Journal of Physical Activity and Health, 2015, 12, 1102 -1111 http://dx.doi.org/10.1123/jpah.2014-0050 © 2015 Human Kinetics, Inc.
ORIGINAL RESEARCH
Trends in Physical Activity and Sedentary Behaviors of United States Youth David R. Bassett, Dinesh John, Scott A. Conger, Eugene C. Fitzhugh, and Dawn P. Coe Background: Increases in childhood and adolescent obesity are a growing concern in the United States (U.S.), and in most countries throughout the world. Declines in physical activity are often postulated to have contributed to the rise in obesity rates during the past 40 years. Methods: We searched for studies of trends in physical activity and sedentary behaviors of U.S. youth, using nontraditional data sources. Literature searches were conducted for active commuting, physical education, high-school sports, and outdoor play. In addition, trends in sedentary behaviors were examined. Results: Data from the Youth Risk Behavior Surveillance System (YRBSS) and other national surveys, as well as longitudinal studies in the transportation, education, electronic media, and recreation sectors showed evidence of changes in several indicators. Active commuting, high school physical education, and outdoor play (in 3- to 12-year-olds) declined over time, while sports participation in high school girls increased from 1971 to 2012. In addition, electronic entertainment and computer use increased during the first decade of the 21st century. Conclusions: Technological and societal changes have impacted the types of physical activities performed by U.S. youth. These data are helpful in understanding the factors associated with the rise in obesity, and in proposing potential solutions. Keywords: active transportation, physical education, sports, television, outdoor play
Obesity levels in U.S. children and adolescents have more than tripled over the past 40 years.1 The percentage of youth with a body mass index (BMI) in the 95th percentile has increased from approximately 5% in 1963–1970 to 18.0% in 6- to 11-year-old children and 18.4% in 12- to 19-year-old children in 2009–2010.2,3 Even young children (2–5 years of age) have experienced a dramatic increase in prevalence of childhood obesity (currently 12.1%). This epidemic has affected children of every age, both genders, and all race/ethnic groups. Physical activity is inversely related to adiposity in children,4,5 suggesting that the increase in childhood obesity rates may be partially due to temporal declines in physical activity levels. The health consequences of childhood/adolescent obesity are quite severe. For instance, obese youth are more likely to suffer from glucose intolerance, joint problems, hypertension, fatty liver, coronary atherosclerosis, gastric reflux, asthma, sleep apnea, and psychological problems (eg, depression, eating disorders, and low self-esteem).6 Cases of type 2 diabetes in youth were first reported around 1980, and the prevalence of this condition has increased over time.7 Furthermore, overweight adolescents have an 83% chance of becoming overweight adults, as evidenced by tracking of BMI from adolescence into adulthood.8 Currently, the relative contribution of changes in energy intake and energy expenditure to obesity trends in children and adolescents is uncertain. A 2011 review concluded that there is a lack of consensus on which of these determinants is the primary driver of time trends in weight gain among U.S. youth.9 In regards to physical activity, there is good evidence that adults have experienced a decline in physical activity levels as a result of rapid changes in Bassett ([email protected]), Fitzhugh, and Coe are with the Dept of Kinesiology, Recreation, and Sport Studies, University of Tennessee, Knoxville, TN. John is with the Dept of Health Sciences, Northeastern University, Boston, MA. Conger is with the Dept of Kinesiology, Boise State University, Boise, ID. 1102
modern technology, transportation, communications, and occupations,10,11 but the same cannot be said of youth.12 Thus, it is important to examine time trends in youth physical activity using nontraditional data sources9 and to determine what is happening in various PA domains. Understanding the magnitude and direction of changes in physical activity can help guide the design and implementation of programs to prevent or treat obesity. The purpose of this study was to review the literature on time trends in physical activities and sedentary behavior in U.S. children and adolescents.
Methods To locate information relevant to obesity and physical activity trends in youth, we conducted computerized literature searches in PubMed, and consulted reviews on this topic.9,12–15 The data for the current review were derived, in part, from peer-reviewed journal articles and data from a large-scale U.S. health interview survey (the Youth Risk Behavior Surveillance System, or YRBSS). Other data were obtained from published articles that used U.S. national travel surveys. Data on sports participation were obtained from the National Federation of State High School Associations, and the U.S. Department of Education. Outdoor activities were assessed by a longitudinal study of outdoor play by Hofferth et al.16 Participation rates for other outdoor activities were obtained from the U.S. Fish & Wildlife service and the U.S. National Park Service.
Results Over the past few centuries, the advent of modern, industrialized civilizations has brought about profound cultural and societal changes that may have affected physical activity habits of youths. It is difficult to assess secular trends in overall physical activity during this 200- to 300-year time period, due to the absence of longitudinal data. However, some evidence can be gleaned from
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Table 1 Steps per Day for Elementary School–Age Children in Different Cultures (Values Reflect the Mean of 4 Weekdays)
studying children and adolescents who live in societies that have not adopted modern, technological advances.
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Comparative Studies of Human Cultures
Country
A number of studies have compared children living in highly technological societies to those living in hunter-gatherer or agrarian societies. Children from the !Kung San and Nharo Bushmen tribes in the Kalahari desert have traditionally accumulated high levels of physical activity playing games, roaming, exploring, and accompanying elders on foraging trips.17,18 Another huntergatherer population is the Inuit, which until 1970 lived an active, hunter-gather lifestyle, contributing to high levels of youth physical fitness and low levels of adiposity. Unfortunately, however, these advantages were lost within a period of 2 decades as they adopted a westernized lifestyle.19 In traditional agrarian societies, youth also tend to be more active than those in modern civilizations. Children in an Old Order Amish farming community accumulate 30% to 50% more steps throughout the week, compared with children in modern, industrialized societies (Table 1).20–25 The Amish in this community refrain from using high-wire electricity and owning motor vehicles; hence, their physical activity habits tend to reflect those of North Americans living 150 years ago. The children walk to-and-from school each day, and perform chores such as milking, collecting eggs, weeding, harvesting vegetables, doing laundry, and sweeping. They also engage in active outdoor play both at school and at home. This may explain, in part, the lower rates of obesity and greater grip strength Old Order Amish youth, compared with their modern Canadian counterparts.20,26
Study
Boys
Girls
Old Order Amish (Canada)
Bassett et
al.20
19,100
16,100
Belgium
Cardon et al25
16,600
13,000
Kenya
Croteau et al23
16,262
13,463
al21
16,200
13,600
Sweden
Vincent, et
16,035
12,729
Australia
Vincent, et
al21
14,300
11,700
United States (Arizona)
Vincent, et al21
13,200
11,000
12,259
10,906
United Kingdom
Rowlands et
Canada
Craig, et
al24
al22
Table 2 Physical Activity Domains in Youth Domains Active transportation (eg, walk or bike to school) School-based activity (eg, physical education, recess, classroom activity breaks) Youth development programs (eg, Boy Scouts, Girl Scouts) Organized sports/exercise Household chores Outdoor play
Secular Trends in Physical Activity, in U.S. Youth
school, 2) school physical education, 3) high school sports, and 4) outdoor play. These physical activity domains were selected due to the existence of longitudinal data in U.S. youth.
U.S. national health surveys, despite showing clear increases in childhood obesity rates over the past 40 years, do not provide clear evidence of a decline in overall physical activity levels.13,27 This failure to document changes over time may be attributable, in part, to the limitations of self-report instruments in measuring physical activity.13 However, by examining societal trends it is possible to glean insights into what has been happening to youth physical activity habits in this country. Physical activity domains in children differ from those seen in adults (Table 2). In this section, societal trends in 4 indicators will be discussed: 1) active commuting to
Active Commuting to School. Data on active commuting to school can be obtained from the National Personal Transportation Survey (NPTS), and its successor, the National Household Travel Survey (NHTS). The survey asks about the number of trips taken, distance, time of day, destination, and mode of transportation. The percentage of youth using nonmotorized transportation (ie, walking or bicycling) to go back and forth to school has declined from 47.7%
Table 3 Usual Mode of Transportation to School, 1969–2009 (%) Grades K–8 (aged 5–14 years)
Elementary (aged 5–11 years)
1969
2009
1969
2009
1969
2009
Auto
12.2
45.3
12.2
47.5
12.3
40.5
Walk/bike
47.7
12.7
49.3
13.1
41.6
11.8
Walk
N/A
11.7
N/A
12.1
N/A
10.7
Middle (aged 12–14 years)
Bike
N/A
1.0
N/A
1.0
N/A
1.1
School bus
38.3
39.4
37.3
37.9
42.3
42.8
Other
1.7
2.6
1.2
1.5
3.8
4.9
Total
100
100
100
100
100
100
Abbreviations: N/A, not assessed. Source: McDonald NC, Brown AL, Marchetti LM, Pedroso MS. U.S. school travel, 2009: an assessment of trends. Am J Prev Med. 2011;41(2):146–51. JPAH Vol. 12, No. 8, 2015
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Figure 1 — Time trends in modes of travel to school. Source: McDonald N. Active transportation to school: trends among US schoolchildren, 1969 to 2001. Am J Prev Med 2007;32(6):509–16.
in 1969 to 12.7% in 2009 (Table 3, Figure 1).28,29 Simultaneously, a large increase occurred in the number of students who rely on automobiles for transportation.28 Urban sprawl, traffic safety concerns, and school siting decisions are contributing factors to diminishing active transport.30 It is unfortunate that active commuting to school has declined, since it could enable many more children to meet the 2008 national physical activity guidelines.31
upwards in the early 1970s following the enactment of Title IX. Thus, high school girls’ sports participation has increased steadily since 1971.39 In summary, this factor likely has not contributed to the rise in obesity rates, and may have even impeded the rise in obesity in girls. Unfortunately, similar data are not available for middle schools, and few elementary schools provide opportunities for team sports.35
School Physical Education. The percentage of high-school youth attending daily physical education classes declined from 42% in 1991 to 31% in 2011 (Figure 2).32 Most elementary and middle schools have policies and practices that support the teaching of physical education.33 Approximately 58% of elementary schools and 65% of middle schools require that physical education be taught in each grade. However, only 4% of elementary schools, 8% of middle schools, and 2% of high schools provide daily physical education for all grades during the entire school year.33 Currently, there is no federal law mandating that physical education be taught in schools, despite evidence that physical education could help children develop healthy and lifelong habits.33,34 Budget constraints35 and a focus on standardized test performance are 2 possible factors contributing to the decline in physical education. However, 2 recent reviews have suggested that enhanced physical education could have the greatest impact of any school-based interventions on children’s physical activity energy expenditure.33,36
Outdoor Play. Trends in outdoor play have received increased
High School Sports. Data on high school sports team participa-
tion are available from the National Federation of State High School Associations (Figure 3).37 A participant is defined as varsity high school sport team member, and thus the numbers are slightly inflated because student athletes who participate on more than 1 team are counted multiple times. Expressed as a percentage of the total high school enrollment in corresponding years,38 the data show that high school boys’ sport team participation has remained relatively constant at about 50% from 1970 to 2012. High school girls’ sport team participation was almost nonexistent in the 1960s, but surged
attention in recent years. Louv40 has proposed that children today spend less time in unstructured, outdoor play, compared with previous generations. A decline in outdoor play is concerning for a number of reasons, and might reflect increased time spent indoors engaged with electronic entertainment. Outdoor play burns calories and thus impacts energy balance, but it also has a calming effect on children and enhances their ability to focus and concentrate.41 Hofferth et al16 used time-allocation methods in which the primary activities of 1343 children 3 to 12 years of age were classified into 18 major categories. They reported a 37% decline in time spent playing out-of-doors from 1997 to 2003 (Figure 4). The percentage of children ages 6 to 12 participating in outdoor recreation declined from 78% to 62% between 2006 and 2010. A similar decline, from 69% to 61%, was seen in adolescents during the same time period (Figure 5).42 Other forms of outdoor activities have also declined over time. The number of visits to national parks also declined over the past decade,43 and children’s participation in hunting and fishing declined by about 30% from 1990 to 2005.44
Secular Trends in Sedentary Behaviors in U.S. Youth The amount of time spent in the use of electronic media has increased over the past 30 years, in U.S. children and adolescents. Among 8- to 18-year-olds, the total amount of time devoted to television, videogames, computers, MP3 players and cell phones now stands at 53 hours per week.45
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Figure 2 — Trends in daily physical education for students in grades 9 to 12. Source: Centers for Disease Control and Prevention. Trends in the Prevalence of Physical Activity, National YRBS: 1991 to 2009. 2010 [cited January 20, 2014]; Available from https://nccd.cdc.gov/youthonline/app/Results. aspx?TT=&OUT=&SID=HS&QID=&LID=XX&YID=&LID2=&YID2=&COL=&ROW1=&ROW2=&HT=&LCT=&FS=&FR=&FG=&FSL=&FR L=&FGL=&PV=&TST=&C1=&C2=&QP=&DP=&VA=&CS=&SYID=&EYID=&SC=&SO=.
Figure 3 — Trends in high school sports participation. Sources: Sports participation data are from the National Federation of State High School Associations (cited January 27, 2014) available from http://www.nfhs.org/ParticipationStatics/ParticipationStatics.aspx. High school enrollment data are from U.S. Census Bureau (cited January 27, 2014) available from http://www.census.gov/hhes/school/index.html. JPAH Vol. 12, No.8, 2015
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Figure 4 — Percentage of time spent in outdoor play in 1997 and 2003. Data are from the Child Development Supplement to the Panel Study Of Income Dynamics. Source: Hofferth SL. Changes in American children’s time—1997 to 2003. Electron Int J Time Use Res. 2009;6(1):26–47.
Figure 5 — Changes in youth outdoor recreation from 2006 to 2010. Source: The Outdoor Foundation’s Outdoor Recreation Participation Report 2010 [cited January 25, 2014]; Available from: http://www.outdoorfoundation.org/.
Television viewing is strongly related to obesity/overweight, in cross-sectional studies.46,47 In 1996, children who watched more than 5 hours of television per day were 4.6 times as likely to be obese than those who watched 0 to 2 hours per day, and the difference persisted after adjusting for previous overweight, maternal overweight, socioeconomic status, household structure, and maternal and child aptitude test scores (Figure 6).46 Television viewing contributes to obesity in 2 ways: a) the energy cost of sedentary television viewing is typically less than that of the activities
it replaces; and b) television viewing is often accompanied by consumption of extra calories.48,49 Do children today watch more television than in the past? One study found that the percentage of middle and high school students who viewed 4 or more hours of television per day declined between 1990 and 2008.50 Similarly, a second study found that television viewing declined from a mean of 4.0 hours per day in the late 1970s to 2.75 hours per day in 1999.51 A third study using YRBSS data found that TV viewing declined between 1999 and 2007.27 However,
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Figure 6 — Relationship between watching television and obesity in U.S. children aged 8 to 16 years, from NHANES III (1988 to 1994). Source: Crespo CJ, Smit E, Troiano RP, Bartlett SJ, Macera CA, Anderson RE. Television watching, energy intake, and obesity in US children. Arch Pediatr Adolesc Med. 2001;155:360–5.
these statistics can be misleading. While the time spent watching live television has declined, the total amount of time devoted to television content has actually risen.45 This is due to the fact that American youth now view television content on digital video recorders, DVDs, computers, MP3 players, and cell phones (Figure 7). Currently, 8- to 18-year-olds view 4 to 4.5 hours of total television content per day.45,52 Television watching is especially high among minority children; black and Hispanic youths watch 2 hours more television per day than white youths.45 Computer time is not as strongly associated with weight status as television/videogame time. Sisson et al53 showed that obese children were more likely to watch ≥ 2 hours of television per day than were overweight or normal weight children. However, all 3 groups had similar levels of computer usage per day. These results suggest that computer use may not impact BMI to the same extent as television and videogames. Preschool children spent over 32 hours per week using television, videogames, and computers in 2009, and elementary school children spent 28 hours per week on these forms of electronic entertainment.54 Most of this time is spent in watching television, and only 1 to 2 hours per week is spent playing videogames. Given the evidence that excessive television watching is associated with obesity, it is surprising that 71% of youth ages 8 to 18 have a television in their bedroom, and 50% have a videogame system in their bedroom. The American Academy of Pediatrics recommends that screen time be limited to no more than 2 hours per day.55 However, only 1 in 10 young people have rules limiting television viewing or videogame use, and 64% of young people say the television is usually on during meals.45 The percentage of high school students using computers 3 or more hours per day for nonschool related activities increased between 2003 (22.1%) and 2011 (31.1%) (Figure 8). While the link between computer use and childhood obesity is weaker than the link between television viewing and childhood obesity, experts are nevertheless concerned by the large amounts of “screen time” accumulated throughout the day, because this is a marker of sedentary behavior.56 Prolonged sitting is associated with health problems,
Figure 7 — Secular trends in total media use among U.S. youth. Source: The Kaiser Family Foundation Report. Kaiser Family Foundation. Generation M2: media in the lives of 8- to 18-year-olds. 2010 [cited 2013 Nov 5]; Available from: http://www.kff.org/entmedia/mh012010pkg.cfm.
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Figure 8 — Percentage of high school students who used computers 3 or more hours per day on an average school day: 2005 to 2011. Source: Centers for Disease Control and Prevention. Trends in the Prevalence of Physical Activity, National YRBS: 1991 to 2009. 2010 [cited January 10, 2014]; Available from: http://www.cdc.gov/HealthyYouth/yrbs/pdf/us_physical_trend_yrbs.pdf
including suppressed lipoprotein lipase activity, high triglyceride levels, and impaired insulin sensitivity.57,58 The International Obesity Task Force (IOTF) attributes the obesity epidemic to an environment that limits opportunities for physical activity, and an abundance of inexpensive, highly palatable, energy-dense foods and drinks.59 We agree with this, and we would add that another factor is the increased time spent on electronic entertainment.
Discussion In reviewing the literature, we observed that there was a decline in youth physical activity within several areas, including active transportation, physical education, and outdoor play. These are domains in which it was once common for children and adolescents to accumulate large volumes of energy expenditure that could be helpful in controlling body weight. Due to declines in these areas, there is now an increased reliance on structured, after-school activities such as individual or team sports to make up the deficit. A consequence of this is that youth today are at greater risk of excessive weight gain, particularly if they are not engaged in sports or other structured activities.
Where Are We Headed? Today’s children and adolescents will become the next generation of adults. In 2003, Foreyt and colleagues60 predicted that at the current rate of increase, every U.S. adult will be overweight or obese by the year 2040. This projection might need to be modified, since the obesity rate in girls 2 to 19 years of age appears to have leveled off between 2007–2008 and 2009–2010; however, the rate of obesity in boys is still climbing.3
Due to the obesity epidemic, the Centers for Disease Control and Prevention have projected that cases of diabetes will double, and perhaps even triple, by the year 2050.61 In 2010, 9% of U.S. adults had diabetes62 and over 90% of adults with diabetes had type 2 diabetes mellitus.63 The prevalence is expected to rise sharply over the next 4 decades, with as many as 1 in 3 adults having the disease. Along with diabetes, increased rates of obesity in the future are expected to lead to increases in the rates of comorbidities (heart disease, stroke, Alzheimer’s disease, and breast and reproductive cancers). In addition, some researchers predict that today’s children may be the first generation whose life expectancy does not exceed their parents’.64 What can be done to increase physical activity levels and decrease sedentary behaviors in American youth? The socioecological model provides a useful framework for thinking about promoting physical activity.65 The model proposes that health behaviors, including physical activity, are impacted at several different levels. Starting at the center, individual factors such as knowledge, skills, and attitudes about physical activity are important factors influencing behavior. Moving outward, interpersonal relationships with parents, teachers, and peers also influence the physical activity of children and adolescents. Organizations (eg, schools, sport/activity organizations, churches and health departments) also have a role to play. The school setting, especially through physical education and after-school sports, plays an important role in increasing physical activity levels of youth. Communities (eg, parks and recreation departments, transportation planning organizations, and health coalitions) provide the infrastructure that provides opportunities for children to be active. Finally, federal, state, and local policies determine funding, laws, and programs that impact all of the previous levels. Recently, the Physical Activity Guidelines for Americans Midcourse Report: Strategies to Increase Physical Activity Among Youth66 examined the importance of 5 settings and their relations
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to youth physical activity. The authors reviewed the research on policy-based approaches, and they summarized the strength of the evidence base for each setting. The report identified several settings where interventions have succeeded at increasing physical activity in youth. The key findings and recommendations were: 1. School settings provide an opportunity to increase physical activity in youth, and should be part of the national physical activity plan. In particular, enhanced physical education and multicomponent school programs involving enhanced physical education, active transport, and activity breaks are effective. (Sufficient Evidence) 2. Preschool and childcare settings are another setting for increasing physical activity in young children. Well-designed programs that include playground equipment, staff training, active learning, and outdoor time can enhance physical activity. (Suggestive Evidence) 3. Community Settings also play a role in the physical activity levels of youth. Through good planning and development (ie, mixed land-use, high-density housing, traffic-calming measures, and infrastructure for active transport), communities can be made safer and more conducive to children’s physical activity. (Suggestive Evidence) 4. Family and home settings hold promise for increasing afterschool and weekend activities. Strategies might include engaging family members to favor physical activity over sedentary pursuits (Insufficient Evidence) 5. Primary care settings also hold promise for increasing youth activity levels, since health care providers could provide counseling on physical activity, and provide integration with community programs. (Insufficient Evidence)
Limitations The current study has a number of limitations. Some of the data (eg, YRBSS, NPTS, and NHTS) are based on self-report, which is subject to bias and may not accurately reflect the physical activity or sedentary behaviors of interest. Other data sources are indicative of participation rates, which lack sensitivity since they do not account for individual variation in the frequency, intensity, and time spent in physical activities. Finally, due to the nature of the studies and datasets used in this review, we could not ascertain whether overall physical activity levels in youth have declined. Although several national health surveys indicate constant levels of overall physical activity over time, these instruments may simply lack the sensitivity to detect changes.
Future Research Future research is needed in this area. The 2014 United States’ Report Card on Physical Activity67 serves as an important baseline for tracking indicators of overall physical activity, specific domains, and sedentary behaviors in youth. This report card provides a grade (A-F) indicating the current status of several indicators of youth physical activity, some of which are the same as the ones identified in our literature search. Continuing to track these indicators over time will provide valuable information on how the nation is doing toward its goals. In addition, there is an urgent need to track secular trends in overall physical activity. The National Health and Nutrition Examination Survey (NHANES) could provide a unique opportunity to follow physical activity trends in youth, using objective methods.
Conclusions Comparative studies of children living in hunter-gatherer, agrarian, and industrial societies suggest that physical activity levels have declined as modern, industrialized lifestyles were adopted. Over the past 4 decades, societal trends suggest that physical activity levels have declined within several key domains. There have been declines in active commuting, school-based physical education, and outdoor play. On the other hand, high school girls’ sports participation increased from 1971 to 2012. In regards to sedentary behaviors, the amount of time devoted to electronic entertainment and computer use rose during the first decade of the 21st century. Knowledge of these societal trends is important for those engaged in shaping policies, programs, and interventions to increase physical activity in youth. Acknowledgments Jennifer I. Flynn (University of South Carolina, Columbia, SC) and Dana L. Wolff (National Cancer Institute, Bethesda, MD) provided helpful comments on the manuscript. Pamela Andrews assisted with finding source material for this review.
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13. Eisenmann JC. Secular trends in variables associated with the metabolic syndrome of North American children and adolescents: a review and synthesis. Am J Hum Biol. 2003;15:786–794. PubMed doi:10.1002/ajhb.10214 14. Sturm R. Childhood obesity—what we can learn from existing data on societal trends, part 1. Prev Chronic Dis. 2005;2(1):A12. PubMed 15. Sturm R. Childhood obesity—what we can learn from existing data on societal trends, part 2. Prev Chronic Dis. 2005;2(2):A20. PubMed 16. Hofferth SL. Changes in American children’s time- 1997 to 2003. Electron Int J Time Use Res. 2009;6(1):26–47. PubMed doi:10.13085/ eIJTUR.6.1.26-47 17. Guenther M. The Nharo Bushmen of Botswana. Hamburg: Helmut Buske Verlag; 1986. 18. Van der Post L, Taylor J. Testament to the Bushmen. New York, NY: Viking Adult; 1984. 19. Rode A, Shephard RJ. Physiological consequences of acculturation: a 20-year study of fitness in an Inuit community. Eur J Appl Physiol. 1994;69:516–524. PubMed doi:10.1007/BF00239869 20. Bassett DR, Tremblay MS, Esliger DW, Copeland JL, Barnes JD, Huntington GE. Physical activity and body mass index of children in an Old Order Amish community. Med Sci Sports Exerc. 2007;39(3):410–415. PubMed doi:10.1249/mss.0b013e31802d3aa7 21. Vincent SD, Pangrazi RP, Raustorp A, Tomson LM, Cuddihy TF. Activity levels and body mass index of children in the United States, Sweden, and Australia. Med Sci Sports Exerc. 2003;35(8):1367–1373. PubMed doi:10.1249/01.MSS.0000079024.40014.91 22. Craig CL, Cameron C, Griffiths JM, Tudor-Locke C. Descriptive epidemiology of youth pedometer-determined physical activity: CANPLAY. Med Sci Sports Exerc. 2010;42(9):1639–1643. PubMed doi:10.1249/MSS.0b013e3181d58a92 23. Croteau K, Schofield G, Towle G, Suresh V. Pedometer-determined physical activity of Western Kenyan children. J Phys Act Health. 2011;8(6):824–828. PubMed 24. Rowlands AV, Eston RG, Ingledew DK. Relationships between activity levels, aerobic fitness, and body fat in 8- to 10-yr-old children. J Appl Physiol. 1999;86(4):1428–1435. PubMed 25. Cardon GM, DeBourdeaudhuij IMM. A pilot study comparing pedometer counts with physical activity minutes in elementary school children. (Abstract) Med Sci Sports Exerc. 2004;36(5):S31. 26. Tremblay MS, Esliger DW, Copeland JL, Barnes JD, Bassett DR. Moving forward by looking back: lessons learned from lost lifestyles. Appl Physiol Nutr Metab. 2008;33:836–842. PubMed doi:10.1139/ H08-045 27. Lowry R, Lee S, Fulton J, Kann L. Healthy people 2010 objectives for physical activity, physical education, and television viewing among adolescents: national trends from the Youth Risk Behavior Surveillance System, 1999-2007. J Phys Act Health. 2009;6(Suppl 1):S36–S45. PubMed 28. McDonald N. Active transportation to school: trends among US schoolchildren, 1969-2001. Am J Prev Med. 2007;32(6):509–516. PubMed doi:10.1016/j.amepre.2007.02.022 29. McDonald N, Brown A, Marchetti L, Pedroso MUS. school travel, 2009 an assessment of trends. Am J Prev Med. 2011;41(2):146–151. PubMed doi:10.1016/j.amepre.2011.04.006 30. Kouri C. Wait for the bus: how low-country school site selection and design deter walking to school and contribute to urban sprawl. Terry Sanford Institute of Public Policy at Duke University. Report prepared for the South Carolina Coastal Conservation League 1999; http://www.eric.ed.gov/ERICWebPortal/search/detailmini. jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=ED451681&E RICExtSearch_SearchType_0=no&accno=ED451681. Accessed Feb 12, 2011. 31. U.S. Department of Health and Human Services. 2008 Physical Activity Guidelines for Americans. 2008; 61. Available at: http://www. health.gov/PAGuidelines/Report/Default.aspx Accessed November 17, 2010. 32. Centers for Disease Control and Prevention (CDC). Youth Online: High School YRBS, United States 1991-2013 Results, Physical Activity, Available at: https://nccd.cdc.gov/youthonline/app/Results.aspx?TT
=&OUT=&SID=HS&QID=&LID=XX&YID=&LID2=&YID2=&C OL=&ROW1=&ROW2=&HT=&LCT=&FS=&FR=&FG=&FSL=& FRL=&FGL=&PV=&TST=&C1=&C2=&QP=&DP=&VA=&CS=& SYID=&EYID=&SC=&SO=. Accessed October 12, 2015. 33. Lee S, Burgeson C, Fulton J, Spain C. Physical education and physical activity: results from the school health policies and programs study 2006. J Sch Health. 2007;77(8):435–463. PubMed doi:10.1111/j.17461561.2007.00229.x 34. Dowda M, Columbia S, Pate R, Saunders R. Guidelines for school and community programs to promote lifelong physical activity among young people. MMWR Recomm Rep. 1997;46:1–36. PubMed 35. Weiss MR, Hayashi CT. The United States. In: Knop PD, Angstrom L-M, Skirstad B, Weiss MR, eds. Worldwide trends in youth sport. Champaign, IL: Human Kinetics; 1996:43–57. 36. Bassett D, Fitzhugh E, Heath G, et al. Estimated energy expenditures for school-based policies and active living. Am J Prev Med. 2013;44(2):108–113. PubMed doi:10.1016/j.amepre.2012.10.017 37. National Federation of State High School Associations. Participation Data. 1969-2014 High School Athletics Participation Survey Results. Available at: http://www.nfhs.org/ParticipationStatics/ParticipationStatics.aspx. Accessed October 12, 2015. 38. U.S. Census Bureau. School enrollment. http://www.census.gov/hhes/ school/index.html. Accessed January 27, 2014. 39. Stevenson B. Title IX and the evolution of high school sports. Contemp Econ Policy. 2007;25(4):486–505. doi:10.1111/j.14657287.2007.00080.x 40. Louv R. Last child in the woods: saving our children from NatureDeficit Disorder. Chapel Hill, NC: Algonquin Press; 2005. 41. McCurdy LE, Winterbottom KE, Mehta SS, Roberts JR. Using nature and outdoor activity to improve children’s health. Curr Probl Pediatr Adolesc Health. 2010;5:102–117. PubMed doi:10.1016/j. cppeds.2010.02.003 42. Outdoor Foundation. Outdoor Recreation Participation Report 2010; http://www.outdoorfoundation.org/. Accessed January 25, 2014. 43. Pergam O, Zardiac P. Evidence for a fundamental and pervasive shift away from nature-based recreation. Proc Natl Acad Sci USA. 2008;105:2295–2300. PubMed doi:10.1073/pnas.0709893105 44. US Fish and Wildlife Service. Fishing and hunting recruitment and retention in the U.S. from 1990 to 2005: addendum to the 2001 National Survey of Fishing. Arlington, VA: Fishing, Hunting, and Wildlife-Associated Recreation; 2007. 45. Kaiser Family Foundation. Generation M2: media in the lives of 8- to 18-year-olds. 2010; http://www.kff.org/entmedia/mh012010pkg.cfm. Accessed November 5, 2013. 46. Gortmaker SL, Must A, Sobol AM, Peterson K, Colditz GA, Dietz WH. Television viewing as a cause of increasing obesity among children in the United States. Arch Pediatr Adolesc Med. 1996;150(4):356–362. PubMed doi:10.1001/archpedi.1996.02170290022003 47. Crespo CJ, Smit E, Troiano RP, Bartlett SJ, Macera CA, Anderson RE. Television watching, energy intake, and obesity in US children. Arch Pediatr Adolesc Med. 2001;155:360–365. PubMed doi:10.1001/ archpedi.155.3.360 48. Epstein LH, Roemmich JN, Paluch RA, Raynor HA. Influence of changes in sedentary behavior on energy and macronutrient intake in youth. Am J Clin Nutr. 2005;81(2):361–366. PubMed 49. Williams DM, Raynor HA, Ciccolo JT. A review of TV viewing and its association with health outcomes in adults. Am J Lifestyle Med. 2008;2:250–259. doi:10.1177/1559827608314104 50. Child Trends Data Bank. Watching Television. 2010; http://www. childtrendsdatabank.org/?q=node/261. Accessed February 13, 2011. 51. Kaiser Family Foundation. Kids and Media @ The New Millenium. 1999; http://www.kff.org/entmedia/1535-index.cfm. Accessed 2011, Feb 13. 52. Nielsen Co. Three screen report: Volume 8, 1st Quarter. 2010. Available at: http://www.nielsen.com/us/en/insights/reports/2010/three-screenreport-q1-2010.html. Accessed October 12, 2015. 53. Sisson SBCT, Martin CK, Tudor-Locke C, et al. Profiles of sedentary behavior in children and adolescents: the US National Health and Nutrition Examination Survey, 2001-2006. Int J Pediatr Obes. 2009;4(4):353–359. PubMed doi:10.3109/17477160902934777
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54. Nielsenwire. TV viewing among kids at an eight-year high. 2009; http://blog.nielsen.com/nielsenwire/media_entertainment/tv-viewingamong-kids-at-an-eight-year-high/. Accessed October 24, 2010. 55. American Academy of Pediatrics; Committee on Public Education. Children, adolescents, and television. Pediatrics. 2001;107:423–426. PubMed doi:10.1542/peds.107.2.423 56. Strasburger VG, Jordan AB, Donnerstein E. Health effects of media on children and adolescents. Pediatrics. 2010;125(4):756–757. PubMed doi:10.1542/peds.2009-2563 57. Alibegovic A, Sonne M, Hojbjerre L, et al. Insulin resistance induced by physical inactivity is associated with multiple transcriptional changes in skeletal muscle in young men. Am J Physiol Endocrinol Metab. 2010;299(5):52–63. PubMed doi:10.1152/ajpendo.00590.2009 58. Hamilton M, Hamilton D, Zderic D. Role of low energy expenditure and sitting in obesity, metabolic syndrome, type 2 diabetes, and cardiovascular disease. Diabetes. 2007;56(11):2655–2667. PubMed doi:10.2337/db07-0882 59. International Obesity Task Force and European Association for the Study of Obesity. Obesity in Europe; the Case for Action. 2002; 30. Available at: http://www.worldobesity.org/site_media/uploads/ Sep_2002_Obesity_in_Europe_Case_for_Action_2002.pdf. Accessed October 12, 2015. 60. Catanese DM, Hyder ML, Poston WSC, Foreyt JP. Future directions in treating obesity. In: Anderson RE, ed. Obesity: Etiology, Assessment, Treatment, and Prevention. Champaign, IL: Human Kinetics; 2004:271–289. 61. Boyle J, Thompson T, Gregg E, Barker L, Williamson D. Projection of the year 2050 burden of diabetes in the US adult population: dynamic
modeling of incidence, mortality, and prediabetes prevalence. Popul Health Metr. 2010;8:29. PubMed doi:10.1186/1478-7954-8-29 62. Centers for Disease Control and Prevention. Crude and Age-Adjusted Percentage of Civilian, Noninstitutionalized Adults with Diagnosed Diabetes, United States, 1980–2011. 2014; http://www.cdc.gov/diabetes/statistics/prev/national/figageadult.htm. Accessed January 26, 2014. 63. NIDDK. National Diabetes Information Clearinghouse, National Diabetes Statistics, 2011. Available at http://diabetes.niddk.nih.gov/ dm/pubs/statistics/ - Types. Accessed January 26, 2014. 64. Olshansky S, Passaro D, Hershow R, et al. A potential decline in life expectancy in the United States in the 21st century. N Engl J Med. 2005;352(11):1138–1145. PubMed doi:10.1056/NEJMsr043743 65. Sallis J, Owen N. Ecological models of health behavior. In: Glanz K, Rimer B, Lewis F, eds. Health Behavior and Health Education. San Francisco, CA: Jossey-Bass; 2002. 66. Physical Activity Guidelines for Americans Midcourse Report Subcommittee of the President’s Council on Fitness SN. Physical Activity Guidelines for Americans Midcourse Report: Strategies to Increase Physical Activity Among Young. Washington, DC: U.S. Department of Health and Human Services; 2012. 67. Dentro KN, Beals K, Crouter SE, et al. Results from the U.S. 2014 Report Card on Physical Activity for Children and Youth. J Phys Act Health. 2014;11(Suppl 1):S105–S112.
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ORIGINAL RESEARCH
Trends in Physical Activity and Sedentary Behaviors of United States Youth David R. Bassett, Dinesh John, Scott A. Conger, Eugene C. Fitzhugh, and Dawn P. Coe Background: Increases in childhood and adolescent obesity are a growing concern in the United States (U.S.), and in most countries throughout the world. Declines in physical activity are often postulated to have contributed to the rise in obesity rates during the past 40 years. Methods: We searched for studies of trends in physical activity and sedentary behaviors of U.S. youth, using nontraditional data sources. Literature searches were conducted for active commuting, physical education, high-school sports, and outdoor play. In addition, trends in sedentary behaviors were examined. Results: Data from the Youth Risk Behavior Surveillance System (YRBSS) and other national surveys, as well as longitudinal studies in the transportation, education, electronic media, and recreation sectors showed evidence of changes in several indicators. Active commuting, high school physical education, and outdoor play (in 3- to 12-year-olds) declined over time, while sports participation in high school girls increased from 1971 to 2012. In addition, electronic entertainment and computer use increased during the first decade of the 21st century. Conclusions: Technological and societal changes have impacted the types of physical activities performed by U.S. youth. These data are helpful in understanding the factors associated with the rise in obesity, and in proposing potential solutions. Keywords: active transportation, physical education, sports, television, outdoor play
Obesity levels in U.S. children and adolescents have more than tripled over the past 40 years.1 The percentage of youth with a body mass index (BMI) in the 95th percentile has increased from approximately 5% in 1963–1970 to 18.0% in 6- to 11-year-old children and 18.4% in 12- to 19-year-old children in 2009–2010.2,3 Even young children (2–5 years of age) have experienced a dramatic increase in prevalence of childhood obesity (currently 12.1%). This epidemic has affected children of every age, both genders, and all race/ethnic groups. Physical activity is inversely related to adiposity in children,4,5 suggesting that the increase in childhood obesity rates may be partially due to temporal declines in physical activity levels. The health consequences of childhood/adolescent obesity are quite severe. For instance, obese youth are more likely to suffer from glucose intolerance, joint problems, hypertension, fatty liver, coronary atherosclerosis, gastric reflux, asthma, sleep apnea, and psychological problems (eg, depression, eating disorders, and low self-esteem).6 Cases of type 2 diabetes in youth were first reported around 1980, and the prevalence of this condition has increased over time.7 Furthermore, overweight adolescents have an 83% chance of becoming overweight adults, as evidenced by tracking of BMI from adolescence into adulthood.8 Currently, the relative contribution of changes in energy intake and energy expenditure to obesity trends in children and adolescents is uncertain. A 2011 review concluded that there is a lack of consensus on which of these determinants is the primary driver of time trends in weight gain among U.S. youth.9 In regards to physical activity, there is good evidence that adults have experienced a decline in physical activity levels as a result of rapid changes in Bassett ([email protected]), Fitzhugh, and Coe are with the Dept of Kinesiology, Recreation, and Sport Studies, University of Tennessee, Knoxville, TN. John is with the Dept of Health Sciences, Northeastern University, Boston, MA. Conger is with the Dept of Kinesiology, Boise State University, Boise, ID. 1102
modern technology, transportation, communications, and occupations,10,11 but the same cannot be said of youth.12 Thus, it is important to examine time trends in youth physical activity using nontraditional data sources9 and to determine what is happening in various PA domains. Understanding the magnitude and direction of changes in physical activity can help guide the design and implementation of programs to prevent or treat obesity. The purpose of this study was to review the literature on time trends in physical activities and sedentary behavior in U.S. children and adolescents.
Methods To locate information relevant to obesity and physical activity trends in youth, we conducted computerized literature searches in PubMed, and consulted reviews on this topic.9,12–15 The data for the current review were derived, in part, from peer-reviewed journal articles and data from a large-scale U.S. health interview survey (the Youth Risk Behavior Surveillance System, or YRBSS). Other data were obtained from published articles that used U.S. national travel surveys. Data on sports participation were obtained from the National Federation of State High School Associations, and the U.S. Department of Education. Outdoor activities were assessed by a longitudinal study of outdoor play by Hofferth et al.16 Participation rates for other outdoor activities were obtained from the U.S. Fish & Wildlife service and the U.S. National Park Service.
Results Over the past few centuries, the advent of modern, industrialized civilizations has brought about profound cultural and societal changes that may have affected physical activity habits of youths. It is difficult to assess secular trends in overall physical activity during this 200- to 300-year time period, due to the absence of longitudinal data. However, some evidence can be gleaned from
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Table 1 Steps per Day for Elementary School–Age Children in Different Cultures (Values Reflect the Mean of 4 Weekdays)
studying children and adolescents who live in societies that have not adopted modern, technological advances.
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Comparative Studies of Human Cultures
Country
A number of studies have compared children living in highly technological societies to those living in hunter-gatherer or agrarian societies. Children from the !Kung San and Nharo Bushmen tribes in the Kalahari desert have traditionally accumulated high levels of physical activity playing games, roaming, exploring, and accompanying elders on foraging trips.17,18 Another huntergatherer population is the Inuit, which until 1970 lived an active, hunter-gather lifestyle, contributing to high levels of youth physical fitness and low levels of adiposity. Unfortunately, however, these advantages were lost within a period of 2 decades as they adopted a westernized lifestyle.19 In traditional agrarian societies, youth also tend to be more active than those in modern civilizations. Children in an Old Order Amish farming community accumulate 30% to 50% more steps throughout the week, compared with children in modern, industrialized societies (Table 1).20–25 The Amish in this community refrain from using high-wire electricity and owning motor vehicles; hence, their physical activity habits tend to reflect those of North Americans living 150 years ago. The children walk to-and-from school each day, and perform chores such as milking, collecting eggs, weeding, harvesting vegetables, doing laundry, and sweeping. They also engage in active outdoor play both at school and at home. This may explain, in part, the lower rates of obesity and greater grip strength Old Order Amish youth, compared with their modern Canadian counterparts.20,26
Study
Boys
Girls
Old Order Amish (Canada)
Bassett et
al.20
19,100
16,100
Belgium
Cardon et al25
16,600
13,000
Kenya
Croteau et al23
16,262
13,463
al21
16,200
13,600
Sweden
Vincent, et
16,035
12,729
Australia
Vincent, et
al21
14,300
11,700
United States (Arizona)
Vincent, et al21
13,200
11,000
12,259
10,906
United Kingdom
Rowlands et
Canada
Craig, et
al24
al22
Table 2 Physical Activity Domains in Youth Domains Active transportation (eg, walk or bike to school) School-based activity (eg, physical education, recess, classroom activity breaks) Youth development programs (eg, Boy Scouts, Girl Scouts) Organized sports/exercise Household chores Outdoor play
Secular Trends in Physical Activity, in U.S. Youth
school, 2) school physical education, 3) high school sports, and 4) outdoor play. These physical activity domains were selected due to the existence of longitudinal data in U.S. youth.
U.S. national health surveys, despite showing clear increases in childhood obesity rates over the past 40 years, do not provide clear evidence of a decline in overall physical activity levels.13,27 This failure to document changes over time may be attributable, in part, to the limitations of self-report instruments in measuring physical activity.13 However, by examining societal trends it is possible to glean insights into what has been happening to youth physical activity habits in this country. Physical activity domains in children differ from those seen in adults (Table 2). In this section, societal trends in 4 indicators will be discussed: 1) active commuting to
Active Commuting to School. Data on active commuting to school can be obtained from the National Personal Transportation Survey (NPTS), and its successor, the National Household Travel Survey (NHTS). The survey asks about the number of trips taken, distance, time of day, destination, and mode of transportation. The percentage of youth using nonmotorized transportation (ie, walking or bicycling) to go back and forth to school has declined from 47.7%
Table 3 Usual Mode of Transportation to School, 1969–2009 (%) Grades K–8 (aged 5–14 years)
Elementary (aged 5–11 years)
1969
2009
1969
2009
1969
2009
Auto
12.2
45.3
12.2
47.5
12.3
40.5
Walk/bike
47.7
12.7
49.3
13.1
41.6
11.8
Walk
N/A
11.7
N/A
12.1
N/A
10.7
Middle (aged 12–14 years)
Bike
N/A
1.0
N/A
1.0
N/A
1.1
School bus
38.3
39.4
37.3
37.9
42.3
42.8
Other
1.7
2.6
1.2
1.5
3.8
4.9
Total
100
100
100
100
100
100
Abbreviations: N/A, not assessed. Source: McDonald NC, Brown AL, Marchetti LM, Pedroso MS. U.S. school travel, 2009: an assessment of trends. Am J Prev Med. 2011;41(2):146–51. JPAH Vol. 12, No. 8, 2015
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Figure 1 — Time trends in modes of travel to school. Source: McDonald N. Active transportation to school: trends among US schoolchildren, 1969 to 2001. Am J Prev Med 2007;32(6):509–16.
in 1969 to 12.7% in 2009 (Table 3, Figure 1).28,29 Simultaneously, a large increase occurred in the number of students who rely on automobiles for transportation.28 Urban sprawl, traffic safety concerns, and school siting decisions are contributing factors to diminishing active transport.30 It is unfortunate that active commuting to school has declined, since it could enable many more children to meet the 2008 national physical activity guidelines.31
upwards in the early 1970s following the enactment of Title IX. Thus, high school girls’ sports participation has increased steadily since 1971.39 In summary, this factor likely has not contributed to the rise in obesity rates, and may have even impeded the rise in obesity in girls. Unfortunately, similar data are not available for middle schools, and few elementary schools provide opportunities for team sports.35
School Physical Education. The percentage of high-school youth attending daily physical education classes declined from 42% in 1991 to 31% in 2011 (Figure 2).32 Most elementary and middle schools have policies and practices that support the teaching of physical education.33 Approximately 58% of elementary schools and 65% of middle schools require that physical education be taught in each grade. However, only 4% of elementary schools, 8% of middle schools, and 2% of high schools provide daily physical education for all grades during the entire school year.33 Currently, there is no federal law mandating that physical education be taught in schools, despite evidence that physical education could help children develop healthy and lifelong habits.33,34 Budget constraints35 and a focus on standardized test performance are 2 possible factors contributing to the decline in physical education. However, 2 recent reviews have suggested that enhanced physical education could have the greatest impact of any school-based interventions on children’s physical activity energy expenditure.33,36
Outdoor Play. Trends in outdoor play have received increased
High School Sports. Data on high school sports team participa-
tion are available from the National Federation of State High School Associations (Figure 3).37 A participant is defined as varsity high school sport team member, and thus the numbers are slightly inflated because student athletes who participate on more than 1 team are counted multiple times. Expressed as a percentage of the total high school enrollment in corresponding years,38 the data show that high school boys’ sport team participation has remained relatively constant at about 50% from 1970 to 2012. High school girls’ sport team participation was almost nonexistent in the 1960s, but surged
attention in recent years. Louv40 has proposed that children today spend less time in unstructured, outdoor play, compared with previous generations. A decline in outdoor play is concerning for a number of reasons, and might reflect increased time spent indoors engaged with electronic entertainment. Outdoor play burns calories and thus impacts energy balance, but it also has a calming effect on children and enhances their ability to focus and concentrate.41 Hofferth et al16 used time-allocation methods in which the primary activities of 1343 children 3 to 12 years of age were classified into 18 major categories. They reported a 37% decline in time spent playing out-of-doors from 1997 to 2003 (Figure 4). The percentage of children ages 6 to 12 participating in outdoor recreation declined from 78% to 62% between 2006 and 2010. A similar decline, from 69% to 61%, was seen in adolescents during the same time period (Figure 5).42 Other forms of outdoor activities have also declined over time. The number of visits to national parks also declined over the past decade,43 and children’s participation in hunting and fishing declined by about 30% from 1990 to 2005.44
Secular Trends in Sedentary Behaviors in U.S. Youth The amount of time spent in the use of electronic media has increased over the past 30 years, in U.S. children and adolescents. Among 8- to 18-year-olds, the total amount of time devoted to television, videogames, computers, MP3 players and cell phones now stands at 53 hours per week.45
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Figure 2 — Trends in daily physical education for students in grades 9 to 12. Source: Centers for Disease Control and Prevention. Trends in the Prevalence of Physical Activity, National YRBS: 1991 to 2009. 2010 [cited January 20, 2014]; Available from https://nccd.cdc.gov/youthonline/app/Results. aspx?TT=&OUT=&SID=HS&QID=&LID=XX&YID=&LID2=&YID2=&COL=&ROW1=&ROW2=&HT=&LCT=&FS=&FR=&FG=&FSL=&FR L=&FGL=&PV=&TST=&C1=&C2=&QP=&DP=&VA=&CS=&SYID=&EYID=&SC=&SO=.
Figure 3 — Trends in high school sports participation. Sources: Sports participation data are from the National Federation of State High School Associations (cited January 27, 2014) available from http://www.nfhs.org/ParticipationStatics/ParticipationStatics.aspx. High school enrollment data are from U.S. Census Bureau (cited January 27, 2014) available from http://www.census.gov/hhes/school/index.html. JPAH Vol. 12, No.8, 2015
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Figure 4 — Percentage of time spent in outdoor play in 1997 and 2003. Data are from the Child Development Supplement to the Panel Study Of Income Dynamics. Source: Hofferth SL. Changes in American children’s time—1997 to 2003. Electron Int J Time Use Res. 2009;6(1):26–47.
Figure 5 — Changes in youth outdoor recreation from 2006 to 2010. Source: The Outdoor Foundation’s Outdoor Recreation Participation Report 2010 [cited January 25, 2014]; Available from: http://www.outdoorfoundation.org/.
Television viewing is strongly related to obesity/overweight, in cross-sectional studies.46,47 In 1996, children who watched more than 5 hours of television per day were 4.6 times as likely to be obese than those who watched 0 to 2 hours per day, and the difference persisted after adjusting for previous overweight, maternal overweight, socioeconomic status, household structure, and maternal and child aptitude test scores (Figure 6).46 Television viewing contributes to obesity in 2 ways: a) the energy cost of sedentary television viewing is typically less than that of the activities
it replaces; and b) television viewing is often accompanied by consumption of extra calories.48,49 Do children today watch more television than in the past? One study found that the percentage of middle and high school students who viewed 4 or more hours of television per day declined between 1990 and 2008.50 Similarly, a second study found that television viewing declined from a mean of 4.0 hours per day in the late 1970s to 2.75 hours per day in 1999.51 A third study using YRBSS data found that TV viewing declined between 1999 and 2007.27 However,
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Figure 6 — Relationship between watching television and obesity in U.S. children aged 8 to 16 years, from NHANES III (1988 to 1994). Source: Crespo CJ, Smit E, Troiano RP, Bartlett SJ, Macera CA, Anderson RE. Television watching, energy intake, and obesity in US children. Arch Pediatr Adolesc Med. 2001;155:360–5.
these statistics can be misleading. While the time spent watching live television has declined, the total amount of time devoted to television content has actually risen.45 This is due to the fact that American youth now view television content on digital video recorders, DVDs, computers, MP3 players, and cell phones (Figure 7). Currently, 8- to 18-year-olds view 4 to 4.5 hours of total television content per day.45,52 Television watching is especially high among minority children; black and Hispanic youths watch 2 hours more television per day than white youths.45 Computer time is not as strongly associated with weight status as television/videogame time. Sisson et al53 showed that obese children were more likely to watch ≥ 2 hours of television per day than were overweight or normal weight children. However, all 3 groups had similar levels of computer usage per day. These results suggest that computer use may not impact BMI to the same extent as television and videogames. Preschool children spent over 32 hours per week using television, videogames, and computers in 2009, and elementary school children spent 28 hours per week on these forms of electronic entertainment.54 Most of this time is spent in watching television, and only 1 to 2 hours per week is spent playing videogames. Given the evidence that excessive television watching is associated with obesity, it is surprising that 71% of youth ages 8 to 18 have a television in their bedroom, and 50% have a videogame system in their bedroom. The American Academy of Pediatrics recommends that screen time be limited to no more than 2 hours per day.55 However, only 1 in 10 young people have rules limiting television viewing or videogame use, and 64% of young people say the television is usually on during meals.45 The percentage of high school students using computers 3 or more hours per day for nonschool related activities increased between 2003 (22.1%) and 2011 (31.1%) (Figure 8). While the link between computer use and childhood obesity is weaker than the link between television viewing and childhood obesity, experts are nevertheless concerned by the large amounts of “screen time” accumulated throughout the day, because this is a marker of sedentary behavior.56 Prolonged sitting is associated with health problems,
Figure 7 — Secular trends in total media use among U.S. youth. Source: The Kaiser Family Foundation Report. Kaiser Family Foundation. Generation M2: media in the lives of 8- to 18-year-olds. 2010 [cited 2013 Nov 5]; Available from: http://www.kff.org/entmedia/mh012010pkg.cfm.
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Figure 8 — Percentage of high school students who used computers 3 or more hours per day on an average school day: 2005 to 2011. Source: Centers for Disease Control and Prevention. Trends in the Prevalence of Physical Activity, National YRBS: 1991 to 2009. 2010 [cited January 10, 2014]; Available from: http://www.cdc.gov/HealthyYouth/yrbs/pdf/us_physical_trend_yrbs.pdf
including suppressed lipoprotein lipase activity, high triglyceride levels, and impaired insulin sensitivity.57,58 The International Obesity Task Force (IOTF) attributes the obesity epidemic to an environment that limits opportunities for physical activity, and an abundance of inexpensive, highly palatable, energy-dense foods and drinks.59 We agree with this, and we would add that another factor is the increased time spent on electronic entertainment.
Discussion In reviewing the literature, we observed that there was a decline in youth physical activity within several areas, including active transportation, physical education, and outdoor play. These are domains in which it was once common for children and adolescents to accumulate large volumes of energy expenditure that could be helpful in controlling body weight. Due to declines in these areas, there is now an increased reliance on structured, after-school activities such as individual or team sports to make up the deficit. A consequence of this is that youth today are at greater risk of excessive weight gain, particularly if they are not engaged in sports or other structured activities.
Where Are We Headed? Today’s children and adolescents will become the next generation of adults. In 2003, Foreyt and colleagues60 predicted that at the current rate of increase, every U.S. adult will be overweight or obese by the year 2040. This projection might need to be modified, since the obesity rate in girls 2 to 19 years of age appears to have leveled off between 2007–2008 and 2009–2010; however, the rate of obesity in boys is still climbing.3
Due to the obesity epidemic, the Centers for Disease Control and Prevention have projected that cases of diabetes will double, and perhaps even triple, by the year 2050.61 In 2010, 9% of U.S. adults had diabetes62 and over 90% of adults with diabetes had type 2 diabetes mellitus.63 The prevalence is expected to rise sharply over the next 4 decades, with as many as 1 in 3 adults having the disease. Along with diabetes, increased rates of obesity in the future are expected to lead to increases in the rates of comorbidities (heart disease, stroke, Alzheimer’s disease, and breast and reproductive cancers). In addition, some researchers predict that today’s children may be the first generation whose life expectancy does not exceed their parents’.64 What can be done to increase physical activity levels and decrease sedentary behaviors in American youth? The socioecological model provides a useful framework for thinking about promoting physical activity.65 The model proposes that health behaviors, including physical activity, are impacted at several different levels. Starting at the center, individual factors such as knowledge, skills, and attitudes about physical activity are important factors influencing behavior. Moving outward, interpersonal relationships with parents, teachers, and peers also influence the physical activity of children and adolescents. Organizations (eg, schools, sport/activity organizations, churches and health departments) also have a role to play. The school setting, especially through physical education and after-school sports, plays an important role in increasing physical activity levels of youth. Communities (eg, parks and recreation departments, transportation planning organizations, and health coalitions) provide the infrastructure that provides opportunities for children to be active. Finally, federal, state, and local policies determine funding, laws, and programs that impact all of the previous levels. Recently, the Physical Activity Guidelines for Americans Midcourse Report: Strategies to Increase Physical Activity Among Youth66 examined the importance of 5 settings and their relations
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to youth physical activity. The authors reviewed the research on policy-based approaches, and they summarized the strength of the evidence base for each setting. The report identified several settings where interventions have succeeded at increasing physical activity in youth. The key findings and recommendations were: 1. School settings provide an opportunity to increase physical activity in youth, and should be part of the national physical activity plan. In particular, enhanced physical education and multicomponent school programs involving enhanced physical education, active transport, and activity breaks are effective. (Sufficient Evidence) 2. Preschool and childcare settings are another setting for increasing physical activity in young children. Well-designed programs that include playground equipment, staff training, active learning, and outdoor time can enhance physical activity. (Suggestive Evidence) 3. Community Settings also play a role in the physical activity levels of youth. Through good planning and development (ie, mixed land-use, high-density housing, traffic-calming measures, and infrastructure for active transport), communities can be made safer and more conducive to children’s physical activity. (Suggestive Evidence) 4. Family and home settings hold promise for increasing afterschool and weekend activities. Strategies might include engaging family members to favor physical activity over sedentary pursuits (Insufficient Evidence) 5. Primary care settings also hold promise for increasing youth activity levels, since health care providers could provide counseling on physical activity, and provide integration with community programs. (Insufficient Evidence)
Limitations The current study has a number of limitations. Some of the data (eg, YRBSS, NPTS, and NHTS) are based on self-report, which is subject to bias and may not accurately reflect the physical activity or sedentary behaviors of interest. Other data sources are indicative of participation rates, which lack sensitivity since they do not account for individual variation in the frequency, intensity, and time spent in physical activities. Finally, due to the nature of the studies and datasets used in this review, we could not ascertain whether overall physical activity levels in youth have declined. Although several national health surveys indicate constant levels of overall physical activity over time, these instruments may simply lack the sensitivity to detect changes.
Future Research Future research is needed in this area. The 2014 United States’ Report Card on Physical Activity67 serves as an important baseline for tracking indicators of overall physical activity, specific domains, and sedentary behaviors in youth. This report card provides a grade (A-F) indicating the current status of several indicators of youth physical activity, some of which are the same as the ones identified in our literature search. Continuing to track these indicators over time will provide valuable information on how the nation is doing toward its goals. In addition, there is an urgent need to track secular trends in overall physical activity. The National Health and Nutrition Examination Survey (NHANES) could provide a unique opportunity to follow physical activity trends in youth, using objective methods.
Conclusions Comparative studies of children living in hunter-gatherer, agrarian, and industrial societies suggest that physical activity levels have declined as modern, industrialized lifestyles were adopted. Over the past 4 decades, societal trends suggest that physical activity levels have declined within several key domains. There have been declines in active commuting, school-based physical education, and outdoor play. On the other hand, high school girls’ sports participation increased from 1971 to 2012. In regards to sedentary behaviors, the amount of time devoted to electronic entertainment and computer use rose during the first decade of the 21st century. Knowledge of these societal trends is important for those engaged in shaping policies, programs, and interventions to increase physical activity in youth. Acknowledgments Jennifer I. Flynn (University of South Carolina, Columbia, SC) and Dana L. Wolff (National Cancer Institute, Bethesda, MD) provided helpful comments on the manuscript. Pamela Andrews assisted with finding source material for this review.
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