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Introduction

  1. Top of page
  2. Introduction
  3. Statement of the problem
  4. Review of the literature
  5. Discussion
  6. REFERENCES

Physical activity is an essential component of a healthy life (1, 2). The importance of physical activity in preventing a number of chronic diseases is highlighted in the 1996 Surgeon General's report on physical activity and health (1). Among the numerous health benefits associated with physical activity are reduced disability and mortality due to coronary heart disease (CHD), diabetes, hypertension, and colon cancer and improved control of the joint swelling and pain associated with arthritis. Unfortunately, many adults do not reach optimal levels of physical activity and are at risk for poor health outcomes (3). Older adults in particular are less likely than younger adults to be regularly active, which is unfortunate because older adults who lead sedentary lives report more physical limitations than their active peers (4, 5). Quality of life is directly related to functional status and the ability to maintain independence, and it appears that physical activity improves health-related quality of life by enhancing psychological well being and improving physical functioning in persons with poor health (1).

Sedentary lifestyles among the general population can result in enormous direct medical costs, individual disability, and societal burden (6–8). According to the National Medical Expenditure Survey (6), the direct medical costs associated with physical inactivity among nondisabled individuals aged 15 years and older were estimated to be $330 per year (1987 dollars). Analysis of these data among persons with arthritis revealed an estimated overall cost associated with physical inactivity of $504 per year (1987 dollars) (7).

In this article we identify problems associated with the measurement of physical activity and difficulties with establishing the true relationship between physical activity and health. In addition, we summarize the scientific evidence characterizing the poor health outcomes associated with physical inactivity or sedentary behaviors, emphasizing the leading causes of mortality and disability (CHD, diabetes, and arthritis). We also provide general physical activity recommendations for individuals with these selected conditions and suggestions for future research.

Statement of the problem

  1. Top of page
  2. Introduction
  3. Statement of the problem
  4. Review of the literature
  5. Discussion
  6. REFERENCES

Some common terminology used in physical activity research studies will be described to provide a consistent framework for discussion. In general, physical activity is defined as any type of bodily movement, whereas exercise denotes a structured, planned activity performed with a fitness goal in mind (9). Both of these terms generally refer to large-muscle activities that may be aerobic (as in walking or running) or anaerobic (as in weight lifting). Physical fitness can refer to cardiorespiratory fitness (important for the prevention of CHD events) or other health-related components, including balance, flexibility, strength, and body composition (10). It is difficult to come up with a common definition of sedentary because it is usually defined relative to activity levels of others, which means the definition can vary from study to study. According to Kent's Oxford dictionary of sports science and medicine, a sedentary individual is one who is “relatively inactive and has a lifestyle characterized by a lot of sitting” (11).

The 3 dimensions that typically are used to characterize physical activity are frequency, duration, and intensity. Frequency is defined as the number of times an activity is performed in a given time frame (usually a week), whereas duration refers to the total amount of time an activity is performed, either continuously during 1 session or accumulated over a specified time (usually a day or week). Intensity refers to the energy expended during a particular activity and is usually measured in metabolic equivalents (METs) where each activity is assigned a value (e.g., no activity, such as sleeping, is assigned a MET value of 1.0). Using a published compendium of activities allows for standardization across several studies (12), but assumes that all individuals have the same physical capacity and are within normal weight range. To avoid such misrepresentations, intensity may be assessed by self report, that is, by asking the participant about a physiologic response (i.e., increased heart rate or breathing) associated with an activity. Having the individual self assess the intensity of an activity may take into account an individual's capacity in a way that is not possible with the objective measure (i.e., specifying an activity and assigning a MET value).

The true measure of the intensity of any physical activity is relative to an individual's capacity and can be accurately measured only in a laboratory setting. Nevertheless, questionnaires have been developed to assess physical activity and can be administered in person, over the telephone, or self administered either on site or by mail. Although questionnaire data are subject to self-report bias, many physical activity questionnaires have been validated and found to provide fairly accurate estimates of physical activity or energy expenditure, especially for the purpose of distinguishing active people from inactive people (13, 14).

Because physical activity behavior is difficult to assess, physical fitness, measured by using a standard treadmill or cycle protocol, often serves as its surrogate. Although physical fitness has a genetic component (as do hypercholesterolemia and hyperglycemia) some evidence indicates that measured physical fitness corresponds well to physical activity levels (15, 16). Physical fitness (measured by treadmill time duration or stage reached on a cycle ergometer protocol) has been shown to be inversely associated with mortality from all causes and a number of other outcomes, including CHD (15–20).

Studying the relationship between physical activity and health is complicated for several reasons. Physical activity is measured in different ways among studies, making it difficult to compare findings among different populations. In addition, the types and intensities of physical activities that are required for a health effect may vary for each health outcome. For example, aerobic activities, such as running or brisk walking, that improve cardiorespiratory fitness may be necessary to prevent CHD events. In contrast, activities that cause less stress to joints and activities that increase flexibility and improve joint lubrication, such as bicycling or swimming, may be necessary to prevent or delay the development or progression of arthritis. Some of the health effects associated with physical activity (e.g., reduced mortality from all causes) may occur through intermediate changes in blood pressure, serum cholesterol, or other metabolic systems that are not easily assessed, especially in large studies. In addition, some of the physiologic changes stimulated by physical activity that lead to beneficial health effects are known immediately, whereas others may not appear for many months or years. Because of these complicating factors, the appropriate time to assess physical activity during the life span is not known. For example, is a person's current activity pattern more important than past activity patterns (during childhood or young adulthood)? Or, does the appropriate activity pattern vary with the health outcome of interest? These are some of the many questions that remain to be answered.

Review of the literature

  1. Top of page
  2. Introduction
  3. Statement of the problem
  4. Review of the literature
  5. Discussion
  6. REFERENCES

Physical activity/fitness and CHD.

Coronary heart disease is the leading cause of death among men and women and accounted for more than 450,000 deaths in the US in 1998 (21). An estimated 12 million individuals have a history of CHD. Risk factors for this major cause of mortality and morbidity include cigarette smoking, high serum cholesterol, physical inactivity, overweight and obesity, and diabetes mellitus (10, 21).

Some of the earliest studies of physical activity and CHD were conducted by observing men who worked in occupations that required varying levels and intensities of movement. Morris and coworkers published a landmark study in which they reported differences in CHD rates experienced by London bus drivers, who had high CHD rates, and ticket takers, who had low CHD rates (22). The drivers sat all day whereas the ticket takers climbed up and down the double-decker buses collecting fares, suggesting an association between physical activity and CHD rates. Other studies also found that rates of CHD-related morbidity and mortality were lower among active occupational groups than among workers with low occupational physical activity (23, 24). Over the past several decades, the general level of physical activity required for many occupations in industrialized countries has declined. During the same time, many studies have primarily measured physical activity by leisure time (nonoccupational) physical activity or exercise.

In 1987 and 1990, 2 comprehensive reviews, in which different methodologies were used, both concluded that the risk of CHD due to physical inactivity was about twice as high among sedentary individuals as among their active peers (25, 26). This strong association is equivalent to that between CHD and smoking or hypertension. However, physical inactivity or sedentary behavior is more prevalent in the general population than either smoking or hypertension, thus accounting for its high population attributable risk (27). Population attributable risk is an important public health measure because its calculation takes into account the strength of the relationship between a risk factor and health outcome, as well as the prevalence of the risk factor in the population. Population attributable risk estimates for physical activity and CHD have ranged from 23% to 46%; the best estimate is approximately 35% (27, 28). Therefore, if all sedentary individuals could achieve at least a moderate level of physical activity, CHD events in the entire population would (theoretically) decrease by 35% (29).

Although few early studies on CHD included women, it is now generally accepted that the heart health benefits of physical activity also apply to women. Recent studies suggest that women may respond equally well to lower intensity activities, such as brisk walking (30, 31). Some studies of men have documented improvements in CHD-related and overall mortality at low levels of intensity or total energy expenditure (32). In general, however, a higher volume or intensity of activity produces greater improvements in men.

Many studies have demonstrated an inverse relationship of CHD to physical fitness—a finding similar to those found with physical activity (17, 19, 20, 33)—suggesting that physical activity and fitness are interchangeable in their effect. However, a metaanalysis using previously published data suggests that being unfit is a risk factor in itself (34). Considering that it is possible to see changes in intermediate biologic markers (e.g., serum cholesterol levels), even without changes in physical fitness, during a program designed to increase physical activity, there is evidence for both inactivity and lack of fitness to be independent risk factors for CHD. Although physical fitness information is difficult to obtain, it is likely to be more accurately measured than physical activity patterns.

Physical activity/fitness and diabetes.

Diabetes mellitus is a chronic disease characterized by elevated blood glucose levels that can lead to organ complications, such as heart, nerve, foot, eye, and kidney damage, resulting in significant disability. The most common form is type 2, also called non–insulin-dependent diabetes mellitus or adult-onset diabetes. In the US, approximately 95% of all persons with diagnosed diabetes (10.5 million) and almost 100% of all persons with undiagnosed diabetes (5.5 million) have type 2 diabetes (10). Although the development of diabetes has a genetic component, behavioral risk factors have also been identified. These include overweight or obesity, physical inactivity, and improper nutrition (10). Regular physical activity (especially when combined with dietary changes) not only lowers the risk of developing diabetes, but also can prevent or reduce related complications (35–39). The biologic mechanism by which physical activity reduces these risks is not clear, but may involve changes in body composition (weight loss), increased insulin sensitivity, or improved glucose tolerance (37). Although the beneficial effects of physical activity may operate solely through improved body composition or weight loss, evidence indicates that increased insulin sensitivity and improved glucose tolerance can occur without weight loss (40). Increased intensity or frequency of physical activity is clearly associated with a lower incidence of diabetes, but it is not clear whether increased intensity or frequency is necessary for effective treatment or glucose control once the condition has developed (41).

Physical activity/fitness and arthritis.

Arthritis represents a group of diseases that is one of the most common causes of disability in the US. The prevalence of arthritis increases with age and is higher among women than among men (10, 42). The most common form of arthritis, osteoarthritis, affects weight-bearing joints and has been studied extensively in relation to physical activity (42, 43). Regular physical activity is critical to maintain normal muscle strength, joint structure, and joint function (44), but there has been some concern that osteoarthritis may develop as a result of many years of vigorous physical activity. However, several studies suggest that physical activity in the amounts and intensities recommended for health are not associated with an increased risk of osteoarthritis. For example, long-term studies have shown that runners do not have a higher rate of joint degeneration than nonrunners (45–47). Several other studies report that moderate-intensity physical activity over time also does not seem to increase the risk of osteoarthritis (48–51). However, long-term participation in high-impact sports (such as some track and field events) and torsional sports (such as football and racquet sports) has been associated with an increased incidence of osteoarthritis (49, 52–54). Some theories suggest that the increased risk of osteoarthritis among sport participants is related to joint injuries, not necessarily the activity itself. Moderate physical activities, such as walking and bicycling, are associated with a lower risk of injury than are strenuous sports, especially among women (55, 56). Further research is needed to clarify the relationship between different components of physical activity and the development of osteoarthritis.

Several studies have demonstrated the benefits of physical activity among persons with osteoarthritis. Physical activity decreases pain, improves self efficacy and physical function, and delays disability among persons with knee osteoarthritis (57, 58). Regular physical activity does not seem to increase the progression (as observed by radiography) of osteoarthritis (59). Further evidence suggests that persons with rheumatoid arthritis may also benefit from regular physical activity to improve aerobic capacity or conditioning (59–62). In addition, a 12-week low-load resistive muscle training program among men and women with functional class II and III rheumatoid arthritis found improvements over a control group in self reported joint count, night time pain, and sit-to-stand time (63). Another study found that patients with fibromyalgia enrolled in a 6-month pool exercise program showed improvements in physical function and quality-of-life measures (64).

Discussion

  1. Top of page
  2. Introduction
  3. Statement of the problem
  4. Review of the literature
  5. Discussion
  6. REFERENCES

Table 1 provides a summary of the health benefits of physical activity for the 3 specific conditions discussed in this report: CHD, the leading cause of death; diabetes, a major cause of disability and death; and arthritis, a major cause of disability. Although much work remains to be done, sufficient information supports the design of programs and infrastructures to assist the most sedentary component of the population to become more active. The individual and societal costs of physical inactivity are high, in terms of both direct medical expenditures and related disability. Physical activity is important not only in the primary prevention of many chronic conditions but also in secondary prevention, to slow the progression and to reduce the symptoms of chronic conditions. Even among sedentary adults and those who have already developed some chronic conditions (e.g., hypertension, heart disease, diabetes, or arthritis), embracing a physically active lifestyle can have documented value. Most people with arthritis are older and likely to have (or be at risk for) an existing chronic condition, thus making physical activity even more important for their general health.

Table 1. Summary of health effects of physical activity on selected conditions
 Coronary heart disease (CHD)Diabetes mellitusArthritis
Overall relationshipPositive relationship between physical activity/fitness and both mortality and morbidity. Inactive persons have about twice the risk of developing or dying from CHD than active persons.Positive relationship for both primary and secondary prevention, even without weight loss.Generally positive relationship when used for secondary prevention of pain and disability for all types of arthritis. Mixed results when considering prevention. Some evidence that sports injuries may lead to osteoarthritis.
AgeBenefits of physical activity occur among adults of all ages; even sedentary adults who begin an exercise program late in life show CHD benefits.Benefits occur among adults of all ages.Most studies focus on older adults with arthritis; some evidence of reduced functional disability (may be due to arthritis) among middle-aged and older adults.
GenderLower CHD mortality with physical activity for both genders, but women may have benefits at lower intensity.Effect of regular exercise similar for both men and women.No apparent gender differences in effect of physical activity on secondary prevention.
Race/ethnicityBenefits of physical activity occur among all race/ethnic groups.Limited information on race/ethnicity.Race/ethnicity issues not well defined.
Overweight/obesityBenefits of physical activity are found even among those who are overweight or obese.Benefits of physical activity are found even among those who are overweight or obese.Effect of physical activity for secondary prevention found even among those who are overweight or obese.

Implications for making physical activity recommendations.

In view of the myriad poor health outcomes associated with a sedentary lifestyle, several agencies and organizations have concluded that everyone should be encouraged to participate in some form of activity designed to increase cardiorespiratory fitness, muscle strength, and flexibility (1, 2, 10). However, for older adults who have not been regularly active, cardiovascular screening may be appropriate (65, 66). This screening can be accomplished by using self-administered questionnaires designed to identify symptoms that require additional medical followup, which may or may not include an exercise stress test (65–68). However, there appears to be consensus that older adults (even those with chronic conditions) who plan to start a low intensity (i.e., walking) program are not candidates for exercise stress testing in the absence of symptoms.

Although healthy adults should participate in various types of physical activity on a daily basis, minimal guidelines can be summarized as follows: adults should perform at least 20 minutes of vigorous activity (such as running or other activities that raise the heart rate) on 3 or more days per week, or 30 minutes of moderate-intensity activities (such as brisk walking) on 5 or more days per week (10). In addition, strengthening and flexibility activities are recommended on at least 2 days per week (10). However, there are caveats for those who have special conditions or who have already developed chronic illnesses. For example, because of the potential for injury with vigorous and impact sports, individuals with a history of injuries or those who have not been regularly active should focus on activities with low joint stress (e.g., walking, swimming, or stationary bicycling). Proper treatment and careful rehabilitation after injury are necessary to maintain a long-term active lifestyle. Those with chronic illnesses should proceed after medical approval, but the overall goal should be to start slowly with a few minutes of light- to moderate-intensity activities and gradually work up to the recommended duration and intensity. A summary of suggestions for appropriate activities for individuals with various health conditions is shown in Table 2.

Table 2. Summary of suggested type, duration, and intensity of physical activity for those with specific conditions
Healthy adultsDo vigorous-intensity activities (such as running) for 20 or more minutes per day 3 or more times per week or do moderate-intensity activities (such as brisk walking) for 30 or more minutes per day, most days of the week. Add strengthening and flexibility exercises 2 or more days per week.
Sedentary adultsBegin with moderate-intensity activities a few minutes at a time and gradually increase to at least 30 minutes per day, most days of the week.
Older adultsAdd strengthening and flexibility activities to aerobic program at least 2 days per week.
Adults who are overweight or obeseWork up to moderate-intensity activities (such as brisk walking) for at least 60 minutes per day, most days of the week (plus attention to diet).
Adults with impaired glucose toleranceDo moderate-intensity activities (such as brisk walking) at least 30 minutes per day, most days of the week; include weight-training programs at least 2 days per week. Monitor blood glucose closely before and after activity sessions.
Adults with diabetesWork up to 30 minutes per day of aerobic activity with precautions to prevent foot injury; include weight-training programs at lest 2 days per week. Monitor blood glucose closely before and after activity sessions.
Adults with functional limitations or disabilityChoose joint-friendly activities such as walking, bicycling, swimming for at least 30 minutes per day, most days of the week; do daily range-of-motion exercises and strengthening activities 2 or more days per week.
Adults with osteoarthritisDo daily range-of-motion exercises, joint-friendly activities such as walking, stationary bicycling, swimming, at least 30 minutes per day, most days of the week; do strengthening activities 2 days per week; avoid activities causing severe pain to a specific joint.
Adults with hypertensionWork up to moderate-intensity activities (such as brisk walking), 30–45 minutes per day, most days of the week.
Adults with high cholesterolBegin slowly and work up to moderate-intensity activities (such as brisk walking), for at least 30 minutes per day, most days of the week.

Suggestions for future research.

The science of physical activity epidemiology is constantly evolving in response to new technology in a quest to further understand the relationship between human movement and disease etiology. Researchers will continue their efforts to determine the types and doses of activity required to reduce the impact of such diseases as arthritis. Three additional areas in physical activity epidemiology will be particularly prominent in the near future: identifying how environmental structure interacts with physical activity behavior, using new technology to enhance the measurement of physical activity, and understanding the complex connection between physical activity and weight control.

Among the up-and-coming research areas are the use of epidemiologic methods to study how environmental structure and policies that support active lifestyles influence physical activity. Although the decision to be active is an individual choice, this behavior is influenced by many factors outside of an individual's control. Furthermore, the knowledge that physical activity has important health benefits is not sufficient to change individual behavior. Studies examining the relationship of physical activity and neighborhood structure may determine that elements of the latter are important in the support of a physically active lifestyle. Examples of environmental structures that promote physical activity are activity-friendly and safe green spaces, walking and jogging trails, and bike paths that lead to commercial areas, schools, and recreational facilities. These types of structural changes in neighborhood environments could support increased physical activity for the whole family and may have more effect on overall physical activity patterns than are interventions targeted at changing individual behavior.

Another major field of research that may enrich epidemiologic studies is the emerging area of measurement. With the technological advances that are expected to materialize in the next several years, questionnaire assessment of physical activity in large-scale epidemiologic studies may be replaced or supplemented by innovative strategies, such as electronic mechanisms that send data directly to researcher's laboratories for analysis. Using these technological advances to improve measurement and reduce recall problems, researchers will be able to sort out key elements of physical activity and determine whether total energy expenditure or activity intensity is the most important parameter for specific diseases and conditions. Advances in measurement will enable epidemiologists to conduct large studies of health outcomes with much more accurate measures of energy expenditure than have been possible in the past.

In the US, participation in physical activity is low and has not changed much in the last decade (69), whereas obesity rates (generally defined for adults as a body mass index of >30 kg/m2) have increased substantially between 1991 and 1998 (70, 71). With the onset of what is now called an obesity epidemic in this country, new attention has been given to physical activity as part of an overall weight management strategy. In 2000, physical activity was included as a separate entry in the dietary guidelines published by the Department of Agriculture (72). This added interest has generated a variety of studies designed to sort out the role of energy expenditure in conjunction with maintaining a healthy diet as a strategy to control body weight. Some studies suggest that weight loss may partially explain some of the health effects that are attributed to physical activity (73). Other studies show that although excess body weight is associated with poor health outcomes and higher medical expenditures (74), there is additional evidence that being physically fit may offset some of the risk of being obese, especially when assessing all-cause mortality (75, 76). These studies are identifying a major lack of understanding of the nature of obesity and suggest that the answer is more complicated than energy intake and energy expenditure. Furthermore, although an individual's metabolic rate is a critical factor in weight control, it may be altered by lifestyle choices (i.e., smoking or physical activity). Identifying characteristics such as diet, geographic location, occupation, and current and past levels of physical activity among normal-weight individuals who become overweight or obese may lead to successful interventions that prevent weight gain as people age.

This article describes the benefits of physical activity in preventing or managing the leading causes of death or disability in this country and provides practical examples of appropriate activities. In addition, the areas highlighted for future research provide a sense of what needs to be done and focus of current research activities. Much work remains to understand more clearly the role of physical activity in reducing the burden of aging-related disability.

REFERENCES

  1. Top of page
  2. Introduction
  3. Statement of the problem
  4. Review of the literature
  5. Discussion
  6. REFERENCES
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