Being regularly active substantially improves outcome and progression of most chronic degenerative diseases . Additionally, an active life style contributes to social participation and quality of life, as expressed in the context of the ��International Classification of Functioning, Disability, and Health�� . In order to understand how PA in daily life is associated with health and functioning in older persons, such behaviour needs to be studied in free-living conditions.Contrary to young active adults, older persons perform most PA as part of every-day life activities related to work, house-holding and leisure time where energy cost is much lower than exercise, such as running. Thus, for many purposes, it is of more relevance to study aspects of PA in older persons, such as postural allocation and type of activity, than the energy expenditure associated with PA.
Against this background, the World Health Organisation considers that PA can be measured by its four main components, which can be abbreviated as FITT: Frequency of the activity (e.g., number of walking periods), Intensity of the activity (e.g., walking speed); Time or the duration of the bout of activity (e.g., duration of walking episodes), and the Type of activity (e.g., lying, sitting, standing, walking) . Where the FITT components apply to the population at large it is reasonable to expect that the weight of the individual FITT components will vary largely for different sub-population. Hence, the daily life performance of mobility related activities (such as standing or walking) can be considered as a key construct of PA in older people.
However, since PA patterns differ so much between different populations, studies addressing PA should carefully define the key construct(s) which correspond to the specific topic and population under study.The formal definition of PA is ��any bodily movement produced by skeletal muscles that results in energy expenditure�� . This definition specifically focuses on the amount and volume of PA and the energy expenditure associated with PA, and thus, a large portion of PA literature has focused on the effect of physical exercise and GSK-3 on energy consumption, mostly from the perspective of health. However, besides activity related energy expenditure, PA is of interest in terms of body posture and movement behaviour.
Assessment of PA has traditionally been done by use of questionnaires, mostly focusing on leisure time levels of PA and on energy expenditure. Questionnaires have known limitations with respects to reliability and their relationship with actual behaviour [8,9], and they do not have the potential to assess all aspects of PA , especially in older persons . Objective, performance-based laboratory tests will neither represent the usual performance of the tested individual .