actigraphcorp.com	  	  	  ActiGraph White PaperEstimating Human Position with the ActiGraph GT3XTriaxial Activity Monitor	  With the introduction of 3-axis accelerometers, it is now possible to expand beyond gross measurement ofphysical activity and classify activity types. The ActiGraph GT3X Triaxial Activity Monitor takes a firststep in this direction with an inclinometer feature that indicates whether a subject is standing, sitting orlying down when the device is worn at the hip as well as indicating that a device is not being worn at all.
This feature provides an additional data point for interpretation of activity levels by researchers and healthprofessionals and is also included in the ActiSleep and 3-axis ActiTrainer Research Model devices.
In the absence of activity, a 3-axis accelerometer senses the acceleration due to gravity. This value iseffectively a down vector from which the orientation of the device (with the exception of heading) isdetermined. Equations 1 and 2 define the calculation used to obtain two important angles for determininga subject's posture where x, y, and z represents the acceleration along each axis.
Equation	  1	  ⎛θ y = cos − 1 ⎜⎜⎝⎞⎟222 ⎟x + y + z ⎠	  yEquation	  2	  ⎛θ z = cos − 1 ⎜⎜⎝⎞⎟x 2 + y 2 + z 2 ⎟⎠	  zWhen worn on the hip and perfectly vertical, the y-axis alone should contain the total acceleration due togravity. As a subject inclines, the offset angle (θy) increases. If the device is not being worn, then oneexpects the z-axis to reflect the total acceleration due to gravity as the device rest on a table-top forexample. Therefore, the addition of the z-axis offset angle (θz) is required to distinguish between lyingand off. Figure 1 contains examples of this y-axis offset angle in the standing (top-left), sitting (top-right),
lying (bottom-left), and z-offset angle in the off (bottom-right) positions.	  	  	  	  49	  E	  Chase	  Street	  	  	  	  |	  	  	  	  Pensacola,	  FL	  32502	  	  	  	  |	  	  	  	  ph:	  850.332.7900	  	  	  	  |	  	  	  	  f:	  850.332.7900	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	
  	  	
  	  	  	  	  	
  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  actigraphcorp.com	  	  	  Standing	  (θy)

Lying	  (θy)	  Sitting	  (θy)	  Off	  (θz)	  	  Figure	  1	  Optimal angle thresholds are required to correctly estimate the posture for the widest range of users. Theacquisition of this data involved collecting 30 Hz raw accelerometer data using a GT3X Activity Monitorfrom numerous subject's for analysis. Each subject was instructed to stand, sit and lie down for 5 minuteperiods for a total of 15 minutes worth of data. Each 30 Hz sample was then categorized and a computerscript was used to determine the optimal θy value for standing, sitting and lying by minimizing the errorbetween the known inclination and the estimate. Raw data was collected using several GT3X devices withand without belt clips and pouches with belt loops and the same script was used to determine the optimalθz value.
When activity exceeds six counts per second, the user is assumed to be standing due to the high activityvalues and thus the inclination angles are ignored. Otherwise, θy < 17º is considered STANDING, 17º <θy < 65º is considered SITTING, and θy > 65º is considered LYING unless θz < 22º when the unit is OFF.
Internal tests have regularly surpassed 95% accuracy with exceptions for outliers with posture at theextremes (e.g. one subject sat up as straight as to indicate standing while sitting). All parameters of thealgorithm will be configurable by the ActiLife software in case broader research finds more optimalvalues.
Estimation of human position using 3-axis accelerometers adds further value to the already solidactigraphy performance of the ActiGraph GT3X. With knowledge of a subject’s disposition, researchersand health professionals can garner further insight from the already accurate activity levels, energyexpenditure and step counting. Detecting off, lying, sitting and standing positions is just a first step ineven more detailed activity classification to be achieved by the research community using high data rate3-axis activity monitors from ActiGraph.	  	  	  	  49	  E	  Chase	  Street	  	  	  	  |	  	  	  	  Pensacola,	  FL	  32502	  	  	  	  |	  	  	  	  ph:	  850.332.7900	  	  	  	  |	  	  	  	  f:	  850.332.7900