obviously vary with the earliness or lateness of leaving home! It is not very likely
that this was perceived or recalled as“physical activity”; the youngster simply had
to get to school and return.
With the preceding as background, this chapter addresses several aspects of
physical activity and inactivity among children and adolescents: (1) assessment and
measurement; (2) current levels of physical activity and inactivity among youth;
(3) recent trends in physical activity across time; (4) recent trends in physical
inactivity across time; (5) correlates of physical activity and inactivity; and (6)fit-
ness and health benefits of regular physical activity.
Assessment of Physical Activity and Inactivity
Methods of assessing physical activity are diverse and include direct observation,
video, interview, diary, questionnaire, movement counters, and most recently ac-
celerometry. Each has advantages and drawbacks (Malina et al. 2004 ).
Nevertheless, the quantification of habitual physical activity is relatively complex,
and it is generally accepted that no single measurement technique accurately reflects
all dimensions of physical activity.
Physical Activity Level (PAL)
Energy expended in physical activity is the most variable component of total energy
expenditure (TEE) which is a composite of resting energy expenditure (REE),
diet-induced energy expenditure (DEE), and activity-related energy expenditure
(AEE). DEE refers to the thermic effect of food, and its contribution to TEE, though
variable, is relatively small (Malina et al. 2004 ).
AEE is very difficult to measure. An approximation of the contribution of AEE
to TEE is provided by the physical activity level (PAL), the ratio of TEE over 24 h
to basal metabolic rate (BMR) or basal (resting) energy expenditure (Food and
Agriculture Organization 2004 ). PAL is expressed as the multiple of basal energy
expenditure. An estimated PAL of 2.0, for example, indicates that the individual’s
TEE is twice that of his/her energy expenditure at rest. Note, however, PAL is not a
measure of physical activity per se.
An estimate of TEE is required to derive the PAL. TEE was derived from heart
rate monitoring and/or calorimetry in earlier studies and is estimated with doubly
labeled water (DLW) in more recent studies (Food and Agriculture Organization
2004 ). At present, DLW is the“gold standard”for the measurement of TEE under
free-living conditions in infants, children, adolescents, and adults. The main limi-
tation of the DLW method is the high cost of the
18
O isotope and the need for a
high-precision isotope-ratio mass spectrometer.
5 Physical Activity and Inactivity Among Children... 69