View Full Version : Re: Qualysis system

Stephen W. Hill, Ph.d.
03-31-2005, 07:41 AM
Hi Renata Kirkwood,

I have never used Qualisys specifically, but have a lot of experience with 3D
gait analysis with Vicon (Oxford Metrics) and Optotrak (Norther Digital), as
well as 3D reconstruction principles in general.

I would expect that as long as your camera system calibration/linearisation
procedures have been done to specifications, the shape of your frontal and
transverse curves would have more to do with marker placement/alignment with
respect to anatomical landmarks, joint axes, etc as per the biomechanical
model. Since frontal and transverse relative joint kinematics for the lower
limbs during gait have smaller dynamic ranges of motion than sagittal ones,
they very sensitive to even slightly erroneous marker placement. The sagittal
angles are also influenced, but the shape of the curve may still
appear 'normal' simply because the signal-to-noise is better for those larger
range measures. Marker placement/alignment is very specific to the
biomechanical model being used, so make sure to follow the specs of your
model. By the way, if participant has pronounced transverse ("rotational") or
frontal ("valgus/varus") deformities, the relative joint angle calculations
may also be distorted... just -how- the 3 angles are portioned out becomes

Regarding the camera positioning question... if you are using 4 cameras for
unilateral gait analysis, I would suggest you start with 1 camera at each end
of the walkway (anterior and posterior views of the participant, but slightly
off axis from one another to avoid reflections), and the two side cameras so
cameras are at about 60 degree intervals around the one side of the room (180
degrees). I found better coverage with the front corner camera a little higher
than the others. You can play with camera height, AP versus ML distances from
camera to calibration volume, camera pitch angle, etc. There is a trade-off
between the dimensions of your calibrated volume (so how many steps you get
per walk) and effective resolution. In addition, false markers due to
glare/reflections from other cameras or other light sources, or noise from
other nearby equipment should be avoided. During the calibration, make sure
that all 4 cameras can see all of the calibration markers in your calibration
cluster. Usually, these camera calibration algorithms are based upon some
initial estimate (even from a tape measure) of the x,y,z coordinates of each
of those calibration markers, and of each camera in a global lab reference

Best of luck; I hope some of this info helps, and please let me know how it
turns out.

Stephen W. Hill, Ph.D.
Post-Doctoral Fellow
Research, University Centre
Toronto Rehabilitation Institute
550 University Avenue, Room 1207
Toronto, Ontario, Canada
M5G 2A2

Quoting Renata Kirkwood :

> Dear subscribers,
> We recently got a four camera motion analysis system (Qualisys) together with
> the Visual 3D software. We are experiencing some problems which we believe
> are related to camera positioning. We tried positioning two cameras on the
> side of the test leg, the third one positioned anteriorly and the last one
> posteriorly to the subject. We just can't seem to find a position that
> allows good data collection. The gait curves from the frontal and transverse
> plane doesn't look good. I tried the literature, but I found nothing
> specifically related to that subject.
> Does anybody have a reference or good experience in how to distribute the
> cameras properly?
> Thanks in advance.
> Renata
> --
> Renata Kirkwood, Ph.D.
> Universidade Federal de Minas Gerais
> Departamento de Fisioterapia
> Belo Horizonte - Brasil
> Tel: 31 3499-4782
> Cel: 31 9985-0707
> email: renatak@forusers.com
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