Assessment of physical activity and energy expenditure in adolescents

Abstract: Physical activity (PA) is a complex, multidimensional human behaviour that includes all bodily movement from fidgeting to participation in marathon running. The detailed relationship (i.e. dose - response) between the type and degree of PA and its effects on various aspects of health remains to be elucidated, not least in young people. The development and evaluation of different PA assessment methods is therefore of high priority. Methods of physical activity assessment include both self-report methods, such as activity diaries, questionnaires and activity recalls, and objective assessment techniques. Objective assessment of free-living physical activity can be based on physiological (energy expenditure, heart rate [HR] monitoring) and biomechanical (accelerometry) principles. The overall purpose with the present studies was to increase our understanding of the suitability of different PA assessment methods for use in normally active adolescents m young athletes, and to gain further knowledge about the amount and pattern of physical activity among adolescents. One hundred and fifty-seven randomly selected boys and girls and eight volunteer young athletes participated in the studies. One self-report method (activity diary) and three objective methods (HR monitoring, accelerometry and the doubly labelled water method, [DLW] were applied in the studies. All measurements of PA and total energy expenditure (TEE) were performed under free-living conditions. In addition, the relationship between HR and energy expenditure was individually measured during rest and standardised exercises in the laboratory. In the group of eight young athletes all measurements were performed under two different training conditions. The main findings were that: 1) there was no significant difference for the data on TEE and time spent at PA of moderate intensity or higher between the HR monitoring method and the activity diary; 2) the fitness level of the subjects affected the use of absolute HR when defining PA intensity levels; 3) in a random sample of 14-to-15-year-old adolescents, approximately 30% of the boys and girls did not achieve appropriate levels of PA according to published recommendations; and 4) the relationship between TEE and its derivatives measured by DLW, on the one hand, and the total amount of physical activity measured with an accelerometer, on the other, was influenced by the different training condition, whereas TEE estimated by minute-by-minute HR monitoring seem to be unaffected both by the definition of the FLEX HR and by the training condition. It is concluded that the activity diary method is a valid method for estimating TEE and time spent at PA of moderate intensity or higher in groups of adolescents. With the HR monitoring method, use of absolute heart rates for defining intensity levels reduces the validity of the data interpretation. Individual calibration for the heart rate - oxygen uptake relationship is therefore necessary. A relatively large group of adolescents does not achieve the amounts of physical activity recommended beneficial for health. This seems to be a public health concern. The HR monitoring method is not affected by different training regimens and is therefore a more favourable way of assessing physical activity in young athletes as compared to accelerometry.