3 Physical Activity Measures in Children – Which Method to Use ?

There is increasing evidence supporting the health benefits of physical activity in children both in the immediate term in terms of body composition and in the longer term in the prevention of risk factors for cardiovascular disease, type-2 diabetes osteoporosis and certain cancers (Lee at al, 2000; Mohan et al, 2005; Blair et al, 1999; Pan et al, 1997; Tuomilehto et al, 2001; Colditz et al, 2005; Samad et al, 2005). Physical activity is defined as body movement produced by skeletal muscles which results in energy expenditure (Caspersen et al, 1985). It can be difficult to measure physical activity as it is a variable with many dimensions including type, frequency, duration and intensity. The context and location where activity takes place may also be of interest. As a behaviour physical activity is unstable, as habitual levels of activity vary during the day, throughout the week and at different times of the year. Children have less regular patterns of activity than adults and therefore a picture of overall activity may be more difficult to capture. Spontaneous un-planned activity is typical of children, particularly younger children, and may be due to opportunities that present within their environment such as play facilities and other children. Even in older children who do engage in regular planned sporting activities, there may be a degree of unplanned activity. The “gold standard” method of measuring energy expenditure as a result of physical activity is by the use of doubly labelled water (DLW). The technique uses stable isotopes of hydrogen and oxygen i.e. deuterium (2H) and oxygen (18O) ingested as water. Oxygen uptake and therefore energy expenditure are then calculated from the rate at which these isotopes are eliminated from the body. The difference between these is the amount of CO2 produced. DLW is not a technique that is generally suitable for use in field studies due to the resources required. Even if cost were not a consideration the information gained pertains to total energy expenditure over a time period e.g. a number of days/ weeks and does not permit the examination of acute patterns of physical activity such as time spent in specific activities or intensity of specific exercise sessions. The methods that can be used in field and most clinical studies are generally divided into the following; subjective methods such as observation and questionnaires and objective methods such as heart rate monitoring and motion sensors. The advantages and limitations of these methods will be outlined below. Before discussing these methods a synopsis of energy expenditure in children will be presented.