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. 2020 Feb 13;8:65.
doi: 10.3389/fbioe.2020.00065. eCollection 2020.

Drift-Free Foot Orientation Estimation in Running Using Wearable IMU

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Free PMC article

Drift-Free Foot Orientation Estimation in Running Using Wearable IMU

Mathieu Falbriard et al. Front Bioeng Biotechnol. .
Free PMC article

Abstract

This study aimed to introduce and validate a new method to estimate and correct the orientation drift measured from foot-worn inertial sensors. A modified strap-down integration (MSDI) was proposed to decrease the orientation drift, which, in turn, was further compensated by estimation of the joint center acceleration (JCA) of a two-segment model of the foot. This method was designed to fit the different foot strike patterns observed in running and was validated against an optical motion-tracking system during level treadmill running at 8, 12, and 16 km/h. The sagittal and frontal plane angles obtained from the inertial sensors and the motion tracking system were compared at different moments of the ground contact phase. The results obtained from 26 runners showed that the foot orientation at mean stance was estimated with an accuracy (inter-trial median ± IQR) of 0.4 ± 3.8° and a precision (inter-trial precision median ± IQR) of 3.0 ± 1.8°. The orientation of the foot shortly before initial contact (IC) was estimated with an accuracy of 2.0 ± 5.9° and a precision of 1.6 ± 1.1°; which is more accurate than commonly used zero-velocity update methods derived from gait analysis and not explicitly designed for running. Finally, the study presented the effect initial and terminal contact (TC) detection errors have on the orientation parameters reported.

Keywords: angles; drift; foot strike; inertial measurement units; orientation; running; validation study.

Figures

FIGURE 1
FIGURE 1
The two-segments model of the foot during the stance phase. Using the RGB convention, {FFrear} represents the FF of the rearfoot segment, {FFfore} the FF of the forefoot segment, and {GF} the room’s global frame. Points p and q are arbitrarily placed on rearfoot, and forefoot segments, c′ and c are, respectively, hypothetic and optimum rearfoot-forefoot joint’s center. ac,q is the acceleration at c estimated from q, ac,p the acceleration at c estimated from p, atreadmill the acceleration of the treadmill, and g the Earth gravitational acceleration. Finally, δ is the orientation difference (i.e., quaternion) between ac,p and ac,q while rpc and rqc are the distance vectors from point p to c and from q to c, respectively.
FIGURE 2
FIGURE 2
(A) Rear/lateral view of the markers’ configuration used in this study. (B) Top scheme of the markers’ configuration required in the definition of the foot’s technical (TF) and functional (FF) frames. Markers illustrated in orange are the one needed to set the TF, in green the FF, and in gray the duplicates which were not used in this study. Also, note that markers 5 and 6 were kept only during the calibration trials.
FIGURE 3
FIGURE 3
Comparison of the pitch angle measured from different measurement systems for a rearfoot (A) and forefoot (B) striker. The blue curve is the estimation from the IMU-based MSDI method (θMSDI), the orange curve from the IMU-based JCA method (θJCA), and the yellow curve from the reference motion tracking system (θref). The IC events are shown using down-pointing triangles, TC events with up-pointing triangles, MS events with squares, and the AC peaks using circles. The black vertical dashed lines accentuate the detection differences, for the IC and TC events, between the IMU and the FP system.
FIGURE 4
FIGURE 4
Boxplot of the intra-trial biases and precision results for the foot pitch activation angle [θJCA(AC)] measured with the proposed method (JCA). In the figure, the intra-trial biases are shown in blue and the precision values in orange. The gray dots represent the statistic of each trial. Note that there are two dots per trial because the feet were considered independently.
FIGURE 5
FIGURE 5
Bland–Altman plot of the activation pitch angle [θJCA(AC)] for the JCA method. The gray dots show the agreement of each step, the blue circles the agreement of the intra-trial mean, and the yellow lines the mean ± STD of the error.

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