approximations. Nevertheless, significant numbers of adolescents 13–15 years of
age throughout the world did not meet the recommendation for MVPA.
The relatively small percentages of adolescents meeting the recommendation for
MVPA were complemented by relatively high percentages of youth 13–15 years in
Europe and North America (66% boys, 68% girls, HBSC data) who spent two or
more hours per day viewing television, an indicator of physical inactivity (Hallal
et al. 2012 ). In the GSHS data for 34 countries, it was estimated that more than
one-third of adolescents spent three or more hours in sedentary activities: television,
computer games, talking to friends and other sitting activities, excluding time at
school, and doing homework (Guthold et al. 2010 ).
Accelerometry
More recent surveys based on accelerometry often focus on epochs of MVPA, the
intensity of activity that is most often associated with health benefits (Strong et al.
2005 ). Observations derived from 4 to 7 days of accelerometry in a nationally
representative sample of US youth indicate a decline in MVPA from 9 through
15 years of age in both sexes, higher levels of MVPA in boys than in girls, and
higher levels of MVPA on weekdays compared to weekend days in both sexes
(Nader et al. 2008 ). Boys exceed the recommended 60 min of MVPA at all ages
except 15 years, while girls exceed the recommendation from 9 to 13 years when
levels of MVPA fell below 60 min.
Corresponding data for European adolescents show a relatively small though
significant decline in MVPA between 13 and 17 years of age in both sexes, but
boys have more MVPA than girls through the limited age range. Time per day
sedentary (<100 counts per minute) increases, on average, across the age range, and
sex differences are small (Ruiz et al. 2011 ). MVPA and sedentary behavior do not
differ by stage of puberty; however, chronological age was not apparently con-
trolled or considered in the analysis. It would make more biological and behavioral
sense to evaluate activity and inactivity levels of youth of the same chronological
age who vary in the stage of puberty and also of youth at the same stage of puberty
who vary in chronological age.
Accelerometry data for children and adolescents 3–18 years from a number of
countries are available in the International Children’s Accelerometry Database
[ICAD] (see Sherar et al. 2011 ). Composite data of accelerometry for children and
adolescents 4–18 years of age from 20 studies in the ICAD databases indicated a
pooled mean of approximately 65 min per day in MVPA, defined as more than
2000 counts per minute adjusted for sex and age to permit comparison with data for
adults (Hallal et al. 2012 ). In another analysis of data from ICAD using different
criteria, pooled data for children and adolescents 4–18 years from 14 studies
indicated the following estimates for MVPA and sedentary time: 37±23 and
24 ±17 min per day of MVPA (>3000 counts per minute), respectively, in boys and
girls, and 345±96 and 363±96 min per day of sedentary time (<100 counts per
min), respectively, in boys and girls (Ekelund et al. 2012 ).
5 Physical Activity and Inactivity Among Children... 75