Actigraph and Combined Heart Rate monitor - the Actiheart
Many instruments designed to calculate energy expenditure use piezoelectric accelerometers as the primary device of measurement. A number of validated actigraphs exist on the market which can be broadly split in to two main camps: those systems that are worn on the wrist such as Geneactiv's self-named device and Respironics' Actiwatch and those that are worn on the hip such as the Actigraph GT3X+, PAL TechNologies' Activpal, McRoberts' Dynaport, the TriTrac R3D and Minisun's IDEEA.
But can accelerometers alone accurately measure AEE in free living? Exercise that is particularly demanding on the upper body (e.g. rowing, boxing or swimming) or strength training and activity that is demanding for the subject but not seen as "vigorous" by the device may not be detected by an accelerometer . Additionally issues can be caused by low level vibrations; a subject travelling in an automobile or sitting in a rocking chair for example may look very similar to someone taking low level exercise.
CamNtech's Actiheart is the new gold standard for measuring energy expenditure and has been validated against Doubly-Labelled water in terms of its accuracy. The Actiheart utilises both a piezoelectric accelerometer and a heart rate monitor recording synchronously; it then calculates AEE by differently weighting the data from the two devices depending on the dominance of activity or heart rate using a validated branched-model calculation (fig. 1). This allows the Actiheart to accurately measure AEE even when the exercise has low body movement (but high heart rate) with exercise such as static cycling that may be otherwise missed by an accelerometer alone. It also prevents false readings when stress or stimulation cause the heart rate to rise, or when low level vibrations are picked up by the accelerometer that could be mistaken for "steps" or exercise.
For more information on the benefits of the Actiheart system over alternative methods of measurement please see Actiheart Whitepaper; 'Calculating Activity Energy Expenditure in Free-Living conditions'
fig. 1- The Actiheart’s branched model for calculating AEE