Physical activity has classically been defined as "any bodily movement produced by contraction of skeletal muscles that results in energy expenditure above rest" (Caspersen et al., 1985). This definition does not necessarily direct one's attention to also studying the absence of movement but this is equally important when the main aim is to understand habitual physical activity.
Another term that is often used in this field is physical behaviour which may be considered more inclusive as it would more readily also include sedentary behaviour and other postures. The issue of definitions is discussed elsewhere (Piggin, 2020).
When it comes to assessment, we need to consider that physical activity is a multidimensional exposure with a complex architecture, and it would be challenging to measure every single subcomponent. These components include:
The type or mode of activity refers to the different specific activities a person is engaged in (e.g. sleeping, sitting, standing, walking, cycling, load bearing).
Number of activity bouts during a specific time period.
Time (seconds, minutes, hours) of participation in a single bout of activity.
The physiological or biomechanical effort per unit time associated with participating in a specific type of activity.
The total amount of activity, e.g. the integrated product of frequency, duration, and intensity.
Domain, physical setting, social context