New thesis about measuring sedentary behaviour

My thesis is all about measuring sedentary behaviour. Too much sedentary time has been linked to all sorts of detrimental health effects, from higher incidences of type 2 diabetes and cardiovascular disease to premature death. There is broad consensus in research that sedentary behaviour consists of a posture component and of a physical activity component: sitting, with minimal physical activity. A synonym for sedentary behaviour is thus inactive sitting. Inactive means that the energy expenditure is a maximum of 50% higher than when lying, while active would mean the energy expenditure is more than 50% higher. However, there is no consensus on how the behaviour is measured. Although posture sensors are recommended, the evidence for the detrimental health effects of sedentary behaviour was collected with physical activity sensors, and we still lack a valid method to measure both components of sedentary behaviour simultaneously. My thesis aimed to close this gap.

Which are the most important results? 

Both posture and physical activity sensors substantially overestimate the time spent sedentary. We were the first ever to identify the reason: active sitting and inactive standing. Both behaviours take an astonishing 2 hours a day on average, and are thus much more common than previously thought. We also observed a large inter-individual variability: Some of the investigated office workers spent a third of their sitting time active, while others spent most of their standing time inactive. Therefore, we urge the research community to be careful with the term sedentary behaviour and specify whether sitting, inactivity, or sedentary behaviour is measured. The interested reader finds in my thesis detailed instructions on how to measure both components of sedentary behaviour simultaneously. In doing so, we may one day be able to identify the behaviour that is responsible for the detrimental health effects currently related to sedentary behaviour.

How can this new knowledge contribute to the improvement of people’s health? 

As a first consequence, the recommendation to reduce sedentary time, as set out in, for example, the WHO’s physical activity and sedentary behaviour guidelines, must be interpreted as a recommendation to reduce inactivity, and the detrimental health effects should be combated with activity rather than standing. For office workers, this questions the use of standing desks, but analysing their impact on people’s health requires another thesis, ideally using a method developed in my thesis.

What's in the future for you? Will you keep on conducting research? 

Yes, I will continue my research. We generated the key findings of my thesis with a new Posture and Physical Activity Index (POPAI), a combination of a posture and a physical activity sensor. However, since two sensors are cumbersome for field studies, we already started a new study to reduce the number of sensors needed for POPAI to only one.