As noted, the PAL is expressed as the multiple of basal or resting energy
expenditure. An expert panel of the Food and Agriculture Organization ( 2004 )
suggested the following criteria for interpreting the PAL: (1) 1.40–1.69—sedentary
or light activity, (2) 1.70–1.99—active or moderately active lifestyle, (3) 2.00–2.40
—vigorous or vigorously active lifestyle.
Questionnaires and Diaries
Most surveys of physical activity (also inactivity) among older children (* 10
- years) and adolescents have historically been based on questionnaires and diaries.
Both methods include variation associated with recall and subjectivity. Diaries
depend on the detail of recording and individual interpretation of specific activities.
Questionnaires vary in structure and detail and in the manner in which questions are
stated. As such, questionnaires are not equivalent and often need to be modified to
accommodate cultural variation. Because of their relative simplicity and low cost,
questionnaires are often used in large-scale surveys. Observation and parental report
are commonly used with young children, while time use diaries completed by
parents provide insights into types of sedentary and active behaviors.
Accelerometry
Accelerometry is the technique of preference for many more recent surveys of activity.
The method is based on both movement counts and acceleration. Overall physical
activity is expressed as counts per minute and also as epochs of MVPA, the intensity
of activity that is most often associated with health benefits (Strong et al. 2005 ).
MVPA is commonly defined as3 METS (Andersen et al. 2006 ). MET refers to
metabolic equivalent; one MET is defined as the amount of oxygen consumed while at
rest, 3.5 ml O 2 per kg body weight per minute in adults. The MET value for a specific
activity refers to the ratio of the energy expenditure in the activity to resting energy
expenditure and indicates the intensity of the activity (Malina et al. 2004 ).
Accelerometry can also document sedentary behavior, e.g., time at accelerometer
counts <100 per minute (Matthews et al. 2008 ). Sedentary counts are commonly
expressed as the percentage of accelerometer wear time or as hours per day.
Studies using accelerometry vary in epochs of activity (5, 15 and 60 s), number
of consecutive blank cells permitted as valid (usually 20 consecutive epochs with
zero), minimum registered time per day (although the majority of studies adopt a
minimum of 600 min), equations used to derive intensities, and also in number of
days of observation. The data also need adjustment for non-wear time. The units are
typically worn during waking hours, except in water-based activities (bathing,
swimming).
Although accelerometry provides an objective estimate of physical activity and
sedentary time, there is a need to relate activity counts and intensities to specific
70 R.M. Malina et al.