TOWARD IMPROVEMENT OF OBJECTIVELY MEASURED SEDENTARY BEHAVIOR IN A FREE-LIVING ENVIRONMENT

Abstract

The objective of this dissertation was to extend our understanding of the objectively measured sedentary behavior (SB) and to explore ways to improve measurement practices of SB in health outcome research. The primary aim of the first study was to explore the measurement issues in objectively measured SB using the National Health Nutrition Examination Survey 2003-2005. The specific aims addressed were 1) the influence of duration of sedentary bout on the association of sedentary time with metabolic risk factors; and 2) the appropriateness of extracting sedentary breaks by counting the number of transitions from sedentary to active phase from accelerometer data. The findings highlighted that the sedentary time with relatively short bout (i.e., ≤ 10 minutes) was in general beneficially associated with health outcomes, which, in turn, had influenced on the dose-response relationships between total sedentary time with health outcomes. Another important finding was that the absolute number of sedentary breaks has a limited measurement property that may not be considered as the interruption of sedentary time, but rather it is related to the patterns of sedentary time accumulation.

The primary aim of the second study was to examine the validity of different types of objective monitoring devices for the assessment of SB in a free-living setting. The specific focus was placed on 1) the overall performance of three accelerometers (Actigraph GT3X, activPal, and SensewearTM Armband) to identify SB in a free-living setting; 2) the influence of sedentary bout restrictions on the validity of the devices to identify the structured SB at given bout condition; and 3) to develop the algorithm to identify SB-bout that may be feasible to identify the sedentary breaks congruent with what has been defined at the conceptual level. The results highlighted that the activPal is the most accurate and precise measure of SB compared with a proxy of direct observation. One possible strategy to improve the performance of threshold-based GT3X measures could be the restriction of short sedentary bout. The developed algorithm was significantly influenced by the functional capability of devices to detect the postural information, and the activPal was the one that can be of useful when using the algorithm to identify the SB-bout.