Theme: The science of sedentary behaviorMeasurement of Adults' Sedentary Time in Population-Based Studies
Introduction
Sedentary behaviors are those pursuits undertaken while awake that involve sitting or reclining and that result in little or no physical activity energy expenditure—typically 1 to 1.5 times the resting metabolic rate.1, 2 Common sedentary behaviors include sitting or lying down while watching TV, using a computer, or driving. Sedentary time can be measured in three ways: (1) in terms of these specific behaviors (e.g., TV viewing time); (2) the amount of sedentary time occurring in a specific domain (e.g., work, leisure, domestic, transport); and (3) the overall sedentary time across the day. As the term sedentary encompasses both sitting and reclining, the broader term sedentary is used in this article, except when sitting specifically is measured.
This paper provides an overview of current methods used to measure sedentary time in free-living, population-based research in adults. The first section provides information on the reliability and validity of self-reported measures, and extends from previous reviews3 to encompass multiple domains of sedentary time. The second section describes device-based measures, with a particular focus on the interpretation and validity of data from the Actigraph activity monitor. The final section uses data from the U.S. National Health and Nutrition Examination Survey (NHANES) to provide an example of how the descriptive epidemiology of sedentary time may differ depending on how it is measured.
Section snippets
Self-Reported Measurement of Sedentary Time
Overall sedentary time can be assessed with either a single item (sometimes asked separately for weekend and weekdays), or by summing responses for the various behaviors or domains (composite measure). Key self-reported methods used are questionnaires (self-administered or interviewer-administered); behavioral logs; and short-term recalls. Questionnaires are a popular method3 because they can be implemented on a large scale, are relatively inexpensive, and do not alter the behavior under
Device-Based Measures of Sedentary Time
Given the errors associated with self-report, the ideal measure of sedentary time would
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be accurate and reliable across different population groups;
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distinguish among sleeping, reclining, sitting, and standing;
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distinguish among different domains and specific behaviors;
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be low-cost, have low participant burden, and be able to be worn continuously for extended periods of time;
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produce data that are easily analyzed and interpreted and can be provided in real time.
No such instrument currently exists.
Descriptive Epidemiology of Sedentary Time in the U.S. As Measured by Self-Report and Accelerometry
In 2003–2004 and 2005–2006, the large, population-representative NHANES included both self-reported (global sitting time, TV viewing time, computer time) and device-based (accelerometer) measures of sedentary time. These data provide the unique opportunity to examine, within one sample, the descriptive epidemiology of sedentary time in the U.S. using a variety of measures. Rather than reporting the relationships of the various sedentary measures (which have been described previously73), the aim
Methods
The relevant NHANES methods are described in at www.cdc.gov/nchs/nhanes.htm.45 The National Centre for Health Statistics Ethics Review Board approved the protocols and written informed consent was obtained. For the current study, 2003–2006 data from adult participants (≥20 years) were used. The study did not vary in protocol and had high response rates across this period.45
Gender Differences
After adjusting for age and race/ethnicity, there were significant gender differences in all measures of sedentary time, with the direction and magnitude of the difference depending on the measure. For the domain-specific measures, prevalence was lower in women than men for high TV time (64.9% [95% CI=63.0%, 66.8%] vs 69.2% [67.6%, 70.7%], p<0.001); computer use (27.1% [25.1%, 29.1%] vs 31.3% [27.9%, 32.8%], p=0.034); and screen time (48.3% [46.2%, 50.3%] vs 52.0% [49.7%, 54.4%], p=0.012).
Summary
In summary, the sedentary measures were consistent to some extent in identifying populations comparatively more or less sedentary, with older (≥60) adults generally the most sedentary and Mexican Americans generally the least sedentary. However, these subgroup differences are not apparent if only a single sedentary time measure is assessed. For example, if NHANES had measured only TV time, then the strong and largely consistent differences between Mexican Americans and non-Hispanic whites would
Conclusion
This paper provides an overview of the reliability and validity of current self-reported and device-based (primarily the Actigraph activity monitor) population-based measures of time spent sedentary. The 2003–2006 NHANES was utilized as an example of how various measures of sedentary time identify different populations as “at risk.”
Given that both self-reported and device-based instruments capture important aspects of sedentary behavior, it is recommended that wherever possible, both measures
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