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BOX 2-4 Continued
Figure for Box 2-4.eps
0
1
2
3
4
5
>7
4-7
1-3
<1
Never/Rarely
Hazard 
Ratio
Hours/Week of Vigorous- or Moderate-Intensity Physical Activity
<1
1-2
3-4
5-6
7+
Hrs/Day
TV
current recommendation for vigorous- or moderate-intensity physical activ-
ity, nearly the whole day comprises sedentary and light-intensity physical 
activities (Treuth et al., 2012). 
Various suggestions have been made about what aspects of youth’s 
sedentary physical activity should be avoided and monitored, including 
limiting television viewing to less than 2 hours per day, limiting television 
viewing to less than 3 hours per day, limiting computer use to less than 
3 hours per day, and limiting total media time to less than 2 hours per day 
(see, e.g., the Healthy People 2020 objectives [HealthyPeople.gov, 2012]).
Not surprisingly, surveys and studies of sedentary behavior among 
children and adolescents have used various methods of data collection 

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Copyright © National Academy of Sciences. All rights reserved.
Educating the Student Body: Taking Physical Activity and Physical Education to School
Status and Trends of Physical Activity Behaviors and Related School Policies
 
55
2011). Cohort studies among youth that have controlled for vigorous- or 
moderate-intensity physical activity have yet to demonstrate a relation-
ship between reductions in sedentary activity (which is the same as 
increases in light-intensity activity) and health or risk factor outcomes 
(Carson and Janssen, 2011; Ekelund et al., 2012).
The implications of these findings for behavioral recommendations 
regarding beneficial physical activity are still under discussion. Sedentary 
activities fill about half the waking hours of children and adolescents 
(probably relatively less for children and more for adolescents). Sedentary 
and light-intensity activities together fill 80-95 percent of waking hours 
(see Figure 2-6). More remains to be learned about the differences in 
health improvement, academic performance, and classroom behavior 
engendered by different intensities of physical activity. It is clear that 
the physiologic effects differ for different intensities. Higher-intensity 
activities, for example, provide greater cardiovascular fitness and lower 
cardiovascular mortality risk at all volumes of sedentary activity. Most 
likely, different intensities provide different benefits. The current consen-
sus is that children and adolescents need some vigorous activity daily for 
maximum current and future health. Light-intensity physical activity can 
now be viewed as a healthy shift away from sedentary physical activity 
as well. Higher rates of energy expenditure are preferred over lower rates 
across the full range of intensities. The behavioral differences between 
sedentary-intensity and vigorous- or moderate-intensity physical activi-
ties are considerable, and interventions to discourage the former and 
promote the latter can be expected to be quite different. 
(e.g., self-report, accelerometer, proxy), have assessed different aspects of 
sedentary-intensity physical activity (e.g., total sedentary time, watching 
television for more than 2 or 3 hours), and have reported the data in differ-
ent formats (e.g., percentage meeting a specified requirement, mean minutes 
of sedentary physical activity). Taken together, the results of these surveys 
and studies indicate that children and adolescents spend a great deal of 
time in activities requiring very low energy expenditure. The results also 
indicate that girls are more likely than boys and older adolescents are more 
likely than children to engage in these low-energy activities, although the 
differences are generally smaller than for vigorous- or moderate-intensity 
physical activity.

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Copyright © National Academy of Sciences. All rights reserved.
Educating the Student Body: Taking Physical Activity and Physical Education to School
56