Exporting Force plate data in terms of fixed point( Electric centre) from Vicon nexus

[Souvik Paul]

Dear All,
iIn our lab, we have a Motek split-belt treadmill with embedded force plates integrated with a Vicon motion-capture system.

Currently, in Vicon Nexus, we are seeing the force-plate data mainly in terms of CoP mapping. We would like to know how to obtain the ground reaction force data referenced to a fixed point, such as the force plate’s electrical center/origin, rather than only through the CoP representation.

If anyone has experience with this setup or knows the correct workflow/settings in Vicon Nexus or Motek, I would really appreciate your insight.
Thank you.

[Manvendra Singh]

You can use C3DFileAdapter ( osimC3D utility in Matlab)-https://opensimconfluence.atlassian.net/wiki/spaces/OpenSim/pages/53090837/C3D+.c3d+Files#Reading-C3D-files-through-Matlab of OpenSIM to read Force Plates' origins, rather than COPs from C3D files exported from MoCap module of D-Flow or Nexus software.
I reproduce:
"The C3DFileAdapter includes an input parameter (ForceLocation) that allows you to choose OriginOfForcePlate or CenterOfPressure. Using the COP is helpful for visualization, but can lead to NaN values. In particular, when the force values go to zero, the COP value is undefined (NaN). This is problematic during Inverse Dynamics analysis as you will get all NaN values for the output moments. This is because OpenSim splines the force and COP data before computing joint moments and the splines are undefined in the presence of NaNs for your input data. If you find that your mot files are containing NaNs for COPs after conversion, you can use the OriginOfForcePlate flag when you import your C3D data. "