What is Visual Parallax and how does it affect gait analysis? : Is your video gait analysis really telling you what you think it is telling you ?

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We recently were asked by a student at a physical therapy school to help with a teaching case. They asked us to look at a gait video to assist in outlining some things in the case.  Here was our response.
“Hello Jane Doe
We are happy to look at the video for you so you and others can learn.
Just please know, as we say all the time here on the Gait Guys, that without an examination that what we are all seeing is not the problem rather the persons compensatory strategy around the dysfunctional parts.
Plus, video negates binocular parallax viewing so things that would stand out in in a exam where we are physically present will be masked quite a bit in/on video or on a computer screen. We try to minimize these visual losses by getting multiplanar gait video views (sagittal from front and back and coronal from left and right sides) but even these will not fill the visual gap from transferring data from 3D to 2D and then trying to interpret a 3D answer from the 2D.  But it is the best one can do with our technology today unless you use a body suit sensor system, and then you still have the limitations of "what you see is not the problem, its their compensation” so one still needs the physical exam to put the puzzle together.
Here…….. read this if you are wondering what we mean.
*This blog article (link below) which we wrote 18 month ago is the heart of what we wanted you to read today. Visual parallax and binocular vision both need to be understood so that you can better understand why what you see on your gait analysis video might not be what  you think you are seeing. Seeing is one thing, knowing what you are seeing is another, knowing the limitations and the “why” of what you are seeing is yet another.
So, we can tell you what we see………but without an exam we cannot tell you with great accuracy why you are seeing what we see.
that make sense ?“

best
shawn and ivo