• Physical activity - a complex multi-facetted exposure
• Dimensions of physical activity
• References

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:

Type

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).

Frequency

Number of activity bouts during a specific time period.

Duration

Time (seconds, minutes, hours) of participation in a single bout of activity.

Intensity

The physiological or biomechanical effort per unit time associated with participating in a specific type of activity.

Volume

The total amount of activity, e.g. the integrated product of frequency, duration, and intensity.

Domain, physical setting, social context

• Behaviour domains include
• Occupational (job, school)
• Transport
• Leisure (incl exercise and hobbies)
• Domestic (incl chores, home maintenance, self-care)
• Spatial setting (e.g. in-doors or out, green space, perceived safe neighbourhood)
• Social context (alone / with peers / with others)

1. Caspersen CJ, Powell KE, Christenson GM. Physical activity, exercise, and physical fitness: definitions and distinctions for health-related research. Public Health Reports (Washington, D.C. : 1974). 1985;100:126-31
2. Piggin J. What Is Physical Activity? A Holistic Definition for Teachers, Researchers and Policy Makers. Frontiers in Sports and Active Living. 2020;2:72