exercisable, adj.
/ek"seuhr suyz'/, n., v., exercised, exercising.
1. bodily or mental exertion, esp. for the sake of training or improvement of health: Walking is good exercise.
2. something done or performed as a means of practice or training: exercises for the piano.
3. a putting into action, use, operation, or effect: the exercise of caution.
4. a written composition, musical piece, or artistic work executed for practice or to illustrate a particular aspect of technique.
5. Often, exercises. a traditional ceremony: graduation exercises.
6. a religious observance or service.
7. to put through exercises, or forms of practice or exertion, designed to train, develop, condition, or the like: to exercise a horse.
8. to put (faculties, rights, etc.) into action, practice, or use: to exercise freedom of speech.
9. to use or display in one's action or procedure: to exercise judgment.
10. to make use of (one's privileges, powers, etc.): to exercise one's constitutional rights.
11. to discharge (a function); perform: to exercise the duties of one's office.
12. to have as an effect: to exercise an influence on someone.
13. to worry; make uneasy; annoy: to be much exercised about one's health.
14. to go through exercises; take bodily exercise.
[1300-50; ME (n.) < MF exercice < L exercitium, equiv. to exercit(us) ptp. of exercere to train (ex- EX-1 + -ercit-, s. of comb. form of arcere to restrain) + -ium n. suffix]
Syn. 1. activity; calisthenics, gymnastics. 2. EXERCISE, DRILL, PRACTICE refer to activities undertaken for training in some skill. EXERCISE is the most general term and may be either physical or mental: an exercise in arithmetic. DRILL is disciplined repetition of set exercises, often performed in a group, directed by a leader: military drill. PRACTICE is repeated or methodical exercise: Even great musicians require constant practice. 3. employment, application, practice, performance. 6. ritual. 7. discipline, drill, school. 9. employ, apply, exert, practice. 13. try, trouble.
Ant. 1. inaction.

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Training of the body to improve health and fitness.

Different types have different purposes, including aerobics for heart and respiratory function and weight loss, weight-bearing exercise for bone strength, weight training for muscle strength, and stretching for flexibility. Specific exercises are used in physical medicine and rehabilitation. Benefits include lower blood pressure, higher HDL cholesterol, improved disease resistance, and better general well-being.

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▪ physical fitness

      the training of the body to improve its function and enhance its fitness.

      The terms exercise and physical activity are often used interchangeably, but this article will distinguish between them. Physical activity is an inclusive term that refers to any expenditure of energy brought about by bodily movement via the skeletal muscles; as such, it includes the complete spectrum of activity from very low resting levels to maximal exertion. Exercise is a component of physical activity. The distinguishing characteristic of exercise is that it is a structured activity specifically planned to develop and maintain physical fitness. Physical conditioning refers to the development of physical fitness through the adaptation of the body and its various systems to an exercise program.

A historical view of exercise

Prehistoric period
      Hominids—human beings and their immediate ancestors—have existed on Earth for at least 2,000,000 years. For more than 99 percent of that time, hominids lived a nomadic existence and survived by hunting and gathering food. It is obvious that this way of life was enormously different from the way people live today in developed countries. Thus, evolutionary history has prepared humankind for one kind of life, but modern people lead another. This fact has profound implications for patterns of disease and for the association between living habits and health. Observation of the few remaining nomadic groups in the world indicates that they are relatively free of chronic diseases and that, in comparison to the populations in developed countries, they are leaner, have a higher level of physical fitness, eat a very different diet, and have different physical activity patterns. Data from the distant past are not available, but it is reasonable to speculate that early humans had considerably higher caloric expenditures per unit of body weight than do modern individuals.

Agricultural period
      As civilization developed, nomadic hunting and gathering societies gave way to agricultural ones in which people grew their own food and domesticated animals. This development occurred relatively recently, approximately 10,000 years ago. Although many aspects of life changed during the agricultural period, it is likely that energy demands remained high, with much of the work still done by human power. Even in cities—which had evolved by about midway through the agricultural period—individuals expended more calories than do most people today.

Industrial (Industrial Revolution) period
      The industrial period began during the mid-18th century, with the development of an efficient steam engine, and lasted to the end of World War II (1945). This relatively brief time span was characterized by a major shift in population from farms to cities, with attendant changes in many areas of life-style. Even though the internal-combustion engine and electrical power were increasingly used to perform work, the great majority of individuals in industrialized societies still faced significant energy demands. In the cities relatively more individuals walked to work, climbed stairs, and had more physically demanding jobs than do most people today.

Technological period
      The post-World-War-II period has been a technological (technology) age, a period characterized by rapid growth in energy-saving devices, both in the home and at the workplace. As an example, longshoremen in the late 1940s worked hard loading and unloading ships; by contrast, most longshoremen in the late 20th century had much lower energy demands from the job, because of the containerization of cargo and the mechanization of the loading and unloading process. Also during this period, the use of labour-saving devices in the home and in yard and garden work became much more widespread. Physical activity became less and less common in industrialized countries, especially among the urban population. Although the level of general physical activity has declined, most observers feel that there have been increases in exercise participation in many countries since the late 1960s. Jogging, racket sports, cycling, and other active recreational pursuits have become much more common. In a sense this is simply humankind's returning to the more active life-style of its distant ancestors.

Types of physical fitness
      Physical fitness is a general concept and is defined in many ways by different scientists. Physical fitness is discussed here in two major categories: health-related physical fitness and motor-performance physical fitness. Despite some overlap between these classifications, there are major differences, as described below.

Health-related physical fitness
      Health-related physical fitness is defined as fitness related to some aspect of health. This type of physical fitness is primarily influenced by an individual's exercise habits; thus, it is a dynamic state and may change. Physical characteristics that constitute health-related physical fitness include strength and endurance of skeletal muscles, joint flexibility, body composition, and cardiorespiratory endurance. All these attributes change in response to appropriate physical conditioning programs, and all are related to health.

      Strength and endurance of skeletal muscles (striated muscle) of the trunk help maintain correct posture and prevent such problems as low back pain. Minimal levels of muscular strength and endurance are needed for routine tasks of living, such as carrying bags of groceries or picking up a young child. Individuals with very low levels of muscular strength and endurance are limited in the performance of routine tasks and have to lead a restricted life. Such limitations are perhaps only indirectly related to health, but individuals who cannot pick up and hug a grandchild or must struggle to get up from a soft chair surely have a lower quality of life than that enjoyed by their fitter peers.

      Flexibility, or range of motion around the joints, also ranks as an important component of health-related fitness. Lack of flexibility in the lower back and posterior thigh is thought to contribute to low back pain. Extreme lack of flexibility also has a deleterious effect on the quality of life by limiting performance.

      Body composition refers to the ratio between fat and lean tissue in the body. Excess body fat is clearly related to several health problems, including cardiovascular disease, type II (adult-onset) diabetes mellitus, and certain forms of cancer. Body composition is affected by diet, but exercise habits play a crucial role in preventing obesity and maintaining acceptable levels of body fat.

      Cardiorespiratory endurance, or aerobic (aerobics) fitness, is probably what most people identify as physical fitness. Aerobic fitness refers to the integrated functional capacity of the heart, lungs, vascular system, and skeletal muscles to expend energy. The basic activity that underlies this type of fitness is aerobic metabolism in the muscle cell, a process in which oxygen is combined with a fuel source (fats or carbohydrates) to release energy and produce carbon dioxide and water. The energy is used by the muscle to contract, thereby exerting force that can be used for movement. For the aerobic reaction to take place, the cardiorespiratory system (i.e., the circulatory and pulmonary systems) must constantly supply oxygen and fuel to the muscle cell and remove carbon dioxide from it. The maximal rate at which aerobic metabolism can occur is thus determined by the functional capacity of the cardiorespiratory system and is measured in the laboratory as maximal oxygen intake. As will be discussed in detail below, aerobic fitness is inversely related to the incidence of coronary heart disease (cardiovascular disease) and hypertension.

Motor-performance physical fitness
      Motor-performance (psychomotor learning) fitness is defined as the ability of the neuromuscular system to perform specific tasks. Test items used to assess motor-performance fitness include chin-ups, sit-ups, the 50-yard dash, the standing long jump, and the shuttle run (a timed run in which the participant dashes back and forth between two points). The primary physical characteristics measured by these tests are the strength and endurance of the skeletal muscles and the speed or power of the legs. These traits are important for success in many types of athletics. Muscular strength and endurance are also related to some aspects of health, as stated above.

      There is disagreement among experts about the relative importance of health-related and motor-performance physical fitness. While both types of fitness are obviously desirable, their relative values should be determined by an individual's personal fitness objectives. If success in athletic events is of primary importance, motor-performance fitness should be emphasized. If concern about health is paramount, health-related fitness should be the focus. Different types of fitness may be important not only to different individuals but also to the same individual at different times. The 16-year-old competing on a school athletic team is likely to focus on motor performance. The typical middle-aged individual is not as likely to be concerned about athletic success, emphasizing instead health and appearance. One further point should be made: to a great extent, motor-performance physical fitness is determined by genetic potential. The person who can run fast at 10 years of age will be fast at age 17; although training may enhance racing performance, it will not appreciably change the individual's genetically determined running speed. On the other hand, characteristics of health-related physical fitness, while also partly determined by inheritance, are much more profoundly influenced by exercise habits.

Principles of exercise training
      Research in exercise training has led to the recognition of a number of general principles of conditioning. These principles must be applied to the development of a successful exercise program.

      The principle of specificity derives from the observation that the adaptation of the body or change in physical fitness is specific to the type of training undertaken. Quite simply this means that if a fitness objective is to increase flexibility, then flexibility training must be used. If one desires to develop strength, resistance or strengthening exercises must be employed. This principle is indeed simple; however, it is frequently ignored. Many fraudulent claims for an exercise product or system promise overall physical fitness from one simple training technique. A person should be suspicious of such claims and should consider whether or not the exercise training recommended is the type that will produce the specific changes desired.

      Overload, the second important principle, means that to improve any aspect of physical fitness the individual must continually increase the demands placed on the appropriate body systems. For example, to develop strength, progressively heavier objects must be lifted. Overload in running programs is achieved by running longer distances or by increasing the speed.

      Individuals frequently make the mistake of attempting too rapid a fitness change. A classic example is that of the middle-aged man or woman who has done no exercise for 20 years and suddenly begins a vigorous training program. The result of such activity is frequently an injury or, at the least, stiffness and soreness. There are no hard-and-fast rules on how rapidly one should progress to a higher level of activity. The individual's subjective impression of whether or not the body seems to be able to tolerate increased training serves as a good guide. In general it might be reasonable not to progress to higher levels of activity more often than every one or two weeks.

Warm-up/cool down
      Another important practice to follow in an exercise program is to gradually start the exercise session and gradually taper off at the end. The warm-up allows various body systems to adjust to increased metabolic demands. The heart rate increases, blood flow increases, and muscle temperatures rise. Warming up is certainly a more comfortable way to begin an exercise session and is probably safer. Progressively more vigorous exercises or a gradual increase in walking speed are good ways to warm up. It is equally important to cool down—that is, to gradually reduce exercise intensity—at the end of each session. The abrupt cessation of vigorous exercise may cause blood to pool in the legs, which can cause fainting or, more seriously, can sometimes precipitate cardiac complications. Slow walking and stretching for five minutes at the end of an exercise session is therefore a good practice. The heart rate should gradually decline during the cool down, and by the end of the five minutes it should be less than 120 beats per minute for individuals under 50 years of age and less than 100 beats per minute for those over 50.

Frequency, intensity, and duration
      To provide guidance on how much exercise an individual should do, exercise physiologists have developed equations based on research. It is generally agreed that to develop and maintain physical fitness, the exercise must be performed on a regular basis. A frequency of about every other day or three days per week appears minimally sufficient. Many individuals exercise more frequently than this, and, of course, such additional exercise is acceptable provided that one does not become overtrained and suffer illness or injury.

      The intensity of exercise required to produce benefits has been the subject of much study. Many people have the impression that exercise is not doing any good unless it hurts. This is simply not true. Regular exercise at 45 to 50 percent of one's maximal capacity is adequate to improve one's physiological functioning and overall health. This level of intensity is generally comfortable for most individuals. A reliable way to gauge exercise intensity is to measure the heart rate during exercise. An exercise heart rate that is 65 percent of a person's maximal heart rate corresponds to approximately 50 percent of his maximal capacity. Maximal heart rate can be estimated by subtracting one's age in years from 220 (or, in the case of active males, by subtracting half of one's age from 205). Thus, a sedentary 40-year-old man has an estimated maximal heart rate of 180 beats per minute. Sixty-five percent of this maximal rate is 117 beats per minute; thus by exercising at 117 beats per minute, this individual is working at about 50 percent of his maximal capacity. To determine exercising heart rate, a person should exercise for several minutes, to allow the heart rate to adjust. The exerciser should then stop exercising, quickly find the pulse, and count the number of beats for 15 seconds. Multiplying this by four gives the rate in beats per minute. The pulse must be taken immediately after stopping exercise, since the heart rate rapidly begins to return to the resting level after work has been stopped. As noted above, exercising at the 50 percent level of intensity will improve physiologic functioning and provide health benefits. This level of exercise will not produce the maximum fitness needed for competitive athletics.

Overall conditioning
      Much emphasis has been given in the foregoing discussion to aerobic fitness, because this form of conditioning is extremely important. It should be noted, however, that other types of conditioning also have benefits. A total exercise program should include strengthening exercises, to maintain body mass and appropriate levels of strength for daily functioning, and stretching exercises to maintain joint mobility and flexibility. The specificity principle described above indicates that no one exercise is likely to produce the overall conditioning effect. In general an exercise plan should consist of aerobics, exercises that increase the strength and endurance of various skeletal muscle groups, and flexibility exercises to maintain good joint function.

Individual differences
      The principles of exercise training discussed above should be viewed as general guidelines. Individuals differ in both physiological and psychological adaptations to exercise. Two people who are similar in many respects and who start the same exercise program may have entirely different impressions of it. One person may feel that the exercise is too easy, while the other may believe that it is much too hard. It is certainly appropriate that the exercise plan be adjusted to account for preferences. Likewise some individuals will progress to more intense training levels far more rapidly than others do. As mentioned earlier, exercise progress should be adjusted according to the exerciser's own assessment.

      Individuals also differ in the type of exercise they like or can tolerate. Jogging, for instance, is not for everyone. Many people who dislike jogging, or who suffer running injuries, can find other satisfactory exercise activities, such as cycling, walking, swimming, or participating in a sport. Many kinds of exercise activities are appropriate and can provide physiological and health benefits to the participant. There is no one best exercise. The important thing is to be regular in exercise participation and to follow the general guidelines outlined in this section.

Physiological effects of exercise

Neuromuscular effects
Strength and endurance
      Appropriate exercise increases the strength and endurance of skeletal muscles. Increases in muscular strength are associated with increases in muscle mass; increases in muscular endurance are associated with improved blood flow to the working muscles. These results are achieved by resistance training. Any exercise that causes the muscle to increase its tension, whether or not the muscle actually shortens during contraction, provides an appropriate strength-training stimulus. Resistance can be applied to a muscle group by attempting to move an immovable object, by working one muscle group against another, by lifting heavy weights, or by using special strength-training machines and devices. There is a wide selection of strength-training equipment that, when used properly, can increase muscular strength and endurance. It is possible that some of the equipment is more efficient in developing maximal performance, which is important for competitive athletes. But for the average individual, who is training to maintain an acceptable level of muscular fitness, any one device or program is probably about as good as another.

      Strength and endurance training is done by performing several “reps” (repetitions) of a given exercise, then moving on to another exercise for a different muscle group. Experts generally recommend that exercisers select a resistance that is approximately 65 percent of the maximum they can lift for that particular exercise. This load should allow the completion of 12 reps of that exercise in 24 to 30 seconds. Each group of eight to 12 reps is called a set, and two or three sets of a given exercise are recommended for each training session. The average individual should perform strength and endurance training two to three days per week. Super circuit weight training refers to a program in which running or other aerobic exercises are performed between sets; this training produces aerobic as well as strength benefits.

      Muscles and tendons (tendon) can be stretched to improve flexibility (the range of motion at a joint). Flexibility training follows a few, simple principles. To improve range of motion, the muscles and other connective tissue around a joint must be stretched. The preferred stretching technique is a slow increase in the range of motion. The exerciser should feel the muscle stretch, but not to the point of pain. The stretch should be performed gradually, and the body should be held for 10 to 20 seconds in the stretched position and then gradually returned to a relaxed posture. By stretching each muscle group in this fashion as a part of the strengthening and conditioning program, the participant will maintain good flexibility. Bouncing or explosive stretching movements should be avoided, as they can result in muscle or tendon tears.

Cardiorespiratory effects
Cardiac effects
      Regular aerobic (aerobics) exercise training has a direct effect on the heart muscle. The muscle mass of the left ventricle, which is the pumping chamber that circulates blood throughout the body, increases with exercise training. This change means that the heart can pump more blood with each beat. In short, the heart becomes a bigger, stronger, and more efficient pump capable of doing more work with less effort.

Circulatory effects
      Regular exercise also produces changes in the circulation. As previously discussed, muscle endurance training serves to increase blood flow to the working muscles. This increased blood flow means that more oxygen and fuel can be delivered to the muscle cells. The number of red blood cells, which carry oxygen in the blood, also increases with training, as does blood volume. Taken together, these changes indicate a greater capacity to transport oxygen to the working muscles.

      The basic function of the lungs is to facilitate the transfer (1) of oxygen from the atmosphere into the blood and (2) of carbon dioxide from the blood into the atmosphere. To accomplish this, air must pass into and out of the lungs, and the respiratory gases must diffuse through the lungs into the circulation and vice versa. Although exercise has not been shown to affect this diffusing ability, exercise training does strengthen the muscles of respiration. This means that a trained individual can move more air through the lungs per time unit, and forced vital capacity (i.e., the maximum volume of air that can be exhaled after a full inspiration) may be increased.

Health effects of exercise

Improved general fitness
      The greatest benefit of a regular exercise program is an improvement in overall fitness. As discussed above, appropriate exercise improves muscular strength and endurance, body composition, flexibility, and cardiorespiratory endurance. The level of maximal oxygen intake or cardiorespiratory endurance is not by itself of great importance to most individuals. What is important is that one's sustained energy-spending ability is directly related to maximal levels of performance. For example, consider the simple task of walking at a rate of three miles per hour. This task involves an energy expenditure of approximately three times the resting metabolic rate. Extremely unfit individuals may have a maximal aerobic power of only six times their resting metabolic rate. For such individuals, a three-mile-per-hour walk requires half of their maximal capacity. A middle-aged person who exercises regularly will have a maximal aerobic power 10 to 12 times resting, so the same walk will represent only 25 to 30 percent of maximal capacity. This example illustrates how any submaximal task is relatively much easier for the conditioned individual. Moreover, a person cannot work throughout the day at much more than about 20 percent of maximal capacity without becoming chronically fatigued (fatigue). The deconditioned person who has a maximal aerobic power of six times resting can comfortably sustain a work level of only about 1.2 times resting throughout the day (6 × 0.20 = 1.2). This low capability for sustained energy expenditure can support only a very sedentary existence: for example, 20 hours of sleep and rest, two hours of personal care, one hour of housework and shopping, and one hour of activity at three times the resting rate each day.

      The point of the preceding discussion is that the average energy-expenditure requirement of anyone's life can be calculated, and a person's maximal cardiorespiratory endurance determines how active a life-style can be sustained. Individuals who attempt to lead more active lives than their fitness level will support become chronically fatigued. Persons with adequate or optimal fitness levels, on the other hand, are able to meet the physical demands of an active life relatively easily. One of the most frequent observations made by individuals who have begun an exercise program is that they feel better, and research studies document an improvement in feelings of general well-being in more active people.

Decreased risk of coronary heart disease
      Coronary heart disease is the leading cause of death in the developed world. Coronary heart disease is defined as myocardial infarction, or heart attack; angina pectoris, or chest pain; or sudden death due to cardiac arrest or abnormal electrical activity in the heart. The basic disease process that underlies coronary heart disease is atherosclerosis, a disorder characterized by the accumulation of cholesterol and the proliferation of smooth muscle cells in the linings of the arteries. This results in a gradual narrowing of the arterial channel, and this narrowing diminishes and may ultimately stop blood flow through an artery. When this occurs in a coronary artery—that is, an artery supplying the heart—one of the manifestations of coronary heart disease occurs.

Epidemiological evidence of exercise benefits
      Studies have linked sedentary living with high rates of coronary heart disease mortality. One study found that San Francisco longshoremen who worked in jobs requiring high levels of energy expenditure had less risk of dying of heart disease than did longshoremen who performed sedentary jobs. This study showed that dockworkers and cargo handlers expended at least 1,000 kilocalories more per day than did clerks and foremen and that sedentary workers, during a 22-year observation, were about twice as likely to die from heart disease. The higher risk of death in the less active men was not due to other coronary heart disease risk factors, such as smoking, obesity, and high blood pressure; nor was it the result of less healthy men's shifting to sedentary jobs.

      Another study followed the health status of approximately 17,000 male graduates of Harvard University for many years. All these men essentially had sedentary jobs, but they differed in the amount of leisure time they spent on physical activities. Men who expended at least 2,000 kilocalories per week on physical activity had only half the death rate from heart disease as did those men who expended less than 500 kilocalories per week. Not all of this energy was spent in exercise programs; some was expended during routine activities such as climbing stairs.

The effect of exercise on coronary-heart-disease risk factors
      One of the important medical achievements of the 20th century has been the development of the risk-factor theory of coronary heart disease. Scientists have discovered that persons who are overweight, smoke cigarettes, have high blood pressure, or show elevated blood levels of certain types of fat- and cholesterol-carrying molecules are much likelier to die from coronary heart disease. Furthermore, combinations of these risk factors result in exponential increases in the risk of death. The discovery and description of risk factors have led to an understanding of the atherosclerotic process and of how to prevent and treat it. Evidence suggests that regular exercise can lower a person's exposure to several of the risk factors.

      Fat and cholesterol are transported by the blood in complex molecules called lipoproteins (lipoprotein). Researchers have identified several classes of lipoproteins and have elucidated their roles in atherosclerotic progression. It is, therefore, possible to describe abnormal, or high-risk, lipoprotein profiles. Diet and heredity are key factors determining a person's lipoprotein profile, and exercise also plays an important role. Regular aerobic (aerobics) exercise improves the lipoprotein profile in most individuals. Although more work is needed to completely understand this association, the dose of exercise necessary to effect a beneficial change in the lipoprotein profile seems to be about eight to 10 miles of running (or its equivalent in other activity) per week.

      Elevated blood pressure ( hypertension) is a second powerful risk factor for coronary heart disease. Sedentary living habits and low levels of physical fitness increase the risk of developing hypertension. Exercise also appears to lower blood pressure in at least some individuals with hypertension. The greatest benefit is probably for younger people (those less than 40 to 45 years of age) whose hypertension is of relatively recent onset.

      Excess body weight is considered by most experts to be an independent risk factor for coronary heart disease, although obesity also indirectly increases the risk via deleterious impact on blood pressure and the lipoprotein profile. Exercise habits are strongly related to body weight. In virtually all studies of large populations, the more active individuals weigh less. One of the most consistent results seen in exercise-training studies is the loss of body weight and fat. Weight-loss programs that incorporate exercise as well as diet are more successful than those that rely on diet alone.

Impact on other chronic diseases
      Although more research is needed to arrive at definitive conclusions, some evidence has suggested that regular exercise may help in the treatment or prevention of other chronic diseases. The control of type II diabetes (diabetes mellitus), for example, appears to be aided by regular exercise. This form of diabetes is a major health problem in which the patient shows elevated levels of blood sugar despite having acceptable levels of insulin, the hormone that normally clears the blood of excess sugar by facilitating its utilization by the body cells. Persons with this disease need to control their blood sugar, but not with insulin injections. Oral medications that lower blood-sugar levels are available, but their usefulness has been questioned. Consequently, dietary modifications and exercise, both of which can lower blood-sugar levels, have become the key measures in controlling type II diabetes. Exercise seems to improve the insulin sensitivity of cells, so that blood sugar can more readily be taken in and used as fuel.

      A few reports have linked low physical activity with a higher risk of developing certain cancers (cancer), particularly colon cancer. These results are intriguing, but more work is needed to firmly establish that sedentary habits are an independent risk factor for cancer.

Risks of exercise
      As can be seen from the foregoing discussion, regular participation in an exercise program can provide several benefits. Yet exercise is similar to other medical or health interventions in that there are also potential costs associated with the activity. These costs range from minor inconveniences, such as time taken up by exercise, to more serious complications, including injury (wound) and even sudden death.

      It is clear that some people who participate in exercise training will develop injuries to their bones, muscles, and joints. Despite unfounded reports in the mass media of extremely high injury rates among adult exercisers, there have been few good studies of exercise injuries in populations. One of the difficulties in performing such studies has been the need to identify both the number of cases (individuals who become injured) and the number of persons at risk for injury (the total number of individuals exercising in the population). These two figures are necessary in order to calculate true injury rates. The best available studies on injury rates suggest that about 25 to 30 percent of adult runners will become injured over the course of a year, if injury is defined as an incident that causes an individual to stop exercising for at least one week. If only more serious injuries, such as those for which the individual seeks medical care, are considered, injury rates are much lower, perhaps in the range of 1 percent per year.

      Little is known about the causes of exercise injuries. One factor that has been linked to injury is the amount of exercise; for example, individuals who run more miles are likelier to be injured than those who run fewer miles. Factors such as age, sex, body type, and experience have not been shown to be associated with risk of injury. It seems logical that structural abnormalities, sudden increases in training intensity, and types of equipment used are likely to be related to injury risk; however, data to support these opinions are not available.

      In view of the limited scientific data on injury risk, the exerciser is advised to follow commonsense practices until such time as the causes of injury are better understood. Exercisers should start their program slowly and gradually progress to more intensive training levels. They should use good equipment and pay particular attention to proper footwear. Exercisers who have had previous injuries should recognize that they may be more susceptible to similar injuries in the future. All exercisers should use caution and should monitor their bodies for the early warning signs of injury. If a problem begins to develop, it is good advice to stop exercising or to reduce the intensity of training for a few days to see if the problem disappears. Exercisers should not be afraid to experiment on themselves to find out what training practices and techniques seem to be more comfortable and less likely to produce injury. Moderation is good advice: few injuries are reported in individuals who run 10 to 15 miles per week, and this level is adequate to provide many health benefits.

Sudden death
      Obviously, the most serious complication from an exercise program is sudden death. This is, fortunately, a rare occurrence. As discussed earlier, several studies have shown that individuals who regularly participate in exercise have a lower risk of dying from a heart attack. There is, however, also evidence that suggests a higher risk of dying during exercise than during sedentary activities. When one considers the total risk of sudden death over a 24-hour period, regular exercisers are much less likely to experience this catastrophe.

      Virtually all individuals who drop dead suddenly have advanced coronary heart disease. It follows, therefore, that the best way to reduce the risk of sudden death during exercise is to avoid getting advanced coronary heart disease. This implies following good health practices in other aspects of life, such as not smoking, eating a prudent diet, and maintaining an ideal body weight. Individuals who are middle-aged or older can probably reduce their risk of sudden death by knowing about their coronary risk status and their general state of health before undertaking an exercise program. There are, of course, no guarantees, but if an individual has a thorough examination by a competent physician, including a maximal exercise test and other procedures that screen for coronary heart disease, that person can probably safely begin an exercise program.

      There has been much progress in the field of exercise and physical conditioning. Concepts about exercise have moved from faddism to scientific legitimacy, thanks to researchers in physical education, exercise physiology, and medicine. Yet much remains to be learned, and experts need to work together to further develop the study and promotion of exercise. There are many items that need further study, from the cellular level to the population as a whole. For example, more information is needed on specifically how exercise affects blood lipoprotein levels, and further research is needed on rates of injuries in populations of exercisers.

Additional Reading
Exercise as a key to health maintenance is found in Kenneth H. Cooper, The Aerobics Program for Total Well-being: Exercise, Diet, Emotional Balance (1982); Michael L. Pollock, Jack H. Wilmore, and Samuel M. Fox III, Exercise in Health and Disease: Evaluation and Prescription for Prevention and Rehabilitation (1984); Philip L. White and Therese Mondeika (eds.), Diet and Exercise: Synergism in Health Maintenance (1982); Bud Getchell and Wayne Anderson, Being Fit: A Personal Guide (1982); John E. Beaulieu, Stretching for All Sports (1980). Specific aspects of exercise for middle-aged or older people are the topic of Herbert A. Devries and Dianne Hales, Fitness After 50 (1982). Other special topics are treated in the Journal of the American Medical Association: Larry W. Gibbons et al., “The Acute Cardiac Risk of Strenuous Exercise,” J.A.M.A., 244(16):1799–1801 (Oct. 17, 1980); John J. Duncan et al., “The Effects of Aerobic Exercise on Plasma Catecholamines and Blood Pressure in Patients with Mild Essential Hypertension,” J.A.M.A., 254(18):2609–13 (Nov. 8, 1985); Ralph S. Paffenbarger et al., “A Natural History of Athleticism and Cardiovascular Health,” J.A.M.A., 252(4):491–495 (July 27, 1984); and Steven N. Blair et al., “Physical Fitness and Incidence of Hypertension in Healthy Normotensive Men and Women,” J.A.M.A., 252(4):487–490 (July 27, 1984). See also Kenneth H. Cooper, Running Without Fear: How to Reduce the Risk of Heart Attack and Sudden Death During Aerobic Exercise (1985); and Sidney Alexander, Running Healthy: A Guide to Cardiovascular Fitness (1980).What happens to the body during exercise and other intense physical activity is explained in many informative sources and texts. See Per-Olof Astrand and Kaare Rodahl, Textbook of Work Physiology: Physiological Bases of Exercise, 2nd ed. (1977); and George A. Brooks and Thomas D. Fahey, Exercise Physiology: Human Bioenergetics and Its Applications (1984). Public health aspects of physical activities and exercise are explored in a collection of articles in Public Health Reports, vol. 100, no. 2 (March–April 1985). Roy J. Shephard (ed.), Frontiers of Fitness (1971), discusses the physiology of exercise and desirable limits of fitness for people of different ages.The usefulness of recreational exercise was studied in Greek antiquity by Galen; see Robert Montraville Greene, A Translation of Galen's “Hygiene” (De sanitate tuenda) (1951). For a historical treatment of exercise and sport, see Richard D. Mandell, Sport, a Cultural History (1984), a scholarly study of physical activity as a component of culture; William J. Baker, Sports in the Western World (1982); and HISPA (International Association for the History of Physical Education and Sport), The History, the Evolution and Diffusion of Sports and Games in Different Cultures (1976).Kenneth H. Cooper Steven N. Blair

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Universalium. 2010.


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