Sedentary behaviour, work, and health-related outcomes : the application of empirically derived accelerometer cut-points to data from the Health Survey for England

Background: During the last century, the technological revolution has contributed to changes in physical behaviour in the workplace. The number of moderate physical activity intensity occupations has decreased significantly over this period, and the introduction of desk dependent, computer-based jobs has resulted in an increase in the number of sedentary occupations. Sedentary behaviour is associated with several health-related outcomes, independent of physical activity; however, the role of occupational sedentary time with health-related outcomes is less clear. Sedentary time in different domains may represent differing associations with health; therefore, there is a need for studies to use more objective, reliable and valid measurements of sitting time in the occupational domain to fully understand the effects of sitting at work and health. Methods: This thesis comprises two main studies: the first study recruited a sample of university employees/postgraduate students (n=30), whose day was spent mostly sedentary. Participants were asked to wear two types of accelerometer (ActiGraph GT3X+ and activPAL3™) during waking hours for seven days: generalised estimating equations were used to derive a counts per minute threshold for sedentary behaviour for the ActiGraph GT3X+, based on the activPAL™ sedentary classification. The derived accelerometer cut-points from the first study were used to complete a secondary analysis using data from the Health Survey for England. In 2008, a sub-sample of participants wore an ActiGraph GT1M accelerometer for seven-days, and these data were used to examine the relationship between occupational sedentary time and health-related outcomes. Results: The derived cut-points from the generalised estimating equations were significantly higher on a Saturday (97 cpm) compared to weekdays (60 cpm) and Sunday (57 cpm). Derived counts per minute for sedentary time during working time were significantly lower compared to non-working time (35 versus 73). Compared to the 100 cpm and 150 cpm thresholds, the empirically derived cut-points were not significantly different in terms of area-under-the-curve, but had lower mean bias for working and non-working times. The amount of sedentary time from the derived and previously proposed cut-points differed significantly; however, this did not affect the beta coefficients and the conclusions drawn from the regression models. In contrast to studies that have found associations with both total sedentary time and leisure-time sedentary behaviour and detrimental health outcomes, there was no evidence that occupational sedentary time is associated with health-related outcomes in the same way. Time spent in moderate to vigorous physical activity was a significant predictor for waist circumference and BMI for occupational sedentary time; furthermore, BMI was a significant predictor of cardiometabolic markers. Conclusions: Accelerometer cut-points for sedentary behaviour can depend on day and also domain, suggesting that the nature of sitting differs depending on the context in which sedentary time is accrued. It is not known if there are underlying mechanisms of sedentary behaviour in different domains that can explain these differences, and the effect that occupational sedentary time has on health.