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(CDC, 2002).

NOTE: BRFSS = Behavioral Risk Factor Surveillance System. NHES = National Health Examination Survey. NHANES = National Health and Nutrition Examination Surveys. YMCLS = Youth Media Campaign Longitudinal Survey. YRBS = Youth Risk Behavior Survey.

NOTE: BRFSS = Behavioral Risk Factor Surveillance System. NHES = National Health Examination Survey. NHANES = National Health and Nutrition Examination Surveys. YMCLS = Youth Media Campaign Longitudinal Survey. YRBS = Youth Risk Behavior Survey.

and an estimated 10 percent report that they are inactive (CDC, 2003b, 2004c; see Chapter 7).

In 2002, the CDC collected baseline data through the Youth Media Campaign Longitudinal Survey (YMCLS), a nationally representative survey of children aged 9 to 13 years and their parents, which revealed that 61.5 percent of youth in this age group do not participate in any organized physical activity during their nonschool hours and 22.6 percent do not engage in any free-time physical activity (CDC, 2003a).

Shifts in transportation patterns can affect energy balance. Many technological innovations have occurred over the past several decades such as the increased availability of labor-saving devices in the home, a decline in physically active occupations, and the dominance of automobiles for commuting to work and personal travel (Cutler et al., 2003). National data tracking trends on the physical activity levels and leisure or discretionary time of younger children and pre-adolescents are limited. However, an analysis of the available data for children aged 3 to 12 years from 1981 to 1997 (Hofferth and Sandberg, 2001) suggests a decline in their free time by six hours per week—attributed to an increase in time away from home in structured settings—and an increase in time spent in organized sports and outdoor activities over this time frame (Sturm, 2005a). However, it is not possible to determine the overall impact of these changes on children's physical activity levels.

One factor that has influenced overall transportation patterns in the United States is the change in the built environment. Through a number of mediating factors, the built environment can either promote or hinder physical activity, although the role and influence of the built environment on physical activity levels is a relatively new area of investigation. The ways in which land is developed and neighborhoods are designed may contribute to the level of physical activity residents achieve as a natural part of their daily lives (Frank, 2000).

There have been many changes in the built environment over the past century or more. For a variety of reasons, Americans moved away from central cities to lower density suburbs, many of the most recent of which necessitate driving for transportation.

In these areas, streets were often built without sidewalks, residential areas were segregated from other land uses, and shopping areas were designed for access by car. These characteristics discourage walking and biking as a means of transportation, historically an important source of physical activity.

Indeed, the amount of time that adults spend walking and biking for transportation has declined in the past two decades, largely because people are driving more (Sturm, 2004). In addition, the more time that Americans spend traveling, the less time they have available for other forms of physical activity. In 2000, Americans spent nearly 26 minutes commuting to their jobs, an increase from 22 minutes in 1990, and the average commuting time was 30 minutes or more in 25 of the 245 cities with at least 100,000 population (Population Reference Bureau, 2004a).

Children's motorized vehicle travel to and from school has increased, though this represents a small proportion of their overall travel. The 2001 National Household Travel Survey (NHTS) indicated that less than 15 percent of children aged 5 to 15 years walked to or from school and 1 percent bicycled (Bureau of Transportation Statistics, 2003). Even children living relatively close to school do not walk to this destination. The 1999 HealthStyles Survey found that among participating households, 25 percent of children aged 5 to 15 years who lived within a mile of school either walked or bicycled at least once during the previous month (CDC, 2002).

From 1977 to 2001, there was a marked decline in children's walking to school as a percentage of total school trips made by 5- to 15-year-olds from 20.2 percent to 12.5 percent (Sturm, 2005b). Based on data collected through the National Personal Transportation Surveys for 1977 and 1990, and the NHTS for 2001, there is little evidence of changes in walking trip length although distance traveled by bicycle has decreased (Sturm, 2005b). Although reduced physical activity has been identified as an unintended consequence of dependence on motorized travel, it is unclear how changes in children's transportation patterns have reduced their overall physical activity levels (Sturm, 2005b).

Media

The presence of electronic media in children's lives, and their time spent with such media, has grown considerably and has increased the time spent in sedentary pursuits, often with reduced outside play time. In 1999, the average American child lived in a home with three televisions, three radios, three tape players, two video cassette recorders (VCRs), one video game player, two compact disc players, and one computer (Roberts et al., 1999) (Figure 1-4). In 2003, nearly all children (99 percent) aged zero to six years lived in a home with a television set and the average number of VCRs or digital video discs (DVDs) in these young children's homes was 2.3 (Rideout et al., 2003). Television dominates the type of specific media used by children and youth and is the only form of electronic media for which trend data are available. In 1950, approximately 10 percent of U.S. households had a television (Putnam, 1995) in comparison with 98 percent in 1999 (Nielsen Media Research, 2000). The percent of American homes with more than one television set rose from 35 percent in 1970 (Lyle and Hoffman, 1972) to 88 percent in 1999 (Roberts et al., 1999). Moreover, there has been a ten-fold increase over the same period in the percent of American homes with three or more television sets (Rideout et al., 2003). In 2003, one-half (50 percent) of children aged zero to six years had three or more televisions, one-third (36 percent) had a television in their bedrooms, and nine out of ten children in this age range had watched television or DVDs (Rideout et al., 2003).

During a typical day, 36 percent of children watch television for one hour or less, 31 percent of children watch television for one to three hours, 16 percent watch television for three to five hours, and 17 percent watch television for more than 5 hours (Roberts et al., 1999) (Figure 1-5).

Two separate national data sources have tracked children's and adolescents' discretionary time spent watching television. Results indicate that the extent of television viewing differs by age, but also suggest an observed decline in television watching by children under 12 years by approximately four hours per week between 1981 and 1997 (Hofferth and Sandberg,

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All Children

2-7 8-18 Girls year olds year olds

Boys White Minority

FIGURE 1-4 Daily media use among children by age. Media use includes television, video games, radios, cassette tape players, VCRs, compact disc players, and computers.

SOURCE: Rideout et al., 1999. This information was reprinted with permission from the Henry J. Kaiser Family Foundation.

2001). Based on the Monitoring the Future Survey from 1990 to 2001, there was a steady decrease in heavy television watching (three hours or more) among adolescents yet an observed increase in television viewing for one hour or less (Child Trends, 2002). Although children are using other types of electronic media including video games and computers (Roberts et al., 1999; Rideout et al., 2003), television viewing represents a significant amount of discretionary time among children and youth, which is a sedentary and modifiable activity (see Chapter 8).

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