Physical Activity and Health
Drawing on the Surgeon General’s 1996 report (DHHS 1996) and subsequent research, this chapter briefly summarizes the state of knowledge about the effects of physical activity on health, as well as recommendations on physical activity levels for adults and for children and adolescents. Methods and data for monitoring physical activity levels are then discussed, and current results for the U.S. population are compared with the recommended levels.
The terms physical activity, physical fitness, and exercise are often used interchangeably but have distinct meanings:
Physical activity can be defined as “bodily movement produced by the contraction of skeletal muscle that increases energy expenditure above the basal level” (DHHS 1996, 20). It is often categorized by the context in which it occurs, such as leisure time, transport, household, and occupation (see Figure 1-2 in Chapter 1).
Physical fitness can be defined as the “ability to carry out daily tasks with vigor and alertness, without undue fatigue, and with ample energy to enjoy leisure-time pursuits and to meet unforeseen emergencies” (Park 1989). Attributes of physical fitness include cardiorespiratory endurance; flexibility; balance; body composition; and muscular endurance, strength, and power. The term can be used to describe either athletic- and performance-
related fitness or health-related fitness (DHHS 1996). Depending on the individual’s performance or health goal, specific attributes of physical fitness become more important (Caspersen et al. 1985; Pate 1983). For example, achieving a certain level of cardiorespiratory fitness—a health-related fitness goal—requires an increase in cardiorespiratory endurance that can either help reduce the risk of cardiovascular disease or improve the life and overall health of a person who already has the disease.
Exercise is considered a subcategory of physical activity and has been defined as “physical activity that is planned, structured, repetitive, and purposive in the sense that improvement or maintenance of one or more components of physical fitness is the objective” (Caspersen et al. 1985).
This study uses the broadest possible definition of physical activity because all types of such activity contribute to health. Health is broadly defined as a “state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity” (WHO 1946). Total activity levels—purposeful physical activity or exercise, as well as utilitarian physical activity that occurs in the home, at work, and in travel—are of interest.
EFFECTS OF PHYSICAL ACTIVITY ON HEALTH
The Science Base
The Surgeon General’s 1996 report (DHHS 1996) examined the results of hundreds of research studies on the effects of physical activity on health from such fields as epidemiology, exercise physiology, medicine, and the behavioral sciences. The research was focused primarily on endurance-type physical activity, that is, on activity that involves repetitive use of large muscle groups, such as those used in walking and cycling.
Observational or epidemiologic studies have been conducted to determine health effects by comparing the activity levels of indi-
viduals who develop specific diseases or health conditions and those who do not. Epidemiologic studies include cohort, case-control, and cross-sectional research designed to estimate the relative magnitude or strength of an association between physical activity or cardiorespiratory fitness levels and a specific health outcome. Cohort studies follow a population to observe how physical activity levels or habits affect the incidence of disease or mortality. In contrast, case-control studies start with a group of individuals (the case group) with a specific disease or health condition of interest, who are then asked to recall their previous level and intensity of physical activity. Their previous level of physical activity is compared with that of a control group that does not have the disease or health condition of interest. Cross-sectional studies assess the association between physical activity and disease at the same point in time; thus they offer limited ability to draw causal inferences between the two variables.
Clinical trials, in comparison, assess the relationship between health outcomes and physical activity by experimentally altering the activity patterns, levels, and intensities within a relatively controlled environment, such as in a laboratory or an exercise program (DHHS 1996). Often considered the gold standard of studies, clinical trials are extremely expensive and time-consuming and require that every precaution be taken to protect the study participants. The high cost and feasibility constraints of large-scale clinical trials have thus far prohibited such designs with major morbidity or mortality as health outcomes. Instead, the effect of physical activity on intermediate outcomes, such as fitness or coronary heart disease risk factors, has been evaluated.
Most studies reviewed in the Surgeon General’s 1996 report were cohort studies; that is, they assessed whether an association existed between some baseline level of physical activity or fitness and the development of specific health outcomes. The majority of the studies controlled for important confounding factors that might have biased the results, such as age, body weight, smoking, blood pressure, blood lipid levels, alcohol consumption, and disease status. Numerous studies were analyzed to determine (a) the
consistency across studies of an association between physical activity and disease incidence; (b) the magnitude or strength of the association; (c) whether there was evidence that the level of activity preceded the development of disease; (d) the presence of a dose-response relationship, that is, whether higher amounts of physical activity conferred greater health benefits; and (e) the biological plausibility of the relationship, that is, the underlying physiological mechanisms that can explain why physical activity has a protective or restorative effect on specific health outcomes.
Benefits of Physical Activity
The Surgeon General’s report concluded that physical activity is causally related to health outcomes. It cited convincing and biologically plausible evidence of consistent and strong inverse relationships between physical activity or fitness and numerous diseases. For most health conditions, longitudinal data confirmed the temporal effects of regular physical activity on disease reduction. Moreover, increasing amounts of physical activity were associated with decreasing risk of disease.
Both older and younger adults who exercise regularly or lead a physically active lifestyle have lower mortality rates and live longer than those who are physically inactive (Kaplan et al. 1996; Paffenbarger et al. 1993; Sherman et al. 1994). More specifically, even in the absence of controlled clinical trials, there is sufficient evidence that regular physical activity reduces the risk of developing or dying from several of the leading chronic diseases in the United States (see Box 2-1). For example, the 1996 report found that regular physical activity or cardiorespiratory fitness reduces the risk of dying from coronary heart disease. The strength of the association is similar in magnitude to that of the relationship between coronary heart disease and smoking, hypertension, or elevated cholesterol. Moreover, the report found that regular physical activity prevents or delays the development of high blood pressure—a risk factor for coronary heart disease—and reduces blood pressure in those who already exhibit hypertension.
Health Benefits of Regular Physical Activity
SOURCES: DHHS 1996; DHHS 2002.
The benefits of regular physical activity to both the individual and society are compelling. Currently, chronic diseases account for 7 of every 10 deaths in the United States and more than 60 percent of all medical expenditures (CDC 2003b). Every year, as many as 255,000 U.S. adults die from causes that may be attributed to physical inactivity alone and 300,000 from inactivity and poor diet combined; these figures do not include others who suffer from chronic disease and impaired quality of life (Hahn et al. 1990; Powell and Blair 1994; McGinnis and Foege 1993). According to another estimate, 32 to 35 percent of all deaths in the United States attributable to coronary heart disease, colon cancer, and diabetes could be prevented if all members of the population were physically active (Powell and Blair 1994).
The benefits of physical activity appear to extend to all segments of the population. For example, even seniors and those with disabilities and chronic disease conditions benefit from physical activity, which improves their mobility and physical, mental, and social functioning (Butler et al. 1998). Regular participation in physical activity during childhood and adolescence helps build and maintain healthy bones, muscles, and joints; helps control weight, build muscle, and reduce fat; prevents or delays the development of high blood pressure and helps reduce blood pressure in adolescents with hypertension; and reduces feelings of depression and anxiety (DHHS 1996; Report to the President 2000). Indeed, more recent research supports stronger conclusions than those presented in the Surgeon General’s 1996 report on the effect of physical activity on mental health (Landers and Arent 2001).1
Although additional studies are needed to demonstrate a conclusive relationship between being physically active as a child and adolescent and achieving higher levels of academic performance, several studies are suggestive in this regard (Pate et al. 1996; Sallis et al. 1999). Participation in physical activity among adolescents has been shown to increase self-esteem, reduce anxiety and stress, and promote a sense of social well-being. Adolescents who participate in interscholastic sports are less likely to be regular or heavy smokers or drug users and to engage in violent behavior, and they are more likely to stay in school and have good conduct and high academic achievement (Escobedo et al. 1993; Pate et al. 1996; Zill et al. 1995).
RECOMMENDED LEVELS OF PHYSICAL ACTIVITY
The direct causal relationship between physical activity and health benefits raises questions about the type and amount of activity needed to produce those benefits. Physical activity regimens can
be designed to meet any individual’s needs, desires, and capacities; however, the primary focus at the national level has been on developing recommendations to help broad segments of the population achieve health benefits. Since the mid-1960s the available scientific evidence has been used as the basis for recommendations on physical activity that vary in their objective (e.g., sports performance, overall health promotion, weight maintenance or loss, disease-specific prevention); type of activity (e.g., endurance, strength, flexibility training); and the activity’s intensity (i.e., rate of energy expenditure), frequency (e.g., number of sessions engaged in over a week), and duration (i.e., length of time spent being physically active) (Bouchard and Shephard 1994; DHHS 1996).
In 1980, the U.S. Department of Health and Human Services (DHHS) stated that adults can achieve significant health benefits from participating in vigorous-intensity activity for at least 20 minutes per day for a minimum of 3 days a week (DHHS 1980).2 Vigorous-intensity activities are defined as those that raise the rate of energy expenditure, expressed as metabolic equivalents (METs), more than sixfold above resting levels.3 Assuming that 1 MET is equivalent to 1 kilocalorie per kilogram body weight per hour, jogging at a 7 MET level burns approximately 7 kilocalories per minute (kcal/min) for a person weighing 60 kilograms (132 pounds). A healthy individual might also burn 7 kcal/min while engaging in heavy yard work, participating in an aerobics class, swimming continuous laps, or bicycling vigorously (Ainsworth et al. 2000).
Over the next decade, epidemiologic evidence not only continued to provide strong evidence of the relationship between physical activity and health but also revealed that substantial benefits
accrue from even moderate amounts and types of activity.4 As a result, the Centers for Disease Control and Prevention (CDC) and the American College of Sports Medicine (ACSM) released a joint statement in 1995—rearticulated in the Surgeon General’s 1996 report—that most adults will achieve substantial health benefits if they accumulate a minimum of 30 minutes of moderate-intensity activity preferably on all days of the week (at least 5 days a week minimum) (Pate et al. 1995).5 Moderate-intensity activity is defined as any activity that raises the rate of energy expenditure three to six times above resting levels (3 to 6 METs). For a 60-kilogram person, activity at these levels burns 3 to 6 kcal/min—an amount a healthy person would burn while walking briskly, mowing the lawn, dancing, swimming for recreation, or bicycling (Ainsworth et al. 2000). The evidence available at the time suggested that this activity could be accumulated over the course of a day in sessions of at least 10 minutes, rather than having to be performed in one longer continuous session, although it was noted that more research was needed on the relative health benefits. It was clear, however, that intermittent episodes of activity are more beneficial than being sedentary (DHHS 1996).
The Surgeon General’s 1996 report acknowledged that for most people, greater health benefits can be obtained by engaging in physical activity of more vigorous intensity or of longer duration than the
In fact, it has been shown that physical activity is related to health outcomes in a dose-response fashion, such that the amount of benefit is proportional to the amount of physical activity; there is no threshold level of activity necessary before health benefits accrue (DHHS 1996). Nevertheless, some studies that have quantified the amount of physical activity associated with improved health outcomes have found that certain levels of kilocalorie energy expenditure from physical activity are associated with different levels of reduction in the risk of mortality and morbidity (Paffenbarger et al. 1986; Leon et al. 1987; Slattery et al. 1989; Helmrich et al. 1991). These studies provided the basis for the finding that a minimum increase in daily energy expenditure of approximately 150 kilocalories per day is associated with substantial health benefits, which can be achieved by moderate levels of physical activity, such as walking briskly for 30 minutes per day (DHHS 1996).
Special physical activity recommendations exist for seniors and those with disabilities and certain illnesses who want to increase their level of physical activity and achieve the associated health advantages. Additional information on recommendations for these populations can be found on CDC’s website at www.cdc.gov/nccdphp/dnpa/physical/recommendations.
30-minute minimum. Evidence from more recent epidemiological, observational, and intervention studies confirms this finding (Manson et al. 2002; Myers et al. 2002; Kraus et al. 2002), although the optimal amount of physical activity to achieve health benefits has not been established and probably varies from person to person.6
Children and Adolescents
The National Association for Sport and Physical Education recommends that children in elementary school accumulate at least 30 to 60 minutes of age- and developmentally appropriate physical activity from a variety of activities on all or most days of the week (NASPE Council on Physical Education for Children 2004). The activity should vary between moderate and vigorous intensity, with appropriate periods of rest and recovery to prevent injury. Children should avoid extended periods of inactivity.
In 1994, the International Consensus Conference on Physical Activity Guidelines for Adolescents recommended that adolescents be physically active daily or nearly every day as part of play, games, sports, work, transportation, recreation, physical education, or planned exercise in the context of family, school, and community activities. Adolescents should engage in three or more sessions per week of activities that last 20 minutes or more at a time and require moderate to vigorous levels of exertion.
National Health Goals
In addition to physical activity guidelines, the federal government has periodically set physical activity goals for the nation for both adults and adolescents. The recent national prevention agenda Healthy People 2010 sets forth various objectives designed to reduce the most significant preventable threats to health (DHHS 2000). The objectives related to physical activity are listed in Box 2-2.
In addition to setting these goals, Healthy People 2010 established physical activity as one of 10 leading health indicators (Box 2-3)
Healthy People 2010’s National Objectives for Physical Activity
SOURCE: DHHS 2000.
Healthy People 2010’s Leading Health Indicators
SOURCE: DHHS 2000.
that will be used to measure the overall health of the nation. In fact, physical activity is listed first, followed by overweight and obesity and tobacco use, respectively. These represent the major public health issues facing the nation and areas in which improvements can have a significant impact on the health of the American population (DHHS 2000).7
MEASURING PHYSICAL ACTIVITY
The ability to demonstrate a relationship between physical activity and health benefits is dependent on measures of physical activity and specific health outcomes that are accurate, precise, and reproducible (NCHS 1989; Wilson et al. 1986). Although limitations still exist, such as the fact that vigorous activities are reported more accurately than moderate or light ones, measures of physical activity have improved over time (Ainsworth et al. 1994; DHHS 1996).
Today, most research on physical activity levels relies on (a) self-reports, such as diaries, logs, and recall surveys; (b) direct monitoring through behavioral observations or mechanical or electronic devices; or (c) indirect measures, such as those of cardiorespiratory fitness based on exercise test tolerance, maximal oxygen uptake, or aerobic power.
Although self-reported measures of physical activity, such as surveys, are subject to recall bias and nonrepresentative sampling of activity depending on the time period about which they inquire, they are relatively easy to administer, inexpensive, and acceptable to study participants. The simplest form of self-reported information about physical activity allows subjects to be placed in hierarchical categories, such as “more or less active than their peers” or “numbers of days on which they were active in the past week.” More complex self-reported information about the frequency, duration, and intensity of physical activities can be converted into estimates of energy expenditure in terms of kilocalories, kilojoules, or METs. Responses are often classified according to an external standard, such as how well subjects meet physical activity recommendations.8
Direct monitoring eliminates many of the limitations associated with self-reported measures. However, it is expensive, time-consuming, and burdensome to study participants, surveyors, or both. As a result, self-reported measures of physical activity have been used more commonly in assessing the relationship between activity and health (DHHS 1996).9 As technology advances, objective monitoring is becoming increasingly available for large-scale
studies, although validity and reliability issues remain (Granner and Sharpe 2004). Measurement issues are discussed in detail in Chapter 5.
Patterns of physical activity in the United States can be gleaned from several national surveys, including the Behavioral Risk Factor Surveillance System (BRFSS), the National Health Interview Survey (NHIS), the National Health and Nutrition Examination Survey (NHANES), the Youth Risk Behavior Survey System (YRBSS), and the National Household Travel Survey (NHTS) (see Table 2-1). Each provides a part of the picture of the activity levels of Americans by different age groups, ethnicities, education and income levels, and geographic locations. Although each survey has its own purpose and uses different questions to inquire about levels of physical activity and inactivity, collectively their results demonstrate that Americans overall are not achieving the recommended levels (CDC 2003a). The primary databases used in this study to examine current levels and trends in physical activity are described below. A more complete discussion of all the relevant databases can be found in the papers by Brownson and Boehmer (2004) and Boarnet (2004) commissioned by the committee.
Two databases in particular provide current and trend data on the extent to which the U.S. population is meeting recommended physical activity guidelines. Begun in 1984, the CDC-coordinated BRFSS is the leading database for both current and longitudinal data on physical activity levels among U.S. adults. Its aim is to track major behavioral health risk factors, such as physical inactivity. Self-reported telephone interviews are used to derive estimates of how many adult Americans meet recommended physical activity levels (either the vigorous-intensity or moderate-intensity activity level recommendations put forward by DHHS, CDC, and ACSM), how many are insufficiently active, and how many are inactive or sedentary.
The strengths of the survey are its sample size, provision of state-and in some cases city-specific data, ability to estimate physical
TABLE 2-1 Data Sources forTrend Analysis of U.S. Physical Activity Levels
activity for subgroups (e.g., women, ethnic groups), and relative consistency of questions over time. Its weaknesses include reliance on self-reported data, lack of coverage for some population subgroups (e.g., low-income individuals who may not have a telephone), and, until 2001, its focus on leisure-time physical activity only. In 2001, survey questions were modified to encompass three types of physical activity—leisure-time, domestic, and transportation.10 A fourth type of physical activity—occupational—is also captured in the BRFSS. Because of technical issues, however, self-reported occupational activity does not contribute to the overall physical activity score for each individual responding to the survey (CDC 2003a). As a result of the changes in 2001, moreover, estimates based on that survey are not comparable with the estimates from 1984 to 2000.11
The YRBSS was developed in 1990 to help determine the onset and national prevalence of key risk behaviors that contribute to death, disability, and social problems among U.S. youth. Conducted biennially, the YRBSS is a national school-based survey of representative samples of students in 9th through 12th grades. Self-reported survey data relevant to physical activity levels among youth include estimates of participation in daily physical education classes and the percentage meeting recommended standards for physical activity. The strength of the survey is its focus on youth as an important population group and oversampling in some cities to allow comparisons by geographic setting. Its limitations include reliance on self-reported data, school-based administration of the survey, and more limited longitudinal data than the BRFSS.
CDC recently launched a new national telephone survey—the Youth Media Campaign Longitudinal Survey (YMCLS)—of children aged 9 to 13 and their parents. The purpose is to obtain data on physical activity levels among children of this age group. Children are asked about their participation in physical activity during nonschool hours, and parents are asked about perceived barriers to their children’s engaging in physical activity.
The NHTS, which combines two former surveys—the Nationwide Personal Transportation Survey and the American Travel Survey—is conducted by the U.S. Department of Transportation to provide data on both the short- and long-term travel behavior of the U.S. population. Conducted every 5 to 7 years, it draws on a nationally representative sample of households and uses travel diaries to derive national estimates of travel type, frequency, mode, and time. These data are the primary source of information on physical activity, particularly walking and bicycling, associated with transportation. Longitudinal data going back as far as 1969 are available for some characteristics. Data on nonmotorized travel (walking and bicycling) have improved in recent years. For example, the most recent survey (2001) gathered additional such data, particularly for walking trips (e.g., trips for walking the dog were to be included in the travel diaries). At the same time, however, the results are not comparable with earlier data.
CURRENT LEVELS OF PHYSICAL ACTIVITY
Results from the 2001 BRFSS indicate that, even with a more complete measure of physical activity than that previously used, fewer than half of all U.S. adults engage in enough physical activity to meet the public health recommendations cited earlier (i.e., at least 30 minutes of moderate-intensity activity per day for at least 5 days a week or at least 20 minutes of vigorous-intensity activity per day for at least 3 days a week) (see Table 2-2). Approximately one-quarter of all U.S. adults reported being completely inactive during their leisure
TABLE 2-2 Summary Statistics on Current Physical Activity Levels Compared with Recommended Guidelines, U.S. Adults and Adolescents
time (CDC 2003d).12 The data show that activity levels decrease with age and are lower among women, ethnic and racial minorities, those with less education and low income levels, the disabled, and those living in the southeastern region of the nation (CDC 2003d).
Insufficient levels of physical activity are not limited to adults. The YRBSS for 2001 revealed that nationwide, nearly one-third of students enrolled in 9th through 12th grades engaged in insufficient amounts of physical activity relative to recommended levels (i.e., had not participated in vigorous physical activity for 20 minutes or more at least three times in the week preceding the survey, or in moderate physical activity for 30 minutes or more at least five times in that week) (CDC 2002) (see Table 2-2). Nearly 10 percent characterized themselves as inactive; that is, they reported not participating in either vigorous or moderate physical activity in the week preceding the survey. Levels of inactivity were higher for female and minority students and increased for all students by grade level (CDC 2002). Nationwide, 52 percent of students were enrolled in a physical education class, but only about one-third attended such a class daily. Approximately 55 percent of students indicated that
they had played on one or more sports teams during the 12 months preceding the survey. Levels of television watching were high; nearly two-fifths of students reported that they had watched television 3 or more hours on an average school day (CDC 2002).
The 2002 baseline assessment of the YMCLS found that 62 percent of U.S. children aged 9 to 13 did not participate in any organized physical activity during their nonschool hours, and 23 percent did not take part in any physical activity during their free time (CDC 2003c). Significantly lower levels of regular nonschool physical activity were found among non-Hispanic black and Hispanic children and children with parents who had lower incomes and educational levels (CDC 2003c). Parents of all races, incomes, and educational levels perceived many of the same barriers to their children’s participating in physical activities. However, transportation difficulties, lack of area opportunities, and expense were reported significantly more often by non-Hispanic black and Hispanic parents than by non-Hispanic white parents. Concerns about neighborhood safety were greater for Hispanic than for non-Hispanic white or black parents (CDC 2003c). Summary data on physical activity levels for younger children are not available.13
Results from the 2001 NHTS, the primary source of data on physical activity for travel,14 showed, not surprisingly, that the vast majority of daily trips (87 percent) were taken by personal vehicle (BTS 2003). Walking, however, accounted for the next highest percentage—almost 9 percent of all trips. Cycling accounted for less than 1 percent of all trips, while trips by transit, including school bus—which involve some walking to reach the transit or bus line—represented slightly more than 3 percent of all trips (2001 NHTS trip results). Because walking and cycling are more prevalent on shorter trips, the NHTS also asked about travel mode for such trips. Adults (those at least 18 years of age) reported walking on 27 percent of
trips of 1 mile or less and cycling on 0.6 percent of trips of 5 miles or less (2001 NHTS special data runs). Children aged 5 to 15 reported walking on 36 percent of school trips of 1 mile or less and cycling on 1.5 percent of school trips of 2 miles or less (2001 NHTS special data runs).
ACSM American College of Sports Medicine
BTS Bureau of Transportation Statistics
CDC Centers for Disease Control and Prevention
DHHS U.S. Department of Health and Human Services
IOM Institute of Medicine
NASPE National Association for Sport and Physical Education
NCHS National Center for Health Statistics
WHO World Health Organization
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Long-Term Trends Affecting Physical Activity Levels
The trend data reviewed in this chapter show that technological innovations, as well as broad social and economic changes, have steadily and substantially reduced the physical demands of work, home, and travel, with a modest and recent offset in increased leisure-time, higher-intensity physical activity for some sectors of the population. Long-term changes in the built environment have also contributed to declining physical activity levels. The suburbanization of the population and employment in lower-density communities and office locations have increased reliance on private vehicles for most trips.
Available trend data, particularly on the role of the built environment in declining levels of physical activity, are limited in their explanatory power. First, the indicators are too general to illuminate how land use patterns and travel affect an individual’s decision to be physically active. Second, some of the key data of interest, for example, information on nonmotorized travel, are available but have not been collected with any reliability until very recently. Where trend data are directly available, more analysis is needed to determine whether observed changes are indeed significantly different in a statistical sense. Finally, with the possible exception of time-use data, available trend data do not provide an integrated perspective on the environmental factors affecting physical activity levels or on the trade-offs among them. For example, walking has declined as a mode of transport to work but may be on the increase for recreational purposes. The net effect on total physical activity levels, however, is unclear. In sum, the longitudinal data are sug-
gestive but not definitive regarding the many factors contributing to changes in total physical activity levels.
Time-use data may best help frame the opportunities and limitations of various strategies for increasing total physical activity to meet the 30-minute daily minimum. For example, doubling the use of nonmotorized transportation would increase daily physical activity levels associated with travel to an average of only 6 minutes at the population level. Work provides another potential opportunity for increasing physical activity, for example, through short exercise breaks and lunchtime walks. Finally, more active use of free time—reducing the time spent watching television and engaging in other sedentary activities or augmenting the time already spent by those who participate in sports and exercise—offers another important opportunity for increasing daily physical activity levels. For many, a combination of small increments in physical activity in many locations may be the most feasible way of meeting the daily requirement.