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Research Papers

Posture and Movement Classification: The Comparison of Tri-Axial Accelerometer Numbers and Anatomical Placement

[+] Author and Article Information
Emma Fortune, Vipul A. Lugade

Motion Analysis Laboratory,
Division of Orthopedic Research,
Mayo Clinic,
Rochester, MN 55905

Kenton R. Kaufman

Motion Analysis Laboratory,
Division of Orthopedic Research,
Mayo Clinic,
Rochester, MN 55905
e-mail: Kaufman.Kenton@mayo.edu

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received August 13, 2013; final manuscript received November 26, 2013; accepted manuscript posted December 12, 2013; published online April 10, 2014. Assoc. Editor: Paul Rullkoetter.

J Biomech Eng 136(5), 051003 (Apr 10, 2014) (8 pages) Paper No: BIO-13-1363; doi: 10.1115/1.4026230 History: Received August 13, 2013; Revised November 26, 2013; Accepted December 12, 2013

Patient compliance is important when assessing movement, particularly in a free-living environment when patients are asked to don their own accelerometers. Reducing the number of accelerometers could increase patient compliance. The aims of this study were (1) to determine and compare the validity of different accelerometer combinations and placements for a previously developed posture and dynamic movement identification algorithm. Custom-built activity monitors, each containing one tri-axial accelerometer, were placed on the ankles, right thigh, and waist of 12 healthy adults. Subjects performed a protocol in the laboratory including static orientations of standing, sitting, and lying down, and dynamic movements of walking, jogging, transitions between postures, and fidgeting to simulate free-living activity. When only one accelerometer was used, the thigh was found to be the optimal placement to identify both movement and static postures, with a misclassification error of 10%, and demonstrated the greatest accuracy for walking/fidgeting and jogging classification with sensitivities and positive predictive value (PPVs) greater than 93%. When two accelerometers were used, the waist-thigh accelerometers identified movement and static postures with greater accuracy than the thigh-ankle accelerometers (with a misclassification error of 11% compared to 17%). However, the thigh-ankle accelerometers demonstrated the greatest accuracy for walking/ fidgeting and jogging classification with sensitivities and PPVs greater than 93%. Movement can be accurately classified in healthy adults using tri-axial accelerometers placed on one or two of the following sites: waist, thigh, or ankle. Posture and transitions require an accelerometer placed on the waist and an accelerometer placed on the thigh.

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Copyright © 2014 by ASME
Topics: Accelerometers
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Figures

Grahic Jump Location
Fig. 4

Positive predictive value (PPV) when identifying static orientations and dynamic movements compared to video identification using (a) a waist accelerometer, (b) a thigh accelerometer, (c) an ankle accelerometer, and (d) thigh and ankle accelerometers. The central line (dashed) represents the median, the edges of the box are the 25th and 75th percentiles, and the whiskers extend to ±1.5 of the interquartile range. Outliers beyond this range are labeled as +. For the PPV of jogging in (a), the median value and the 75th percentile are equal to 100%.

Grahic Jump Location
Fig. 3

Sensitivity when identifying static orientations and dynamic movements compared to video identification using (a) a waist accelerometer, (b) a thigh accelerometer, (c) an ankle accelerometer, and (d) the central line (dashed) represents the median, the edges of the box are the 25th and 75th percentiles, and the whiskers extend to ±1.5 of the interquartile range. Outliers beyond this range are labeled as +. Trans: Transitions.

Grahic Jump Location
Fig. 2

Decision algorithm for the possible posture and movement classifications determined from the accelerometer data when using (a) single waist or ankle, (b) a single thigh, and (c) thigh and ankle accelerometers. SMA is signal magnitude area and CWT is continuous wavelet transform

Grahic Jump Location
Fig. 1

Mean signal magnitude area per second and corresponding movement and jogging thresholds for (a) waist, (b) thigh, and (c) acceleration data from one subject chosen at random during the simulated free living protocol

Grahic Jump Location
Fig. 5

Bland-Altman plots demonstrating error in identifying walking/fidgeting and jogging activities when using accelerometer compared to video identification for (a) a waist accelerometer, (b) a thigh accelerometer, (c) an ankle accelerometer, and (d) thigh and ankle accelerometers. The dashed line represents the mean, while the solid lines represent the repeatability coefficients (± 1.96 SD). ICC(A,1) values are also presented.

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