Measurement of energy expenditure by activity monitors: is it feasible to measure energy expenditure using tiny portable monitors?

Abstract

Physical inactivity has a major health effect worldwide. It is stated in the currently published physical activity recommendations for adults to promote and maintain health, that all healthy adults need moderate intensity physical activity for a minimum of 30 minutes, 5 days per week or vigorous intensity aerobic physical activity for a minimum of 20 minutes, 3 days per week. Physical activity can be defined as any bodily movement produced by the contraction of skeletal muscles that substantially increases energy expenditure and is characterised by its intensity, duration, frequency and mode of activity. Ideally, all these aspects should be recorded during physical activity measurements. However, it is important to make a distinction between energy expenditure measured in kJ/minute and physical activity, which is behaviour. The validity of portable monitors giving an estimated energy expenditure needs to be carefully examined. Ryan and Gormley provide a review related to the measurements of energy expenditure using portable activity monitors in children and adults. A comprehensive overview of the literature is given focussing on validity, reliability as well as feasibility (adherence). Several monitors give estimated energy expenditure; however, the authors present results for the following three activity monitors: the TritracR3D accelerometer (RT3, Triaxial Research) Tracker (StayHealthy, Inc., Monrovia, CA, USA), the Sensewear Pro Armband (SWA, BodyMedia Inc., Pittsburgh, PA, USA) and the Intelligent Device for Energy Expenditure and Activity (IDEEA, MiniSun LLC, Fresno, CA). A brief overview of the included monitors is given. RT3 measures the magnitude of acceleration in three dimensions (accelerometry) to provide vector data in ‘counts’. Counts are converted into energy expenditure with the use of algorithms based on the wearer’s characteristics. SWA combines accelerometry, heat loss, skin temperature and galvanic skin response collected from five sensors. Algorithms, taking into account the wearer’s characteristics, convert raw signals from the sensors to step count and energy expenditure, among other variables. IDEEA consists of five sensors that are attached to the lower limbs and a data collection device that is clipped to the user’s belt. Information regarding body and limb motions is carried through thin, flexible wires from the sensors to the recorder. IDEEA uses inbuilt algorithms to estimate energy expenditure. There are a couple of limitations in the review. First, a presentation of pooled data or a metaanalysis in addition to pros and cons with respect to the included monitors are not provided by the authors, but would have been helpful for the readers when deciding what monitor to choose. Measured or estimated physical activity level and energy expenditure varies with methods of assessment. The activity monitors included in the review record different aspects of physical activity such as acceleration, position changes, skin temperature etc., and a direct comparison is challenging. However, objectively recorded physical activity or energy expenditure is still preferred because subjects may recall only y50% of physical activity in the previous week when using, e.g. questionnaires. Furthermore, self-reported physical activity by obese subjects have been over-reported compared to objective measurements, particularly after attending management programmes for weight reduction. Secondly, Ryan and Gormley excluded monitors that did not directly provide an output in terms of energy expenditure. It is important to keep in mind that the included monitors do not directly measure energy expenditure, but estimate energy expenditure with the use of ‘unknown’ algorithms based on the wearer’s characteristics and ‘raw-data’. For example, SWA is a ‘black box’ because we, as researchers, do not know the algorithms provided by the manufacturer. Direct measurement of energy expenditure by heat production, or indirect measurement by oxygen consumption ( : VO2), is limited to small populations or short time periods because of the cost of assessment to both the investigator and the participant. Correspondence to: S Berntsen, Department of Public Health, Sport and Nutrition, Faculty of Health and Sport Sciences, University of Agder, Post Box 422, NO-4604 Kristiansand, Norway. Email: sveinung.berntsen@ uia.no