[Technical aspects and relevance of physical activity assessment in children and adolescents in free-living conditions].

Physical activity accounts for 25-30 % of total daily energy expenditure. Total energy expenditure varies according to the physical activity, which can be voluntarily modulated. Assessment of physical activity is therefore an important factor in the promotion of health and in several childhood and adolescent pathological situations. Questionnaires are widely used for assessing physical activity patterns in youth because of their low cost and ease of use, but they systematically overestimate physical activity. Heart rate monitoring requires individual calibration in the laboratory and a number of factors other than physical activity can affect heart rate (i.e., stress, temperature, etc.). Pedometers are objective devices but give only information on the number of steps and do not assess physical activity patterns. Accelerometers seem the best compromise between feasibility and validity in the assessment of physical activity in children in free-living conditions because this is an objective and accurate method whose cost is low. The choice of the type of device depends on the objectives of the clinician or researcher.