Purpose of review: This is a review on the measurement of physical activity under daily life conditions. The focus is on the doubly labelled water method and accelerometry. Doubly labelled water is a gold standard and the reference for the validation of field methods to assess physical activity. Accelerometry is the most objective and precise technique to assess activity patterns in terms of frequency, duration and intensity. Applications of the two techniques are illustrated with the limits of physical activity and energy expenditure and with activity intensity as a determinant of the physical activity level.
Recent findings: The upper limit of the physical activity index (total energy expenditure as a multiple of basal metabolic rate) of 2.5, as derived from cross-sectional data, is confirmed by training intervention studies. Exercise training, in which total energy expenditure was measured before and at the end of the training programme, showed no increase in physical activity index when training was combined with an energy restricted diet and in elderly subjects. In children, the distribution of time spent at activities with low and high intensity determines the physical activity index while in adults moderate-intensity activities are the main determinant.
Summary: In adults, within the normal physical activity index range, the distribution of time spent at activities with low and moderate intensity determines the physical activity level. High-intensity activity does not have much impact on daily energy expenditure. High-intensity activity is not required to increase the activity energy expenditure.