GRF moment converting from Vicon to Opensim

[聂 倩文]

Hello there!

I was going to conduct a muscle force simulation. So I need a GRF .mot file from my erxperimental data. I use a Vicon system(Nexus version 2.5.0) to collect markers trajectory and two AMTI (series OR-6) force plate to collect ground react force. At first, I used the Lee-Son toolbox to convert .csv file to .mot file to be used in OpenSim. Then, I found that Nexus GUI can export .mot file and .trc file directly. However, I was astonished that the ground reaction moment was very different between the two ways, as the figure shown.



As the difference is large, I tried to know the dedfinition of the ground reaction moment and the relationship between the Opensim and the Vicon. However, I didn't found the definition in the two system and the relationship between them. I also tried to contact Sangjun Lee, the developer of the Lee-Son's Toolbox.But I haven't receive a reply till now.

I add my data file to the attachment so that you can unstand my question.
How do I get around this? I' m hungry for your reply!

Best regards,

Qianwen

[jimmy d]

Hi-

Typically, motion capture systems often export COP and forceplate moments in millimeters (mm) while OpenSim works in meters (m). So if you were writing a transform function you would have to convert the units.

OpenSim 4.0 has methods to read C3D data into OpenSim format, you can read about it here. Or you could use packages from other people, a list can be found here.

Cheers,
-J