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Perspectives in Disease Prevention and Health Promotion Workshop on Epidemiologic and Public Health Aspects of Physical Activity and Exercise
In response to increased evidence that regular physical activity produces substantial physical and emotional benefits, the U.S. Public Health Service has specified "Physical Fitness and Exercise" as one of the 15 areas of greatest importance for improving the health of the public (1,2).
To provide the public health and scientific communities with a summary of the current knowledge in this area and with recommendations for future research, CDC organized the preparation of 10 scientific papers and conducted a Workshop on the Epidemiologic and Public Health Aspects of Physical Activity and Exercise on September 24-25, 1984. The papers, all of which are published in the March-April issue of Public Health Reports* (3-12), and workshop deliberations are summarized below. SUMMARIES OF THE INDIVIDUAL PAPERS
definitions and distinctions for health-related research (3). Physical activity is movement produced by skeletal muscles that results in energy expenditure. Exercise is a subset of physical activity that is planned, structured, repetitive, and has the improvement or maintenance of physical fitness as an objective. Physical fitness is a set of attributes, some of which are health related, that people have or achieve. Extra attention was given to "exercise" because it often is used interchangeably with "physical activity"; however, common usage suggests that exercise has characteristics that separate it from many other physical activities.
The main thrust of the paper is that physical activity has many dimensions or components. To evaluate and compare published reports, investigators need to recognize and describe the specific components of the physical activity they have studied.
2. Assessment of physical activity in epidemiologic research: problems and prospects (4). Physical activity is a complex behavior with many interrelated dimensions. It has been measured in a variety of ways ranging from direct calorimetry to a single query about how active one is. Each method captures only part of the entire physical activity spectrum. In addition, different dimensions of activity may be related to different dimensions of health. Therefore, the specific concerns of a survey or study determine the most appropriate method. At present, recall procedures seem the best method for large population studies.
In spite of the many methods currently in use, little has been done to determine the reliability and validity of the various measurement methods. This crucial area must receive more attention to assure the accuracy of research efforts.
3. A descriptive epidemiology of leisure-time physical activity (5). Inconsistent and inadequately detailed measurement of activity in population surveys badly hampers a thorough description of the active population. It appears, however, that, during their leisure time, only about 20% of adults perform the amount of physical activity generally recommended for cardiovascular fitness. In the United States, leisure-time physical activity is positively associated with male sex and socioeconomic status and inversely associated with age.
4. The determinants of physical activity and exercise (6). The factors causally associated with a physically active lifestyle are poorly understood. The behavior is determined, at least in part, by characteristics of the person, the environment, and the activity itself. Important differences in determinants probably exist between the adoption and the maintenance of activities and between supervised and unsupervised activities. Whereas the paper describes many potentially predictive associations, the final conclusion is that the determinants of physical activity are very uncertain. Previous experience in sports, family and peer support, self-motivational characteristics, and positive feelings resulting from the activity seem important; the evidence supporting the importance of accessible facilities, time restraints, and various climatic conditions is less conclusive.
5. Relationships between exercise or physical activity and other health behaviors (7). The expectation that physical activity, particularly exercise, may favorably influence other important health behaviors is firmly established only for weight control. In other areas, such as for smoking cessation, the evidence to date is only suggestive. Reported associations are small, insignificant, or both, and causality cannot be determined. Research in this area is hampered not only by the difficulties of measuring physical activity but also by those of measuring the other behaviors.
6. The disease-specific benefits and risks of physical activity and exercise (8). Habitual vigorous physical activity reduces the risk of coronary heart disease (CHD) and sudden death. The reduction is the result of protection and not merely the selection of less-susceptible individuals. The protective effect is independent of other risk factors, such as hypertension, obesity, smoking, and family history, and may actually provide relatively more protection for those with hypertension and obesity than those without these risk factors. The temporarily increased risk of sudden death during vigorous physical activity is outweighed by the overall reduced risk of CHD from habitual vigorous activity. Several studies suggest that habitual exercise or physical activity may prevent or control hypertension, osteoporosis, or Type II diabetes. Little or no information is available on the effect of habitual activity on cancer, respiratory diseases, or arthritis.
For each of the disorders, the dose-response effect needs to be explored in more detail. It is of great importance to determine the effects of beginning an exercise program in early, middle, or later life. Distinction needs to be made between the effect of vigorous physical activity and that of less-vigorous activity performed over a longer period of time, yet results in an equivalent expenditure of energy.
7. The risks of exercise: a public health view of injuries and hazards (9). The potential hazards of physical activity or exercise are many. They may be acute or chronic, mechanical, metabolic, or psychologic. They may be specific to the activity, to the age or sex of the participant, or to both. Data permitting the calculation of incidence rates for any of these potential problems are essentially nonexistent. Even for the six most commonly reported aerobic activities among U.S. adults--walking, jogging, swimming, cycling, calisthenics, and racket sports--there is almost no information about the incidence of acute mechanical injuries, let alone metabolic, psychologic, or chronic effects.
8. The relation of physical activity and exercise to mental health (10). The beneficial effects of physical activity or exercise on various aspects of mental health are potentially large. Unfortunately, few studies have been performed or reported with sufficient care that valid conclusions can be drawn. Physical activity and exercise do alleviate the symptoms of mild to moderate depression and, in the general population, reduce the symptoms of anxiety. Particularly fruitful areas of research are in the areas of substance abuse, psychologic stress, and coronary-prone (Type A) behavior.
9. Physical activity and exercise to achieve health-related physical fitness components (11). Physically active persons have fewer health problems. The most diverse benefits on health accrue from physical activity characterized by the rhythmical contraction of large muscle groups that move the body over distance or against gravity. The activity can be performed at moderate intensity (50%-70% maximal oxygen capacity, i.e., at about "half-speed") and should be done at least every other day. Not known, however, is whether the health benefits are mediated through improvements in physical fitness or are achieved through some other pathway, such as improved serum lipoprotein profile, fibrinolytic activity, decreased platelet adherence, or other metabolic changes. Some health benefits seem to be achieved through activity that does not improve cardiorespiratory endurance.
10. The promotion of physical activity in the U.S. population: the status of programs in medical, worksite, community, and school settings (12). Exercise programs at the worksite, exercise recommendations or prescriptions by health-care providers, and physical education in the schools have potential for beneficially modifying exercise behaviors of large numbers of people of all ages. A community-based program to promote physical activity can provide support for behaviors triggered in the worksite, medical setting, or school programs and also may provide the primary contact for persons who may not otherwise be reached. Behavioral change seems most likely when these forums and others provide overlapping encouragement for the adoption and maintenance of regular exercise behavior. There is evidence that worksite programs, medical professionals, and schools favorably influence exercise behavior. However, the components of an effective program in any setting are unknown. "Success" may differ both between and within settings and depend on the program-specific objectives, which may not stem from health-related concerns. The community setting is the most complex and, to date, community-based programs have not demonstrated community-wide changes. Individuals responsible for programs need to establish clear objectives before initiating the pro-gram. Researchers need to help evaluate the individual programs and identify components of success across different programs.
RECURRENT TOPICS OF DISCUSSION
Conceptual issues. Physical activity, physical fitness, health, and disease are complex, multidimensiona concepts that relate to each other via an equally complex array of demographic and cultural variables. Meaningful discussion of their interrelationships, causal or otherwise, requires that the specific dimension(s) under discussion be described as carefully as possible. The complexity of the potential interrelations should not deter investigation of these relationships or unduly hamper utilization of current knowledge. It does mean that future researchers should carefully consider and describe the component(s) of physical activity, fitness, or health with which they are concerned. Equally important, utilization and dissemination of information must be accurate lest unrealistic expectations be engendered Methodologic issues. Throughout the set of workshop papers, there is a repeated call for reliable and valid measurement instruments. The complexity of the concepts under study precludes the Possibility that a single instrument will be suitable in all situations. What is needed is (1) that the instrument be selected or developed with the specific dimensions of activity, health, or fitness to be studied firmly in mind and (2) that the accuracy of the instrument be determined.
Major gaps. In almost every paper, the paucity and necassity of dose-response information are mentioned. This should not be mistaken for the search for a single optimal level below which there is no benefit and above which one reaps full reward. On the contrary, the interest in dose-response information stems from the recognition that dose is probably inversely related to likelihood of participation and from the necessity to compare benefits and risks, both of which are almost certainly dose-related. The increase in benefits may be greatest at low levels and diminish with increasing activity. On the other hand, risks may be less at lower levels and become increasingly more frequent and severe at higher levels. The effect of low-intensity activity, such as walking, is an area of great interest. it appears likely that the greatest gain in the risk-benefit relationship per unit change in physical activity occurs at the lower end of the activity spectrum (11). As a whole, the population is likely to benefit more if the least active begin to do a little than if the more active do even more. There may be different levels at which the rate of improvement diminishes for different diseases. Although low-level activity is a particularly important area for study, it is, unfortunately, also the place at which current measurement instruments are least discriminatory.
Another frequently mentioned deficiency of currently available data is the lack of information pertaining to specific subgroups within the population, such as children and adolescents, elderly, the disabled, and others. Not all individuals are likely to achieve equal benefit from an activity program. Some groups of persons are more likely to become injured than others, and come are more likely to respond favorably to a specific intervention than others. Overall, greater attention to the differing effects on population subgroups is very important. In particular, the patterns and determinants of childhood and youth physical activity and the behavioral patterns that are more likely to carry over into adulthood should be ascertained.
The near absence of data about secular trends in physical activity patterns at the national level is not surprising given the relatively recent interest in this area shown by the public health community. The few data available from national surveys in Canada, opinion polls in the United States, and studies of selected groups suggest a recent increase in leisure-time physical activities (5, 13). However, the increase cannot be quantified and may not apply to all groups. It is hoped that future surveillance systems will address these issues with a more systematic and quantitative approach.
Miscellaneous. A recurrent theme of discussion was that the benefits and risks cannot be considered in isolation. It may be necessary to study them separately, but the overall effect of physical activity on the health of the population requires that both be known, studied with equal care, and considered objectively. The potential overall beneficial impact of physical activity on health will be poorly served if activity patterns are recommended indiscriminately for all groups without regard for the subgroup-specific benefits and risks. Reported by Behavioral Epidemiology and Evaluation Br, Div of Health Education, Center for Health Promotion and Education, CDC.
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