activities and types of inactivity. In many ways, accelerometry is the“visible,”an
objective indicator (counts) of time spent in, and intensity of, different behaviors
during the course of a day or over several days. The types of active and sedentary
behaviors are the“invisible.”There is a need for simultaneous study using both
objective and subjective measures. As a corollary, there is also a need for study of
the agreement between self-reported physical activities and accelerometry estimates
of activity energy expenditure. For example, Portuguese youth 13–16 years of age
completed a 3-day physical activity diary and wore an accelerometer on the same
days. The relationship between estimated activity energy expenditure based on
accelerometry and the 3-day diary was moderate. Agreement was significantly
higher in boys than in girls. In addition, the diary underestimated activity energy
expenditure in overweight and obese youth (Machado-Rodrigues et al. 2011 ).
Accelerometry does not capture all types of activities (e.g., activities that involve
water or activities in which wearing an accelerometer may present physical risk).
On the other hand, self-report protocols may be affected by the specific details of
recalled behaviors. Most daily activities, for example, are intermittent and may
involve substantial periods of rest, which may lead to significant overestimation of
time spent in vigorous activities (Trost et al. 2002 ). Physical activity questionnaires
and diaries in children and adolescents present additional challenges. Youth are less
time conscious and may be less able to process and recall information than adults;
youth are also more likely to engage in sporadic activities (Baquet et al. 2007 ).
There is a need for multi-method protocols that provide both quantitative and
qualitative physical activity data in children and adolescents.
Current Levels of Physical Activity and Inactivity Among
Youth
The physical activity of children and adolescents can be viewed from several
perspectives, e.g., overall status, age- and sex-associated variation, activities of
specific types, and intensity. Care is needed in comparing results based on surveys
using self-report and accelerometry. Data are derived largely from youth in
developed countries, especially in North America and Europe. Where appropriate,
data from lesser developed or developing areas of the world are noted.
Physical Activity Level (PAL)
Data for school age children and adolescents are summarized in Table5.1. The data
span the 1980s through thefirst decade of the twenty-first century. Allowing for
relatively small sample sizes, variable age groups, and compilation across studies,
several trends are apparent. Among children* 5 – 13 years, sex differences in PAL
5 Physical Activity and Inactivity Among Children... 71