Model and coordinate system

[Sandra Salvador]

Hello,

I have a couple of questions about OpenCap that I was hoping you could help me with.

The first question is why, when I opened the LaiArnoldModified2017 model in OpenSim, the feet appeared to be below the floor. Why is this happening?

For the second question, I'm interested in synchronizing OpenCap with both the force plate and marker-based motion capture system. Do the coordinate systems need to be in the same position, direction, and orientation for both the OpenCap coordinate system and the force plate coordinate system?

Thank you really much,

Sandra

[Scott Uhlrich]

Hi Sandra,

When you open a model in opensim, there is no motion associated with it, so how it appears relative to the floor is determined by the default pelvis_ty. You can change pelvis_ty in the coordinates pane.

It depends what you intend to do with this synchronization. If you want to perform inverse dynamics with ground reaction forces from the force plates, then yes the data needs to be synchronized in time and space. We have an example for using OpenCap with force plates (https://github.com/stanfordnmbl/opencap ... ntegration). Keep in mind, however, the time sync'ing method is specific to the task (jumping in this example). You could also time synchronize using marker data.

[Sandra Salvador]

Hi,
Thank you very much for your reply!!

I would like to compare the kinematics and kinetics (including force moments) estimated with OpenCap with those estimated using a marker-based MOCAP system and force plates.

In this case, is it also necessary to synchronize the data in time and space?

Thank you very much,

Sandra

[Scott Uhlrich]

If you only want to compare joint angles, GRFs, and joint moments, you would only have to synchronize in time. You would need the coordinate systems aligned to compare GRFs, but the origin of the coordinate systems wouldn't necessarily have to align.