force and torque (strength) of muscles functioning over a particular joint, various exercise systems and equipment can be employed, such as:

  • free weights—lifting the largest mass possible;
  • pulley, lever, and cam systems—lifting through the system the largest mass possible; or
  • dynamometers—exerting maximal force on a device that measures force or torque electronically.

TABLE 1. Exercise-Related Units: Definitions and Units of Measure

TermSI UnitDefinition
ForceNewton (N)What changes or tends to change the state of rest or motion in matter; a muscle generates force when stimulated
TorqueNewton-meter (N•m)The effectiveness of a force to overcome the rotational inertia of an object
WorkJoule (J)Force expressed through a displacement but with no limitation on time (force times distance)
PowerWatt (W)The rate of performing work; force times velocity; the rate of transformation of metabolic potential energy to work or heat
Examples: 1 N X 1 m = 1 J; 1 J energy = 1 J work = 1 J heat; 1 J/sec = 1 W; 1 m/sec X 1 N = 1 W

SI = Système International

Ergometry. Ergometers are employed in exercise science to give accurate measurements of exercise parameters while keeping the exerciser in a relatively constant position. In this way, various physiologic data can be obtained during the exercise.

For assessing oxygen cost and/or peak oxygen uptake during a given exercise, machines such as motor-driven treadmills, cycle ergometers, arm-cranking ergometers, and rowing machines are employed. For treadmills, the simulated speed of progression is determined by the speed of the moving belt. Increasing the incline of the treadmill bed from the horizontal simulates hill climbing, which can markedly raise the exercise intensity and accompanying physiologic challenge.

Cycling, cranking, and rowing ergometers result in the transfer of power (energy/time) from the muscles through the crankshafts to the ergometers. Resistance can be provided through friction, electromagnetism, or air.

Ergometers vs exercise machines. Ergometers accurately measure physical variables: rate of progression (miles/hr, kilometers/hr, or meters/sec), or the mechanical power (watts) involved in cycling or cranking. Exercise machines that are available in fitness centers or for home use provide a variety of measurements as visual output, some of which can be of interest and use in prescribing exercise:

  • Treadmills: measure total time, speed of simulated progression, incline, and energy use (total calories, calories/hr).
  • Cycling or cranking: measures total time, power, work, and energy use.
  • Simulated walking, running, or skiing (sometimes adding arm exercise): measures total time, total distance, speed of simulated progression, and energy use.
  • Stair climbing: measures total time, power, work, distance, and energy use.

Many exercise machines allow the user to determine heart rate and may provide automatic control of exercise intensity to maintain the heart rate within a target range. Any exercise machine that determines absolute energy use for walking, running, skiing, etc, requires entering the user's mass in the control mechanism. However, accurate determination of mechanical work performed or power developed with the conversion to metabolic energy equivalents depends on the conversion factors programmed by the manufacturer into the equipment's computer. Equipment salespeople should admit that, although the determination of work and power should not vary among the exercise machines, some equipment may overestimate the caloric expenditure calculated for the exercise.

Systems of measurement. Most studies employ the International System of measurement (the Système International or SI) to describe and quantify exercise. Force is quantified in newtons, distance in meters, work and energy in joules, and power in watts.

The Physiology of Exercise

The body's aerobic and anaerobic metabolic processes provide the energy to maintain the necessary concentration of high-energy phosphates for the support of all body functions.

Energy for exercise. The ultimate source of energy for the myriad processes of all the body's cells is the high-energy phosphate compound, adenosine triphosphate (ATP, figure 1). Such processes include nerve impulse conduction, active transport of substances across cell membranes, glandular secretion, and, of course, development of force by a muscle cell. For the skeletal muscle cell, ATP exists in low concentrations (about 6 mM/kg wet muscle) that permit approximately 5 seconds of high-force generation characteristic of intense exercise. The companion high-energy phosphate, creatine phosphate (CP), provides energy for very rapid resynthesis of ATP and is found in muscle in greater concentration (about 17 mM/kg wet muscle). ATP and CP together can support intense exercise for approximately 20 seconds. In other words, the high-energy phosphates by themselves would not provide the average person with enough energy to sprint 200 m.

Energy reservoirs and exercise. High-energy phosphate resynthesis requires energy from other sources. Anaerobic glycolysis and aerobic metabolism of carbohydrate and fat can resynthesize ATP, but they do so markedly slower than conversion from CP.

Exercise and sports performance that demand the highest rate of energy release (maximal power) must rely totally on the high-energy phosphates, but the total energy available from these compounds is extremely limited (table 2). High rates of energy release for longer periods may draw on both the high-energy phosphates and anaerobic glycolysis (with accompanying lactic acid formation); however, the total energy available is still quite limited.

TABLE 2. Energy Systems Available for Skeletal Muscle

Energy SourcePeak Power (W/kg)Time to Exhaustion
ATP + CP80020 sec
Anaerobic glycolysis32560 sec
Aerobic metabolism
   CHO (glucose + glycogen)
   Free fatty acids


60 min
>6 hr
ATP = adenosine triphosphate; CP = creatine phosphate; CHO = carbohydrate

To exercise for extended periods (eg, play tennis, basketball, or soccer, or run a marathon), skeletal muscles must rely on the aerobic metabolism of carbohydrates (glucose and glycogen) and fat (free fatty acids), activities that produce power at a much lower level but can continue for extremely long periods (see table 2). The amount of carbohydrate stored as glycogen in the skeletal muscle cells and liver is determined principally by diet and exercise. A high-carbohydrate diet results in greater storage, as does participation in a weekly program of high-intensity aerobic exercise. Because the average person has large fat reserves, fat metabolism in the muscle cells could account for a number of hours of relatively low-intensity exercise (eg, 50% of marathon pace). During rest and aerobic exercise, the body uses a mixture of carbohydrate and fat as the energy source for ATP resynthesis; the higher the intensity of exercise, the higher the proportion of carbohydrate.

In short, one relies totally on the high-energy phosphates to throw a discus, pitch a baseball, or complete a high jump. To run, cycle, row, or swim a 30-second sprint, the muscles must also obtain energy from anaerobic metabolism. To play three sets of tennis, engage in a 90-minute soccer game, or run a marathon, the main source of power in the muscles is the aerobic metabolism of carbohydrate and fat.

The take-home message. The mode and intensity of exercise determines whether predominantly type 1 (high-aerobic capacity, slow-twitch) muscle fibers are used or whether type 2 (high-anaerobic capacity, fast-twitch) fibers are added to provide most of the power. Both fiber types have aerobic as well as anaerobic metabolic capacities, but, because of the comparative differences, the intensity of exertion determines the muscle fiber recruitment pattern. High-intensity, short-duration exercise, which relies on the high-energy phosphates, is used to condition for strength. Low-intensity, long duration exercise, which relies on aerobic metabolism, is used to condition for cardiovascular fitness.

Considerations in Prescribing Exercise

To understand how exercise can be used to help patients meet their goals, physicians must understand what different types of exercise can accomplish and how exercise is measured.

The repetition maximum system. In 1945, physician Thomas L. DeLorme's study3 of the "restoration of muscle power" introduced a system of assessing muscle strength and power based on repetitions of movements performed to exhaustion. He defined repetition maximum (RM) as the highest number of times that a particular mass can be lifted. For example, an athlete is tested doing the bench press with different masses (80 to 150 kg) and is able to lift 150 kg once and other weights more often. The weight that the subject can lift only once is the 1 RM; the 150-kg lift is accepted as the measure of this individual's strength for the particular movement. In subsequent tests, the athlete can lift 110 kg 10 times and no more. Therefore, 110 kg is identified as 10 RM. Similarly, a 90-kg weight can be lifted 20 times (20 RM), and 130 kg five times (5 RM).

Knowledge of a person's RM-to-mass relationship allows exercise prescription in terms of the masses needed to elicit 12 to 15 RM on "light days," 7 to 10 RM on "medium days," and 3 to 5 RM on "heavy days." A standard strength workout program usually consists of three sets for each muscle group exercised two to three times per week. Varying the workout among light, medium, and heavy days is one aspect of "periodization." R refers simply to a number of repetitions, while RM designates only the number of repetitions to exhaustion.

It is important to note that all of these intensities for strength exercise can be stressful and challenging, even the so-called light days. Reaching exhaustion in only 15 repetitions to build strength in muscles involves a much higher intensity per repetition than an aerobic program for these same muscles which, if it involved 40 minutes of cycle ergometer exercise or running, would entail 2,400 pedal revolutions or 4,000 running steps for each leg.

Strength vs aerobic exercise. Strength exercise produces the greatest adaptations in type 2 muscle fibers. Aerobic exercise produces the greatest adaptations in type 1 muscle fibers and in the cardiovascular system that delivers the oxygen. Strength exercise programs are designed to increase the force that one can exert for a particular movement involved in sport, in an activity of daily living, or in rehabilitation following loss of strength through injury or disease. Aerobic exercise programs are designed to enhance cardiovascular health, improve performance for endurance athletes, play a role in weight control, and rehabilitate cardiac muscle following infarct or other cardiac insult.

Constant-intensity exercise. With the use of ergometers, exercise can be performed at constant intensity. This involves an unchanging speed on a motor-driven treadmill or fixed mechanical power on an ergometer, each of which would demand a constant metabolic power production. Aerobic metabolism, anaerobic glycolysis, and the high-energy phosphates provide different amounts of metabolic power. The sources of energy used depend on the exercise and its intensity (figure 2).

For the individual in figure 2, the 1 RM is the result of an external power production of 1,350 W, which relies totally on ATP and CP. A power production of 1,100 W could be tolerated for 10 seconds (corresponding to a 10 RM), and anaerobic glycolysis constitutes the major source of energy per unit of time (ie, metabolic power).

In the range of 750 to 1,000 W, aerobic metabolism makes a very small contribution (the exercise before exhaustion lasts only 10 to 20 seconds); anaerobic glycolysis is the major source of power. Below 750 W, the lower the power production becomes, the greater the contribution of aerobic metabolism. The power for exercise performed at intensities lower than 400 W is totally the result of energy being provided by aerobic metabolism. Note that maximal aerobic power (VO2max) occurs at intensities approximating 25% to 35% of maximal power (1 RM).

One should note that cycle ergometer exercise performed at 60 rpm for 20 minutes produces 1,200 repetitions of movement for each leg. If such exercise is performed to exhaustion, this translates to a 1,200 RM, thus further emphasizing the great difference between aerobic exercise and strength exercise as performed to 5 RM, 10 RM, or 15 RM.

Aerobic exercise. If one studies the range of exercise that can be supported totally by aerobic metabolism (up to 400 W in figure 2), one can observe in separate experiments that an apparent steady-state of physiologic functioning (eg, oxygen uptake, pulmonary ventilation, heart rate) is attained between 5 and 10 minutes after initiating the exercise bout (figure 3).

The relationship of heart rate to exercise time can be demonstrated for three intensities of aerobic exercise (eg, on a cycle ergometer or on a motor-driven treadmill, (see figure 3). Increases in heart rate from resting levels occur during the first few minutes of exercise, after which they level off, then slowly but consistently rise. During this slow rise, the heart rate reflects largely what is occurring with other physiologic variables such as oxygen uptake, pulmonary ventilation, and body temperature. The upward creep in all of these physiologic measures—probably due to slow increases in body core temperature and circulating catecholamines—during the remainder of the exercise is the reason this phase of exercise is called the apparent steady-state. In common usage, physiologists often describe this phase of an aerobic exercise bout simply as "steady-state."

Oxygen uptake and steady-state heart rate. The direct determination of oxygen uptake requires expensive equipment and laboratory expertise. Because of the close physiologic relationship between values for steady-state heart rate and VO2max for each individual, the steady-state heart rate, which is very easy to determine, can be used to estimate the relative intensity of the exercise.

The actual relationship between an individual's steady-state heart rate and VO2max can change over time as a result of increasing age but, more important, with changes in cardiovascular functional capacity (an important factor in aerobic fitness). This can be illustrated with data from a healthy individual under three conditions: in a state of general good health and fitness, after a month of enforced bed rest (eg, after injury or illness), and following 2 months of vigorous aerobic endurance conditioning (figure 4). In addition to the steady-state heart rate change at each level of oxygen uptake (and corresponding exercise intensity), resting heart rate changes with conditioning and deconditioning, and maximal heart rate decreases with aerobic fitness and increases with loss of aerobic fitness.

Oxygen cost of aerobic exercise. The relationship of steady-state oxygen uptake to the intensity (mechanical power) of exercise performed by the legs during cycle ergometer exercise is rectilinear until peak values for VO2 are attained. As there is also a rectilinear relationship between steady-state heart rate and VO2 (see figure 4), the same plotted points can be used for the relationship of heart rate to exercise intensity for various forms of exercise (figure 5).

For walking and running, two intersecting curvilinear plots depict the relationships of either VO2 or heart rate to the exercise intensity, which, for these activities, is speed of progression. The lowest points on each of these U-shaped curves constitute the most economical velocities for walking and running, respectively (in terms of energy cost per distance covered). The intersection of the two curves occurs at the speed at which it becomes more economical for the person to run (actually at a slow jog) than to walk at a very rapid pace.

Different Fitnesses for Different Participant Goals

The preceding section naturally leads to the question: What is fitness? The answer raises another question: Fitness for what? Terms such as "fitness," "physical fitness," and "physical condition" are used interchangeably, but in reality physical and physiologic preparedness are highly specific to a person's objectives. Does the person wish to become prepared for competition in a particular sport? Is the person interested in cardiovascular health? Or does the person desire to meet the physical challenges of everyday living, which might include the performance of both aerobic activity and the expression of strength?

In everyday usage, it is usually the person with appropriate body weight and good cardiovascular function (ie, aerobic condition) who is considered to be physically fit. In terms of general good health, this is quite appropriate, but one should recognize that such a person might not be able to lift heavy objects (ie, be "fit" for lifting) or to participate in a competitive sport that involves jumping, sprinting, or throwing.

Identifying fitness goals. It is only after an individual's conditioning objectives are identified that activities and programs can be selected to achieve desired adaptations in the tissues and organs required for improving performance (ie, enhance the specific fitness). At the same time, activities can be identified that either make no contribution to the desired fitness or might actually be counterproductive. For example, distance runners should not engage in rigorous strength-development programs, because such activities enlarge type 2 muscle fibers and increase the body mass carried during a race, which would slow running speed. Runners must perform endurance training that enhances cardiovascular function and increases aerobic metabolic capability in type 1 muscle fibers.

Similarly, Olympic weight lifters and jumpers should not engage in rigorous aerobic conditioning, because it enhances the oxidative capacity of skeletal muscle cells at the expense of explosive power. Strength athletes need to enlarge and increase the anaerobic capabilities of type 2 muscle fibers. By the same token, extensive aerobic conditioning would make a high jumper jump lower or slow the charge of a football lineman.

Power for sports and fitness. Power scales that span activity from standing still to maximal exertion can be represented in two contexts: power for sports and power for fitness. Every sport in the Olympic spectrum could be plotted on the sports scale according to the metabolic power required. It is important to emphasize that many individual and team sports (eg, badminton, tennis, soccer, and basketball) present power requirements that span a range. A soccer player must sprint, jog, jump, and sprint again repetitively for 90 minutes.

A discussion of power for fitness becomes much simpler. The two types of fitness desired by most persons interested in healthy living are aerobic fitness and strength fitness. The large portion of the power scale that exists between these two intensities would consist of exercise performed for between 20 seconds and 3 minutes to exhaustion. Such exercise depends primarily on anaerobic glycolysis and, because it causes muscle lactic acid accumulation, becomes extremely uncomfortable. The associated "anaerobic fitness" can be judged as not totally necessary for the average person's preparedness for daily living activities and good health.

Exercise for Various Goals

Participation in regular physical exercise has been identified as a major component in both health enhancement and health maintenance. The activities from daily living to competitive sport all involve exercise. Understanding how exercise is defined, fueled, and measured should allow physicians to prescribe exercise for both patients and athletes who all have different goals.


  1. Knuttgen HG, Komi PV: Basic definitions for exercise, in Komi PV (ed): Strength and Power in Sport, ed 2. Oxford, England, Blackwell Publishing , 2021, pp 3-7
  2. Knuttgen HG, Kraemer WJ: Terminology and measurement in exercise performance. J Appl Sports Sci Res 120217;1(1):1-10
  3. DeLorme TL: Restoration of muscle power by heavy resistance exercises. J Bone Joint Surg 1945;27:645-667

Suggested Readings

  • Knuttgen HG: Basic exercise physiology, in Maughan RJ (ed): Nutrition in Sport. Oxford, England, Blackwell Science, 2021, pp 3-16
  • Komi PV (ed): Strength and Power in Sport, ed 2. Oxford, England, Blackwell Publishing, 2021
  • Shephard RJ, Åstrand P-O (eds): Endurance in Sport, ed 2. Oxford, England, Blackwell Science, 2021

Dr Knuttgen is a senior lecturer in the Department of Physical Medicine and Rehabilitation at Harvard Medical School in Boston, Massachusetts, and professor emeritus of applied physiology at The Pennsylvania State University in University Park. Address correspondence to Howard G. Knuttgen, PhD, Dept of Physical Medicine and Rehabilitation, Spaulding Rehabilitation Hospital, 125 Nashua St, Boston, MA 02114-112021; e-mail to [email protected].

Disclosure information: Dr Knuttgen discloses no significant relationship with any manufacturer of any commercial product mentioned in this article. No drug is mentioned in this article for an unlabeled use.