Inertial measures of motion for clinical biomechanics: comparative assessment of accuracy under controlled conditions - changes in accuracy over time.

Karina Lebel,Christian Duval,Patrick Boissy,Mathieu Hamel,Stefan Glasauer

doi:10.1371/journal.pone.0118361

Abstract

BackgroundInterest in 3D inertial motion tracking devices (AHRS) has been growing rapidly among the biomechanical community. Although the convenience of such tracking devices seems to open a whole new world of possibilities for evaluation in clinical biomechanics, its limitations haven’t been extensively documented. The objectives of this study are: 1) to assess the change in absolute and relative accuracy of multiple units of 3 commercially available AHRS over time; and 2) to identify different sources of errors affecting AHRS accuracy and to document how they may affect the measurements over time.MethodsThis study used an instrumented Gimbal table on which AHRS modules were carefully attached and put through a series of velocity-controlled sustained motions including 2 minutes motion trials (2MT) and 12 minutes multiple dynamic phases motion trials (12MDP). Absolute accuracy was assessed by comparison of the AHRS orientation measurements to those of an optical gold standard. Relative accuracy was evaluated using the variation in relative orientation between modules during the trials.FindingsBoth absolute and relative accuracy decreased over time during 2MT. 12MDP trials showed a significant decrease in accuracy over multiple phases, but accuracy could be enhanced significantly by resetting the reference point and/or compensating for initial Inertial frame estimation reference for each phase.InterpretationThe variation in AHRS accuracy observed between the different systems and with time can be attributed in part to the dynamic estimation error, but also and foremost, to the ability of AHRS units to locate the same Inertial frame.ConclusionsMean accuracies obtained under the Gimbal table sustained conditions of motion suggest that AHRS are promising tools for clinical mobility assessment under constrained conditions of use. However, improvement in magnetic compensation and alignment between AHRS modules are desirable in order for AHRS to reach their full potential in capturing clinical outcomes.

Highlights

Functional mobility is an important aspect of a person’s quality of life
Mean accuracies obtained under the Gimbal table sustained conditions of motion suggest that AHRS are promising tools for clinical mobility assessment under constrained conditions of use
AHRS are composed of a set of inertial sensors whose outputs are fed into a fusion algorithm in order to determine the orientation of a rigid body in a global reference frame, defined from gravity and magnetic North

Summary

Introduction

Its evaluation plays an important role in numerous clinical decisions throughout the continuum of care, in various fields of practice ranging from rehabilitation to geriatrics. Because of their simplicity, self-report questionnaires, scales and performance-based clinical tests (e.g. Timed-Up and Go) are most commonly used for mobility evaluation. AHRS offer a flexible and lower cost alternative to traditional motion capture systems and their convenience opens up possibilities for different applications in clinical biomechanics. This technology has limitations, which have not been extensively documented. The objectives of this study are: 1) to assess the change in absolute and relative accuracy of multiple units of 3 commercially available AHRS over time; and 2) to identify different sources of errors affecting AHRS accuracy and to document how they may affect the measurements over time.

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Conclusion