Units for force platform

Hi

I am using the mocap model to analyze my data which was captured via Codamotion. The forceplate (Type 2) units is in mV instead of N/N.m. Is this a problem? and if so how can I go about rectifying it?

I have attached the c3D file to this message.

Regards,
Kirsten

Hi Kirstin

Many system register the ‘units’ incorrectly when saving c3d files. My guess is that your data is in N and Nmm. Try to check the size of the data.

However, when I load your data it looks like the orientation of the plate is not stored correctly in the c3d files. It seems the like the plate is rotated 90 degrees. I have a little experience with Coda systems. I think that you may made a mistake when setting up the plate. It is possible to fix without redoing the experiments. If you only have few trials you can easily do it manually, using ‘Mocca’ (Open source software). Just create a new correct recording in your lab. Then use Mocca to open the correct c3d file and write down the force plate meta data. Then open your other files with Mocca and change them accordingly.

If you have many trials with that problem, then there are also tools to automate these things.

/Morten

Hi Morten

Thanks for your reply. I will look at the force plate setup again when I go back into the laboratory.

How did you see that the plate may be rotated 90 degrees? I have mokka installed on my computer.

Regards,
Kirsten

Also, as you have had experience with Coda before. Would this be an alignment issue or an issue with the actual setup of Codamotion?

Hi Kirsten

Look at how the ground reaction force vector moves across the force plate. It moves from left to right instead of the direction of gait. Try to right click the force plate and select “Toggle GRF butterfly”. Then you will easily see the problem.

Regarding the Coda problem. I am not sure. It is a long time since I worked with it. In principle I think the problem could be both places. In the lab with coda that I visited, they had a error in their plate configuration. The plate where simply two centimeters from their true location. So in that case the problem were in the coda configuration. That kind of error is critical for musculoskeletal simulations, but not for the type of analysis they had previously done in the lab. However, a 90deg error would probably have been noticed by other users of the lab. So I guess it must be an error in the initial alignment of the plates.

/Regards
Morten