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360 cm (schematic representation in Fig. S1 1 ). The camera was
used both in a stationary position (a length of 130 cm of the
walkway could be captured reliably within the field of view)
and as a moving camera on a 7-m long rail (dolly track) placed
parallel to the walkway, over which the camera could be moved
manually (Fig. 1). An overview of the SGAS requirements and
costs is presented in Appendix S1 1 .
The SGAS measures calibrated positions on the floor of the
walkway. The calibration procedure is based on the method
proposed by Zhang (31). Intrinsic camera parameters are
determined from approximately 30 images of a planar 6×10
chequerboard pattern of 9-cm squares (Fig. 2). These para-
meters characterize the camera’s optical system. Placing this
chequerboard pattern vertically in a well-defined location on the
walkway sets up an orthogonal laboratory coordinate system in
which the y-axis runs along the walkway, the x-axis is perpendi-
cular to the walkway, and the z-axis points vertically upwards.
The position and orientation of the camera with respect to this
coordinate system are determined from a single image of the
chequerboard pattern at this location. This image provides the
information for the camera’s extrinsic parameters. Combining
the intrinsic and extrinsic parameters results in the camera’s pro-
jection matrix, which describes how the coordinates of a point
in the laboratory coordinate system are converted into the pixel
coordinates of the camera’s image plane. In the current study, the
set-up and calibration process of the SGAS took approximately
10 min; calibration was repeated after assessing 8 subjects and
in a clinical setting requires one calibration for the day.
The y-coordinates of an object on the floor are determined
manually from the video image. On the basis of the projection
matrix, the SGAS software draws a thin red line in the video
image representing the projection of the line in the x-direction
for a given y-coordinate in the plane z = 0 (Fig. 3). The user
adjusts the y-coordinate of this line by moving the computer’s
mouse until the projection matches the position of the object on
the floor in the image; for instance, the location of heel contact
of a foot. The user reads the corresponding y-coordinate from
the SGAS user interface. The time of the event is derived from
the video frame rate (time resolution 0.02 s). Counting the
number of frames yields the time difference between 2 events
in the video recording.
Simultaneously with the SGAS, the gait of the subject was
recorded with the GAITRite® system. The GAITRite® system
(GAITRite® Platinum 488P, CIR Systems Inc., New York,
USA) consists of a portable carpet walkway embedded with
pressure-activated sensors that sample at 60 Hz. The walkway is
488 cm long and 61 cm wide and contains an active sensor area
of 384 × 48 sensors arranged 1.27 cm from each other (centre
on centre, 18,432 sensors in total).
Experimental set-up
During a 30-min test session, subjects were tested in 4 different
walking conditions in a fixed order under stationary camera set-
up: barefoot walking at comfortable speed; barefoot walking at
slow speed; barefoot toe walking at comfortable speed; and shod
walking at comfortable speed wearing their own comfortable
flat-soled shoes. Data collection with the SGAS and GAITRite®
system was conducted by one investigator (MVB).
Ten valid gait trials, 5 in which a left and 5 in which a right
footstep was visible within the 130-cm field of view, per walking
condition were collected with the GAITRite® system and SGAS
in a stationary position. Gait strides were collected in the given
field of view by the stationary SGAS camera for the conditions
of slow speed and toe walking.
In addition to the stationary camera conditions, while walking
barefoot at a comfortable walking speed, 4 gait trials, including
4–8 strides per trial, were collected with the SGAS camera being
moved along the walkway by the investigator.
Inter-rater reliability was assessed with 3 trained observers
who were instructed in the definitions of the spatiotemporal gait
parameters (Table SI 1 ) and gait analysis method. To assess intra-
rater reliability, one observer (MVB) assessed the same data on
2 different occasions (minimally 1 month apart). Inter-rater and
intra-rater reliability were assessed for the barefoot comfortable
walking condition with the stationary SGAS camera.
Data processing and analysis
The initial contact (heel or toe) and toe-off distance and time-
points during each trial were identified manually from the video
images of the SGAS by 5 trained observers and were recorded in
a Microsoft Excel spreadsheet format (Microsoft Corporation,
Washington, USA) that was designed to automatically calculate
the spatiotemporal gait parameters (https://github.com/Mvan-
Bloemendaal/SGAS). Analysed data from the SGAS and the
GAITRite® system were: step length, step time, stance time,
double support time, stride length, stride time, and swing time.
http://www.medicaljournals.se/jrm/content/?doi=10.2340/16501977-2559
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Fig. 3. Video analysis using the spatiotemporal gait analysis system (SGAS).
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