Physical activity is a group of complex behaviours rather than a single act. An activity type or physical behaviour type is a specific label used to collectively describe a pattern of motion and posture associatiated with a specific behaviour, as well as some physiological states such as being awake or asleep. It typically includes a qualitative dimension which describes the nature of physical activity. The label is likely to be the first word that springs to mind when we observe a behaviour with our eyes, and it may help us imagine what the activity would look and feel like. Activity or behaviour type is a categorical variable, the classification of which is parallel to the quantitative characterisation of the behaviour such as its rate of energy expenditure or the acceleration and angle of an anatomical segment of the body. For example, walking is characterised by a cyclical pattern between the double-stance phase and the swing phase and shifting between left and right leg, at least one foot would always be in contact with the ground and the thigh would be largely upright, and the arms and shoulder girdle typically moving in opposite direction to the legs, whereas running which is distinctly characterised by a cyclical pattern between the flight phase and the one-foot contact phase.
There is almost an infinite number of different types of activity/behaviour undertaken by human beings. Examples of types of physical behaviour are
One can easily argue that many behaviour types also contain well-defined subtypes within them, for example a person can be lying down and be asleep in either REM or non-REM stage 1, 2, or 3 sleep. We can walk up or down a flight of stairs, holding or not holding the railings. And we can run on level ground on a smooth firm surface, or an athletic track, or through rough terrain, up or downhill.
Activity types can also be combined in sequence and considered a broader activity type, for example playing football is a combination of standing, walking forwards, sideways and backwards, running, kicking, jumping, heading and likely other specific activity types during the course of a game, and whilst we may be able to measure each one of these with some methods, we are likely to recall this activity as simply a game of football. In fact, most activities across occupation, transport, and leisure-time are characterised by being made up of several subtypes of activities, for example:
The concept of physical behaviour types includes the subset of static postures, as well as dynamic activity types. The classification of behaviour type is usually parallel to the quantitative characterisation of the behaviour such as its rate of energy expenditure or the acceleration and angle of an anatomical segment of the body. These dimensions are often correlated with each other, e.g. during walking the limbs of the body would accelerate and decelerate within a limited range of values, and this range of values would be shifted upwards if we changed to the activity type running. Intensity often differs substantially between different types of behaviour, and the metabolic costs of many activity types are listed in the Compendium of Physical Activities (Ainsworth et al., 2011), which is often used to estimate energy expenditure from an initial assessment of activity type.