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. 2018 Jun 21;18(7):1980.
doi: 10.3390/s18071980.

Validity, Test-Retest Reliability and Long-Term Stability of Magnetometer Free Inertial Sensor Based 3D Joint Kinematics

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Free PMC article

Validity, Test-Retest Reliability and Long-Term Stability of Magnetometer Free Inertial Sensor Based 3D Joint Kinematics

Wolfgang Teufl et al. Sensors (Basel). .
Free PMC article

Abstract

The present study investigates an algorithm for the calculation of 3D joint angles based on inertial measurement units (IMUs), omitting magnetometer data. Validity, test-retest reliability, and long-term stability are evaluated in reference to an optical motion capture (OMC) system. Twenty-eight healthy subjects performed a 6 min walk test. Three-dimensional joint kinematics of the lower extremity was recorded simultaneously by means of seven IMUs and an OptiTrack OMC system. To evaluate the performance, the root mean squared error (RMSE), mean range of motion error (ROME), coefficient of multiple correlations (CMC), Bland-Altman (BA) analysis, and intraclass correlation coefficient (ICC) were calculated. For all joints, the RMSE was lower than 2.40°, and the ROME was lower than 1.60°. The CMC revealed good to excellent waveform similarity. Reliability was moderate to excellent with ICC values of 0.52⁻0.99 for all joints. Error measures did not increase over time. When considering soft tissue artefacts, RMSE and ROME increased by an average of 2.2° ± 1.5° and 2.9° ± 1.7°. This study revealed an excellent correspondence of a magnetometer-free IMU system with an OMC system when excluding soft tissue artefacts.

Keywords: 3D joint kinematics; drift; inertial sensor; soft tissue; test-retest reliability; validity.

Conflict of interest statement

The authors declare no conflict of interest. The authors alone are responsible for the content and writing of this paper. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Figures

Figure A1
Figure A1
Biomechanical model of the lower body with segments (magenta lines), joint centers (red spheres), four ground contact points on each foot (green spheres), IMU placement, and involved coordinate frames. A technical coordinate system is associated to each IMU (I). The segment coordinate systems (S) are drawn at the proximal ends of the segments. The six DoF transformations, each in terms of an orientation (quaternion) qSI and a translation IS, between the IMU coordinate frames and the associated segment coordinate frames are called IMU-to-segment calibrations. The symbol G denotes the global coordinate system. The figure has been taken from [14].
Figure A2
Figure A2
BA diagrams of 3D joint rotations of hip, knee and ankle of the left side for condition 1. The solid line indicates the mean difference. The dashed lines indicate the LoA (95% CI of the mean difference).
Figure A3
Figure A3
BA diagrams of 3D joint rotations of hip, knee and ankle of the right side for condition 1. The solid line indicates the mean difference. The dashed lines indicate the LoA (95% CI of the mean difference).
Figure A4
Figure A4
BA diagrams of 3D joint rotations of the pelvis for condition 1. The solid line indicates the mean difference. The dashed lines indicate the LoA (95% CI of the mean difference).
Figure A5
Figure A5
BA diagrams of 3D joint rotations of hip, knee, and ankle of the left side for condition 2. The solid line indicates the mean difference. The dashed lines indicate the LoA (95% CI of the mean difference).
Figure A6
Figure A6
BA diagrams of 3D joint rotations of hip, knee, and ankle of the right side for condition 2. The solid line indicates the mean difference. The dashed lines indicate the LoA (95% CI) of the mean difference.
Figure A7
Figure A7
BA diagrams of 3D joint rotations of the pelvis for condition 2. The solid line indicates the mean difference. The dashed lines indicate the LoA (95% CI) of the mean difference.
Figure 1
Figure 1
Instrumentation of one exemplary subject. Marker set and rigid marker clusters.
Figure 2
Figure 2
Left (LT) ankle flexion of a representative subject. Soft tissue artefacts (STA) error excluded (a) and included (b). At 60 to 100% Gait Cycle (GC) appears a typical offset between optical motion capture (OMC) and inertial measurement unit (IMU)-derived data.
Figure 3
Figure 3
Bland-Altman (BA) diagrams for the most affected joint angles of every plane. The plots show the agreement between the 100% GC normalized joint angle waveforms of OMC and IMU system (averaged over all 28 participants). A normalized joint angle waveform contains 100 data points, which results in 100 data points in the BA diagrams. Upper row (a) shows condition 1, the lower row (b) condition 2. The solid line indicates the mean difference. The dashed lines indicate the limits of agreement (LoA) (95% CI of the mean difference).
Figure 4
Figure 4
Mean coefficient of multiple correlation (CMC) values over all subjects of section A for condition 1 (a) and condition 2 (b). Exemplary only the joint angles of the left lower extremity are shown.
Figure 5
Figure 5
Error of the global pelvis rotation in the transversal plane at minute 0, 1, 2, 3, 4, 5, and 6 for all subjects. The drift in the global transversal plane showed a linear trend but no consistent dimension.
Figure 6
Figure 6
Evaluation of linear regression of RMSE for condition 2 over time exemplary for the left lower extremity and global pelvis angles in sagittal and frontal plane. On the vertical axis is the slope of the regression line in degree. Median slope values, quartiles, and whiskers are above and below zero indicating no increasing trend of RMSE over time.
Figure 7
Figure 7
Evaluation of linear regression of ROME for condition 2 over time exemplary for the left lower extremity and global pelvis angles in sagittal and frontal plane. On the vertical axis is the slope of the regression line in degree. Median slope values, quartiles as well as whiskers are above and below zero indicating no increasing trend of ROME over time.

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