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Validation of tri-axial accelerometer for the calculation of elevation angles.

Amasay-T; Zodrow-K; Kincl-L; Karduna-A
Proceedings of the 31st Annual Meeting of the American Society of Biomechanics, August 22-25, 2007, Stanford, California. Rochester, MN: American Society of Biomechanics, 2007 Aug; :265
Introduction: One of the main issues in occupational studies focusing on musculoskeletal disorders of the upper extremity is to quantify workers' exposures to risk factors during a workday. It has been shown that workers are more susceptible to shoulder injuries when they have a high lifetime exposure to arm elevation above 90 degrees (Svendsen et al. 2004). For whole day ambulatory recordings, body-mounted transducers, in combination with data loggers are used. It has been shown that accelerometers with a DC response can measure static acceleration and therefore can detect orientation relative to the line of gravity (Hansson et al. 2001). However, most of these devices are clumsy, complicated to mount, not self-contained and not commercially available. The Virtual Corset (Microstrain Inc, Williston, VT) is a battery powered tri-axial accelerometer with an integrated data logger, all contained within a pager casing with no cables, intended to measure 2 planes of trunk motion, flexion and lateral bending. The purpose of the study was to derive an equation to convert accelerometer data to shoulder elevation angles and to validate this equation with data collected with the Virtual Corset. Methods: The first step was to derive an equation to convert accelerometer data to elevation angles. In a Cartesian coordinate system the angle 0 between a vector (x y z) and its projection on the XY plane represents the elevation angle of the vector relative to that plane. When measuring acceleration with a triaxial accelerometer (x y z are the component of the acceleration) in static position the resultant vector is the gravitational acceleration, thus, equation 3 can be used to calculate the elevation angle at different orientation of a tri-axial accelerometer relative to gravity. To validate equation 3, the Virtual Corset was mounted on a vise which can be rotated through 360 degree of elevation and 90 degree of axial rotation, where 0 degree of axial rotation represents shoulder abduction and 90 degree of axial rotation is shoulder flexion. A PRO 3600 digital protractor (Macklanburg Duncan, OK), with a reported accuracy of 0.1 degree, was attached to the vise to identify the elevation angles. The vise was rotated through 360 degree of elevation in 10 degree increments. At each elevation angle, the axial rotation was varied from 0 degree to 90 degree in 15 degree increments. Each position was held for 10 seconds and the x y z accelerometer data were recorded and averaged. Elevation angles were calculated using equation 3. This procedure was repeated two different days for the Virtual Corset. The root mean square (RMS) error was calculated for each position between the known inclination angles and the calculated elevation angles. Results and Discussion: The RMS error of the calculated elevation angles, for the whole range, was found to be less than 1 degree for both trials. The maximum difference between the calculated and the actual elevation angles was less than 2degree. The Virtual Corset manufacture reports a typical accuracy of +/- 0.5 degree of projection angles which are limited to 360 degree of trunk flexion and +/- 70 degree of trunk lateral bending; not reporting angles accuracy when the movement occurs in different planes. Our results showed that the calculated angles error was similar in the different axial rotation angles that were tested. Summary/Conclusions: The tri-axial accelerometer (Virtual Corset) can be used to accurately reconstruct elevation angles. Future studies will be conducted to measure the elevation angle during dynamic conditions and in-vivo scenarios for the upper extremity.
Mathematical-models; Musculoskeletal-system; Musculoskeletal-system-disorders; Extremities; Workers; Work-environment; Exposure-levels; Risk-factors; Injuries; Measurement-equipment; Analytical-processes
Tal Amasay, Orthopaedic Biomechanics Lab, University of Oregon, Eugene, OR 97403
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Proceedings of the 31st Annual Meeting of the American Society of Biomechanics, August 22-25, 2007, Palo Alto, California
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University of Oregon