simtk.org

Hello

I'm settung up a model to analyze mocap marker trajectories and ground reaction forces and moments. My goal is to perform inverse dynamics and calculate joint power and work. I would also like to estimate muscle forces using e.g. static optimization. The motion I'm analyzing is running.


I have used the "lee-sonstoolbox" to transform marker trajectories from my lab orientation to the reference frame used in the model called "leg39" that is available in the model repository. The toolbox didn't work to extract GRF&Ms from the .csv file, and i have therefore exported the .mot file from Vicon Nexus.

I think i have solved the kinematics correctly. I have RMS errors for the markers at 0.0319, and the pose of the model looks correct.

The inverse dynamic results on the other hand looks completely off comparing the results i have calculated using other musculoskeletal softwares. Looking at the raw data in the .mot file from Vicon, GRF&Ms look correct. I have attached the joint angels, moments and pelvis forces in plots. I would assume the pelvis forces should be around 0 and they should only be active to take up small residuals to maintain equilibrium for the analysis solved bottom-up.

I hope one of you guys can help

Cheers
Frederik

I assume that you have scaled your model. It might be that you are not applying the external ground reaction forces correctly. I advice you to look at the configuration files that come with gait2392 (subject01_walk1_grf.xml) to see how the external forces are applied. Furthermore, make sure that the muscles are excluded from the inverse dynamics setup file.

hey

thanks for your reply. looking at the setup file (gait2392 (subject01_walk1_grf.xml)), i think i have applied the forces correctly.


why do i need to exclude the muscles? the gait2392 example works without i exclude any muscles.

any other suggestions?

Frederik

Hi Frederik Larsen,
I used the "lee-sonstoolbox" to transform marker trajectories too. However, I found I didn't known the theory about moment conversion in the tool. It doesn't seem to be just as simple as changing the magnification and positive or negative. Therefore, I am not sure the GRF moment is true or not. So I want to learn how to export the .mot file from Vicon Nexus.
Anything will be helpful. Hope your reply.
Best wishes!
Qianwen Nie