InvDyn w. GRF as external load - what is torque field?

[Vivian Allen]

Hey guys, quick question from a beginner. I'm running inverse dynamics in OpenSim, using xromm kinematics and GRF's from a kistler forceplate to estimate joint torques and power. I've got the GRF as a point force applied to the pes, with the force and CoP expressed in the ground. However, the tool will not run unless I also specify a torque (I get an error message to do with being unable to find the label 'unassigned' in my forces storage file). This torque effects the calculated moments (I've tried using faked constant zero and 1 torques), but what torque is this referring to? The free vertical torque about the plate? The individual component torques about the plate centre? Some kind of vertical torque about the CoP? Or is it supposed to be the GRF torque at the MTP joint (which would be strange, as that is what I'm asking the tool to estimate).

I can calculate the component torques from the plate ouput and geometry with no trouble, but I am not sure why the inverse dynamics tool would need that information - should just be the force vector and the point of application, right?

Anyways, anyone with any pointers would be much appreciated! I'm sure it's something pretty simple, it's just not something I have come across before. This is my first inverse dynamics study. Cheers guys,

Dr Vivian Allen (Uni. Jena, DE & Royal Vet. College, UK)

[Rudolf Klemetti]

Hi Dr. Allen,

T_y is free vertical torque about COP exerted to the model. T_x and T_z are usually zeros.

Regards,

Rudi

[Vivian Allen]

Excellent! Many thanks Rudi.