Definitions and Measurement of Physical Activity
"Physical activity" has been defined as any bodily movement produced by skeletal muscle action17. That definition includes virtually all forms of human motion, including occupational activities (e.g., lifting, digging, walking), household tasks (e.g., mowing the lawn, vacuuming), transportation (e.g., walking to school or work, bicycling to the store), and leisure pursuits (e.g., golf, basketball). It also includes "exercise," i.e., physical activity that is performed for the purpose of enhancing health or physical fitness. Common forms of exercise are jogging, weight lifting, aerobic dance, and brisk walking. Active play is perhaps the most common form of physical activity in young children.

Before researchers and health professionals can help youth become more active, they need to understand who is already active and who is not, when are they active, how often, and for how long. Several methods of measuring youth physical activity, each with benefits and drawbacks, are currently used. Population surveys use questionnaires that ask subjects to recall their physical activity during the previous day, week, month, or year. These surveys are cost effective and allow a large number of people to be counted, but they are limited by recall errors. Children, in particular, have difficulty accurately recalling how active they were in previous days or weeks. Objective measures of physical activity, such as direct observation by a trained observer and mechanical activity monitors (heart rate monitors, pedometers, and accelerometers) avoid the recall limitations of questionnaires but are more costly for researchers and more inconvenient for subjects. Despite their limitations, a number of accurate and acceptable methods are used in research, public health, and program settings to collect data on physical activity levels in youth. Table 1 provides information on various instruments used to measure physical activity.

Physical Activity Recommendations
Over the past decade physical activity experts and other health professionals have considered the question, "How much physical activity is needed for good health?" The traditional answer, for adults and adolescents, is regular participation in vigorous, continuous exercise. This perspective is best exemplified by the American College of Sports Medicine's "exercise prescription," which recommends that adults participate in vigorous exercise lasting at least 20 minutes, at least three times per week 2. While the benefits of complying with this prescription are clear, new scientific evidence indicates that other ways of being physically active can also provide health benefits. One way is to accumulate moderate physical activity throughout the day.

Based on the new evidence about the health benefits of different types of physical activity, expert groups issued several new guidelines during the 1990s. The U.S. Centers for Disease Control and Prevention and the American College of Sports Medicine 61 issued a joint recommendation that Americans accumulate at least 30 minutes of moderate physical activity each day. A similar recommendation, also directed at adults, was included
in the 1996 Surgeon General's Report on Physical Activity and Health 91. Participation in moderate intensity physical activity also shows downward gender and age trends, although they are less pronounced than those observed for vigorous physical activity.

This newer approach to recommending physical activity foradults has prompted a reconsideration of recommendations for young people. Because regular participation in structured, vigorous exercise also produces health and fitness benefits for youth 87, some expert groups have recommended that the "exercise prescription" guideline be applied to adolescents 73. A structured approach to physical activity is not practical or appropriate for many young people, so experts have also developed accumulation guidelines for children and youth. For example, a combined group of physical activity professionals from the U.S. and the Health Education Authority from the United Kingdom issued guidelines recommending that young people accumulate at least 60 minutes of moderate to vigorous physical activity per day 37. The rationale for this recommendation is the rapid increase in the prevalence of childhood and adolescent obesity that is occurring in industrialized nations. The same group also recommended that youth regularly participate in resistance exercise, particularly for the upper extremities. Table 2 provides the specific guidelines issued by this expert panel.

How Active Are American Children and Youth?

" Participation in moderate intensity physical activity also shows downward gender and age trends, although they are less pronounced than those observed for vigorous physical activity."

Professionals in education and health have long been interested in monitoring the physical fitness of America's young people. Many national studies of youth physical fitness were completed in the U.S. on a regular basis between the late 1950s and the mid-1980s. Since then, however, researchers have turned their attention to monitoring physical activity of young people and have developed measurement procedures and surveillance systems to describe physical activity at the population level.

The most established surveillance system for monitoring the physical activity of the nation's youth is the Youth Risk Behavior Survey (YRBS). This survey is administered in alternate years to nationally representative samples of high school students. Table 3 lists the physical activity questionsfrom the 1999 YRBS 19.

Table 3: Physical Activity Questions from the 1999 YRBS (19) On how many of the past 7 days did you exercise or participate in physical activity for at least 20 minutes that made you sweat and breathe hard, such as basketball, soccer, running, swimming laps, fast bicycling, fast dancing, or similar aerobic activities? On how many of the past 7 days did you participate in physical activity for at least 30 minutes that did not make you sweat or breathe hard, such as fast walking, slow bicycling, skating, pushing a lawn mower or mopping floors? On how many of the past 7 days did you do exercises to strengthen or tone your muscles, such as push-ups, sit-ups, or weight lifting? On an average school day, how many hours do you watch TV? In an average week when you are in school, on how many days do you go to physical education (PE) classes? During an average physical education (PE) class, how many minutes do you spend actually exercising or playing sports? During the past 12 months, on how many sports teams did you play? (Include any teams run by your school or community groups.

"The 1999 YRBS found that the percentage of youth enrolled in physical education classes decreased approximately 40% from ninth grade to twelfth grade for both boys and girls."

Results of the 1999 YRBS, which was completed by 15,349 high school students, are summarized in
Figures 1 to 3. Students reported participating in bouts of vigorous physical activity between 3.0 and 4.5 days per week (Figure 1A). These rates compare favorably with traditionalexercise guidelines. It is important to note, however, a clear gender difference: girls reported substantially lower rates of participation. Average participation rates decline steadily across the high school years in both girls and boys. Participation in moderate intensity physical activity (Figure 1B) also shows downward gender and age trends, although they are less pronounced than those observed for vigorous physical activity. Participation in strengthening exercises also shows decreasing age and gender trends (Figure 1C).

Figure 1: Mean number of days in the past week children performed vigorous physical activity (A), moderate physical activity (B) and muscle strengthening exercises (C) - U.S., YRBS, 1999

The data presented in Figure 1 could be interpreted in a generally positive light. Indeed, the average rates of participation appear to conform reasonably well to accepted physical activity guidelines. The data, however, should be interpreted cautiously and with some skepticism. First, YRBS information is collected by self-report, which is subject to recall limitations and bias due to social desirability. Data from other physical activity studies that have used objective measures of physical activity suggest that YRBS and comparable self-report surveys significantly over-estimate participation rates, particularly for bouts of vigorous physical activity. Second, physical activity levels are highly variable in all age and gender groups. For example, the standard deviation for bouts of vigorous physical activity approximates two days per week. This indicates that a substantial number of youth reported very low frequencies of participation in physical activity. This point is exemplified by the data in Figure 2. Between 10% and 33% of youth reported participating in bouts of moderate and/or vigorous physical activity on less than three days per week (Figure 2A). In some age/gender groups over 10% reported no participation in either moderate or vigorous physical activity (Figure 2B).

Figure 2: Percent of children reporting less than 3 days of moderate-to-vigorous physical activity (A) and the percent of sedentary children (B) - U.S., YRBS, 1999

School-based physical education programs are potentially important sources of physical activity. The same is true of organized sports programs, which are available in virtually all high schools and many community organizations. Unfortunately, the 1999 YRBS found that the percentage of youth enrolled in physical education classes decreased approximately 40% from ninth grade to twelfth grade for both boys and girls (Figure 3A). Girls' participation in sports also decreased with grade level, while boys' participation remained stable at approximately 60% (Figure 3B). For all grades, girls consistently reported less participation in physical education classes and sports teams than boys.

Figure 3: Percent of children reporting less than 3 days of moderate-to-vigorous physical activity (A) and the percent of sedentary children (B) - U.S., YRBS, 1999