2.2. Reference System
This protocol is extracted from research article:
Orientation-Invariant Spatio-Temporal Gait Analysis Using Foot-Worn Inertial Sensors
Sensors (Basel), Jun 7, 2021; DOI: 10.3390/s21113940

As a reference system, we used an optical motion capture system (Vicon, Oxford Metrics). It consisted of 10 infrared cameras (Vicon Vero v2.2), plus 2 optical cameras (Vicon Vue), installed as illustrated in Figure 1, which resulted in a capture volume of around 3 per 8 m. Optical markers were placed on participants’ shoes as illustrated in Figure 1.

The trajectories of the markers were captured at 100 Hz and post-processed using Vicon Nexus (Vicon Motion Systems Ltd., Version 2.10.1). Post-processing included automatic marker labelling, manual marker swapping correction and gap filling operations, using the methods of spline, rigid body and pattern fill available in Vicon Nexus.

After post-processing, gait parameters were automatically extracted from trajectories using a Python routine. The pipeline started with the identification of steady periods to segment signals into strides, which involved the application of thresholds to the velocity of the markers. It was followed by horizontal plane correction, where the normal (vertical) vector corresponded to the second principal axis obtained using Principal Component Analysis (PCA); PCA was applied to the position vectors–obtained from the heel (down) marker–formed between two successive steady states. Before determining gait events and calculating gait parameters, trajectories were low pass filtered using a zero-lag bidirectional first order Butterworth filter (cutoff of 20 Hz).

Initial foot contact (FC) and foot off (FO) were automatically detected, using as reference the trajectories from the markers on the heel (down) and toe. The FC instant was considered a minimum in heel vertical velocity [25], whereas the FO event was considered a maximum in vertical toe acceleration [26]. Temporal gait metrics—stride, swing and stance duration—were calculated as defined in [14]. Cadence was obtained as the inverse of stride duration, converted to the units of steps per minute.

The trajectories of the heel (down), toe and sensor centroid were used to evaluate spatial parameters, as illustrated in Figure 2: (i) Stride length (SL) was described as the linear distance obtained between two successive horizontal mid-stance sensor positions (as defined in [27]); (ii) Turning angle was defined as the angle (yaw) between two successive horizontal mid-stance foot vectors that were obtained from the positions of the heel and toe at mid-stance; (iii) Swing width (SW) was considered the maximum lateral excursion of the feet during swing, corresponding to the maximum-size vector perpendicular to the stride length direction; (iv) Minimum toe clearance (MTC) was obtained directly from toe trajectories, considered as the minimum peak vertical displacement during swing, to which the toe height at toe off was subtracted. Gait speed was obtained by dividing stride length by its corresponding stride duration.

Illustration of spatial gait parameters: (a) Stride length, swing width and turning angle. (b) Minimum toe clearance (MTC).

All parameters were reported per stride, and calculated individually for each foot side.

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