External Forces for Joint Moments

[Francesca Santini]

Hi,

I have a question on how to calculate the joint moments in OpenSim from a simulation of Scone. I have loaded the .sto file in OpenSim, and I am using the Inverse Dynamics tool. I am not sure what are the external loads to add. In the .sto file, there are the right and left leg grf, normalized, the grm and the foot forces and moments.

Until now, I added both legs normalized grf, as point forces on the cop, and grm, is that correct? Is it correct to select 'applied to ground', 'force expressed in ground' and 'point expressed in ground'? Do I need to add the foot force and moment too?

Thank you in advance!

Francesca

[Kirsten Veerkamp]

Hi Francesca,

Have you tried running the tool without adding external loads? From my experience, when adding the external loads (following instructions from https://simtk-confluence.stanford.edu:8 ... amics+Tool for a point force, using the non-normalized GRF), the GRFs were applied twice, resulting in joint moments which were twice as high as they should be. Also, the vertical residuals were around -750, indicating that the GRFs were applied twice. I am not entirely sure why this is, my guess is that the GRF is already taken into account internally when the SCONE output file is loaded.

Let me know how it goes!

Cheers,
Kirsten

[Francesca Santini]

Hi Kirsten,

Thank you for your reply! I have tried without adding external loads and the resulting joint moments are in the correct range. I agree with you when you say the GRF is already taken into account then.

Cheers

Francesca