Sedentary behaviour is an important risk factor for a number of chronic diseases. While gaps remain in our knowledge of the elements of sedentary behaviour most associated with reduced health outcomes, measuring it is important, especially in less active patient populations where treatment-related changes may be seen first in changes in sedentary behaviour.

Sedentary behaviour is defined as participation in activities such as sitting and reclining during waking hours that do not substantially increase energy expenditure. Long periods of sitting are associated with increased health risks including reduced pulmonary oxygen uptake, reduced blood flow and nerve signalling, reduced fat metabolism, and reduced concentration. Recent work has shown that this can lead to a higher risk of chronic diseases including cardiovascular disease, diabetes, kidney and liver disease.

Measuring sedentary behaviour may be particularly important in understanding the wider impact of treatments in conditions in which patients are often quite inactive, including for example obesity and COPD. Small improvements in overall health status that improve the ability and motivation to perform modest discretionary activities, may be better observed in changes in sedentary behaviour compared to changes in overall daily activity measures.  Changes in sedentary behaviour and increased motivation to perform modest discretional activities are likely to be directly related to quality of life.

Tri-axial accelerometers have the potential to act as inclinometers to help identify body posture. In particular, accelerometers worn on the thigh, for example, provide the ability to robustly distinguish between standing (thigh vertical) and sitting/lying (thigh horizontal). Some thigh-worn devices, such as the ActivPAL, can be worn conveniently and unobtrusively in this way for a number of days continuously, under a thin waterproof (Tegaderm) dressing. 

In our review of current practice with Professor Gareth Stratton (Head of the Research Centre in Applied Sports, Technology, Exercise and Medicine, University of Swansea, UK) we have proposed a set of recommendations on the outcome measures derived from accelerometers to study when measuring sedentary behaviour and changes over time. These include:

• Total sedentary time (h/day)
• Weighted median sedentary bout length (min), and maximum bout length (min)
• Number of postural transitions
• Alpha parameter describing the distribution of bouts.

The rationale for the choice of these outcome measures, and supporting scientific literature, can be found in our article published in The International Journal of Obesity. 

While there is more work required to determine and validate the most clinically relevant and sensitive measures of sedentary behaviour, there is enough understanding of how to measure it to enable its inclusion in study protocols.