Levels of physical activity and inactivity in children and... : Medicine & Science in Sports & Exercise (original) (raw)
Regular physical activity is an important contributor to a healthy lifestyle and preventing chronic disease (27,28,47). Physical activity may be especially important in preventing obesity (47,49). Assessment of current levels of physical activity and inactivity is critical for defining the extent of the problem, guiding public health efforts, and evaluating progress toward national health objectives (14,30,45,46,48).
Measures of physical activity have been included in the three major national surveys used to assess the health and health behaviors of the U.S. population: National Health and Nutrition Examination Survey (NHANES), National Health Interview Survey (NHIS), and the Behavioral Risk Factor Surveillance System (BRFSS). A national survey of physical activity and fitness of children and adolescents aged 6–18 yr was carried out between 1984 and 1986: National Children and Youth Fitness study I and II (NCYFS I and II) (33,34). This survey has not been repeated. Current estimates of youth physical activity are derived from the Youth Risk Behavior Survey (YRBS). Each of the current national surveys is described briefly below.
NHANES.
Different questions on physical activity have been included in NHANES I, II, and III. These questions have not assessed the duration of participation in physical activity and thus cannot be used to assess the overall quantity of physical activity or progress toward national objectives for vigorous or moderate physical activity (16).
NHIS.
The Health Promotion/Disease Prevention supplements of NHIS in 1990, 1991, and 1995 included detailed assessment of participation in as many as 24 specific activities. Results for several activity categories and many demographic groups are published in Physical Activity and Health: A Report of the Surgeon General (23,47).
BRFSS.
The BRFSS is a random-digit-dialed telephone survey of health behaviors of the noninstitutionalized U.S. population 18 yr of age and older. The BRFSS is conducted year round in all 50 states and the District of Columbia, and it is the only system that provides data at the state level (10,32). The 1996 survey results presented in this report included data from approximately 120,000 respondents. The median response rate for eligible persons contacted in the state-based surveys was 77.9% in 1996.
YRBS.
The YRBS was developed by the Centers for Disease Control and Prevention (CDC) in 1990 to monitor health behaviors, including physical activity, in young people. It is a school-based survey of high school students in grades 9–12 carried out in the spring of odd numbered years. In 1997, 16,262 questionnaires were completed in 151 schools with an overall response rate of 69% (24). In 1992, the YRBS was administered as part of the NHIS to assess health behaviors of young people aged 12–21, including those not attending school. Results from the 1992 survey were reported in 1996 in Physical Activity and Health: A Report of the Surgeon General (47).
For this report we have relied on weighted estimates from the most recent national data sources: the 1996 BRFSS for adults and the 1997 YRBS for high school students (12,24,44). The reliability of both surveys has been demonstrated to be very good (4,38,40,41). Validity has not been established for any of the national surveys of physical activity, although similar physical activity questionnaires for adults appear to be valid (25). Prevalence estimates for physical activity and inactivity are influenced not only by survey design and administration but by the analytic approaches and definitions used in presenting the data (5,6). The data presented here will use the following definitions of physical activity.
Vigorous physical activity.
Persons are categorized as vigorously physically active if they report 3 or more sessions per week of 20 min or greater duration of activity at an intensity equal to 50% or more of their age and gender specific maximum cardiorespiratory capacity (46,47).
For high school students, vigorous physical activity is defined as responding positively to a question on the YRBS assessing participation in activities that made them sweat and breathe hard for at least 20 min on 3 or more days during the past 7 days (24).
Moderate physical activity.
Persons are defined to be moderately physically active if they report 5 or more sessions per week of any activity included in the BRFSS, and the total activity time for the week equals 150 min or more. All activity sessions must be at least 10 min long (5,9).
Recommended physical activity.
Recommended physical activity is defined as meeting either the requirements for moderate or vigorous physical activity. This group includes all adults participating in physical activity at or above levels recommended for health benefits (5,9,28,29,47).
Inactivity.
Inactivity or no physical activity refers to no reported leisure-time physical activity during the past month (5,6,47). This definition does not address the many sedentary behaviors that contribute to total time spent at low levels of energy expenditure (1). These behaviors may be important to health and obesity prevention but are not assessed well in current national surveys (18).
In the following section, levels of physical activity and inactivity from the most current national data sources are presented for adults and young people by a variety of demographic categories. These results are summarized in Tables 1 and 2 for adults and in Tables 3, 4, and 5 for high school students.
Participation in physical activity, U.S. adults, BRFSS 1996.
Participation in physical activity, U.S. adults, BRFSS 1996.
Levels of Physical Activity among Adults
The most recent national data from the 1996 BRFSS indicate that 29.2% of adults are inactive in their leisure time, 43.1% participate in some activity but not enough to ensure health benefits, and 27.7% are physically active at recommended levels (Fig. 1). Of the 27.7% who are regularly active, 23.4% report 5 or more sessions of activity totaling at least 150 min and are estimated to meet the recommendation for accumulating 30 min of moderate intensity activity 5 or more days per week. Among adults, 12.6% meet the vigorous activity recommendation (20 min or more of vigorous intensity activity 3 or more times per week). Although it is difficult to make comparisons between countries because of differences in survey questions and methodology, similar patterns of activity and inactivity are seen in all developed countries that have national health surveys that assess physical activity (43).
Physical activity, U.S. adults, BRFSS, 1996.
Sex.
Gender differences in participation in physical activity in the United States are relatively minor as can be seen in Table 1. More women (30.9%) than men (27.2%) report no physical activity, but levels of vigorous, moderate, and total (recommended) activity are quite similar. Increased participation in physical activity by women over the past decade has closed what was once a noticeable gender gap (7,8,23).
Age.
Patterns of participation in physical activity by age vary between men and women and are complex (Table 1). Inactivity increases progressively with age for both men and women. Above 75 yr of age 36.9% of men and nearly half (47.3%) of women report no activity. Vigorous physical activity also increases progressively with age for men, whereas it peaks in middle-aged for women. Vigorous activity rates may be overestimated for older men and women, due to over adjustment for age-related declines in maximal cardiorespiratory capacity (47). The same absolute level of activity (i.e., walking) may be classified as moderate for a younger person and vigorous for an older person with a reduced cardiorespiratory capacity. There is little variation in total and moderate activity for women until after age 75 when participation declines. For men, levels are highest in the 18–29 and 65–74 age groups with a marked dip during middle age, perhaps associated with demands of employment. There is a corresponding increase in physical activity when men reach retirement age (65–74). The absence of this increase for women may reflect a cohort effect. Social acceptance of physical activity in 1996 was likely to be less for women in the 65 and over age groups than men of that age.
Race-ethnicity.
National surveys of physical activity consistently show large differences in participation in physical activity by race-ethnicity (7,23,46,47). The 1996 BRFSS is no exception to this pattern. Whites report less inactivity (26.8%) and more total activity (29.0%) than either blacks (38.9% and 22.0%) or Hispanics (38.9% and 22.3%). Physical activity levels among Asian and Pacific Islanders and Native Americans are similar to those for whites (Table 1). Adjusting for age and socioeconomic factors diminishes, many, but not all, of the observed differences in physical activity by race (51).
Education and income.
These closely linked variables both reflect socioeconomic status and a cluster of factors (knowledge, time, social support, access to facilities, and neighborhood safety) that appear to influence participation in physical activity (35,36,47). Of the demographic factors assessed in the BRFSS, education is most closely associated with participation in physical activity. Nearly half (48.9%) of persons with less than a high school education report no physical activity, whereas only 17.3% of college graduates are inactive. The gradient is almost as steep for family income: inactivity falls from 42.6% to 15.1% from the lowest to highest income categories (Table 1). Because the BRFSS and other national surveys of physical activity focus on assessing leisure time physical activity and do not adequately address occupational activity, there is likely to be some misclassification of lower income or education persons with physically demanding jobs as inactive (12,23).
Body mass index (BMI).
Persons with higher levels of self-reported physically activity usually have lower indices of body weight or fat (13,47,50). In the 1996 BRFSS, persons with BMI of less than 25 reported less inactivity and more regular physical activity than overweight (BMI 25–29.9) or obese (BMI ≥ 30) persons (Table 1). Participation in physical activity differed only slightly between normal weight and overweight persons but was substantially lower among the obese. Increases in inactivity with increasing BMI (normal, overweight, obese) were more marked for women (27.8%, 31.7%, and 40.9%) than for men (26.8%, 25.6%, and 32.7%). Patterns of declining participation in recommended physical activity by BMI category also differed between genders: men (normal weight 29.3%, overweight 29.2%, obese 23.5%), women (normal weight 30.7%, overweight 26.0%, obese 18.9%).
Geographic analyses.
Regional differences in participation in physical activity are consistent across different surveys and years (12,47). In the 1996 BRFSS, the usual pattern of the greatest reported participation in physical activity in the west, lowest in the southeast, and intermediate levels in the midwest and northeast regions of the United States was observed (Table 2). A gradient in participation in regular physical activity has also been seen between urban and rural counties (12). Inactivity levels rise from 27.4% in metropolitan counties with populations greater than one million to 35.7% in rural counties (Table 2). This pattern is most marked in the south, present in the midwest and northeast, and absent in the west (12). The trend toward declining participation in regular physical activity as county population decreases is less clear. Regional and urban-rural differences in physical activity persist after adjustment for age, sex, education, and income (12).
Seasonality.
There are clear and consistent seasonal differences in participation in physical activity (11,47). Vigorous, moderate, and recommended activity peaks in the summer months, whereas inactivity peaks in the winter. The range between the highest and lowest months is 10.4% for inactivity (January vs August) and 7.6% for recommended physical activity (August vs December). The seasonal patterns for inactivity and recommended physical activity are displayed in Figure 2.
Physical activity patterns by month, U.S. adults, BRFSS, 1996.
Trends.
It is widely believed that participation in physical activity is declining among adults in the United States. However, national survey data are not able to support this contention. Levels of physical activity and inactivity as measured by the BRFSS (Fig. 3) and NHIS have been remarkably stable during the past decade (8,39,47). Data sources consistent enough to adequately assess trends before 1985 do not exist. A synthesis of noncomparable surveys suggests that leisure-time physical activity probably increased between the 1960s and 1980s (42). The stability of leisure time physical activity over the past decade or more may mask an overall decline in physical activity. Several authors have postulated that there have been substantial decreases in overall energy expenditure driven by large decreases in the amount of physical activity required for work, transportation, and routine daily tasks (22,31).
Participation in physical activity: adults aged 18+, BRFSS, 1986–1996.
Levels of Physical Activity Among Children and Adolescents
The 1997 YRBS provides the most recent estimates of participation in physical activity by young people. However, this survey is limited to high school students in grades 9–12. The 1992 NHIS included a YRBS supplement that was administered to a nationally representative sample of young people aged 12–21 yr. This paper primarily reports data from the 1997 YRBS with some additions from the 1992 YRBS and the 1995 National College Health Risk Behavior Survey (NCHRBS) (19). Variation in prevalence estimates between these surveys reflect not only the differences in populations but differences in season and mode of administration and question wording (47). Vigorous physical activity is assessed on these surveys with questions that allow a general comparison with adult levels of vigorous physical activity. There are additional questions on walking and bicycling, stretching, and strengthening exercises that are not included in this paper (24,47).
Overall, 63.8% of high school students responding to the 1997 YRBS reported participating in vigorous physical activity. On the 1992 NHIS-YRBS, 53.7% of young people aged 12–21 reported participating in vigorous physical activity, and on the 1995 NCHRBS, 41.8% of students 18–24 and 30.6% of students 25 and older were vigorously active. Vigorous activity levels among young people are clearly much higher than those for adults, although direct comparisons are hampered by different questions and survey methodology.
Sex.
All surveys of children and adolescents in the United States show higher levels of vigorous physical activity among boys than girls (19,24,47). In the 1997 YRBS 72.3% of boys and 53.5% of girls report vigorous physical activity (Table 3). Male students are also more vigorously active than female students in the NHIS-YRBS (60.2% vs 47.2%) and the NCHRBS (43.7% vs 33.0%). This difference is present in all grade, age, and race-ethnicity groups. The gender difference in participation in regular physical activity appears to be much greater among high school and college-age youth than among adults.
Percentage of high school students who participated in vigorous physical activity, by sex, race/ethnicity, and grade—United States, Youth Risk Behavior Survey, 1997.a
Age-grade.
There is a progressive decrease in the prevalence of reported participation in vigorous physical activity with both grade and age. Between grades 9 and 12 reported vigorous physical activity falls from 72.7% to 57.5% (Table 3). The decline is more marked among girls (66.1% to 43.6%) than among boys (78.7% to 57.5%). Similarly large decrease between ages 12 and 21 are observed in the NHIS-YRBS for both boys (70.8% to 42.2%) and girls (66.2% to 30.2%) (46). Data from the NCHRBS is consistent with a continued decline in participation with age during the college years. Vigorous physical activity is reported by 41.8% of students ages 18–24, and 30.2% of students age 25 and older (19). There appears to be a steady decline in participation in vigorous physical activity beginning at age 12–14 and extending into early adulthood. Preliminary results from a regional study suggests that this decline may begin as early as first grade (37). However, current data to assess physical activity or fitness at the national level simply do not exist for children less than 12.
Race-ethnicity.
White respondents to the 1997 YRBS reported greater participation in vigorous physical activity (66.8%) than either Hispanic (60.4%) or black (53.9%) students (Table 3) (24). These differences were greater for girls than boys.
Education and income.
Parental education and income are not collected on the YRBS. In the NHIS-YRBS increased participation in vigorous physical activity was associated with higher family income and education level of the responsible adult (26,47). This relationship persists after standardization for age, sex, race-ethnicity, and school status (26). The relative difference in participation in physical activity between low and high education and income groups is less for adolescents than for adults.
Geographic analyses.
Regional differences in participation in physical activity based on national surveys have not been reported for young people. Based on data from 33 states participating in the state-based component of the 1997 YRBS, it appears that regional patterns of participation in vigorous physical activity among high school students may be similar to that for adults, with higher levels in the west (24). This is an area in which much additional work is needed.
Trends.
The public perception of an increasingly sedentary way of life among children is even more widespread than for adults (3,15). However, there is even less good information available on national trends in youth physical activity or fitness than for adults. No fitness data exist since the completion of NCYFS II in 1986 (34). The YRBS physical activity questions have been standardized only since 1993 and provide information only on young people attending school in grades 9–12. There has been no significant change in reported vigorous physical activity between 1993 and 1997: 1993, 65.8%; 1995, 63.7%; and 1997, 63.8%.
Special Issues for Youth
Television watching.
Physical inactivity appears to be a distinct behavior from physical activity, and may be an important health promotion target (21). TV watching is a major contributor to sedentary activity, especially among children, and may be a useful marker for overall levels of sedentary behavior (18). It may also be important to track because of the positive association between hours of TV watched and indices of body fat and weight in children (2,17,20). Among children aged 8–16 yr examined in NHANES III, 61.0% reported watching 2 or more hours of television per day.
Physical education (PE).
PE is a unique setting that provides instruction and education about physical activity and the opportunity to participate in regular physical activity. There has been a clear decline in both enrollment in PE and daily participation in PE since 1991 (Fig. 4) (24,47). In 1991, 41.6% of high school students participated in daily PE, but by 1997 this had fallen to 27.4%. PE attendance also declines markedly by grade, especially for girls (Table 4).
Attend physical education daily, grades 9–12, YRBS, 1991–1997.
Percentage of high school students who were enrolled in a physical education (PE) class and attended PE class daily, by sex, race/ethnicity, and grade—United States, Youth Risk Behavior Survey, 1997.
Organized sports.
In contrast with the very small proportion of adults who participate in organized sports, a large number of high school students play on organized sports team during the course of a year (24,47) (Table 5). Nearly half (49.5%) of students play on school teams and 38.3% play on teams outside of school. Unlike for PE, there is minimal decline in participation on sports teams between grades 9 and 12 (53.2% to 45.5%). Organized sports teams appear to be an unrealized public health opportunity for maintaining and promoting physical activity for young people.
Percentage of high school students who played on sports teams sponsored by the school, and played on sports teams unaffiliated with the school,a by sex, race/ethnicity, and grade—United States, Youth Risk Behavior Survey, 1997.
MAJOR CONCLUSIONS
All of the conclusions which follow are based on large national surveys that have been adjusted to be representative of the overall U.S. population. They are observational studies and fall into Evidence Category C.
Adults
• 29% report no physical activity during leisure time.
• 28% engage regularly in recommended levels of physical activity.
• Physical inactivity increases with age for both men and women.
• Gender differences in physical activity are small in adults.
• Higher levels of education and income and white race are associated with greater participation in physical activity.
• Obese persons (BMI ≥ 30) report more inactivity and less regular physical activity than overweight (BMI 25–29.9) and normal weight (BMI <25) persons.
• Participation in regular leisure-time physical activity has been stable during the past decade. Trends in overall physical activity cannot be assessed with currently available data.
Children and Adolescents
• Approximately 60% of adolescents participate in regular vigorous physical activity. This level is much higher than for adults.
• Boys report participating in vigorous physical activity substantially more than girls. Other differences by race-ethnicity and socioeconomic factors are similar to those seen in adults.
• Vigorous physical activity declines progressively and significantly with advancing age and grade.
• Enrollment and daily participation in physical education have declined since 1991
• National survey data is inadequate to assess trends in physical activity and fitness among children and adolescents.
Research Issues
• Improve the assessment of moderate intensity and lifestyle physical activity.
• Improve the assessment of occupational, housework, and transportation related physical activity in population-based surveys.
• Develop reliable and valid measures of sedentary behaviors that can be used on population-based surveys.
• Investigate the role of activity monitoring as a component of national population-based surveys of physical activity.
• Routinely monitor physical activity among children less than 12 yr of age and periodically assess fitness among children and adolescents.
• Improve physical activity survey methodology, reliability, and validity for children
• Standardize questions, sampling, definitions, and analysis to facilitate comparisons of physical activity behaviors between countries, surveys, and over time.
• Carry out longitudinal studies to examine changes in physical activity and sedentary behaviors during the critical transitions from childhood to adolescence and from adolescence to young adulthood.
• Improve our understanding of age and gender related differences in participating in physical activity throughout life.
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Keywords:
OBESITY; EXERCISE; SEDENTARY BEHAVIOR; PUBLIC HEALTH
© 1999 Lippincott Williams & Wilkins, Inc.