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Hi all,

When I run inverse dynamics on my model, the model lifts off the ground and then floats randomly in space.

I'm wondering if anyone knows what could cause this to happen.

I also ran inverse kinematics and the motion and the location of the ground reaction force looks fine. The kinematic and ground reaction force I'm using is from the Gait2354 example model folder that comes with OpenSim, which may be a problem.

Thanks
Laurence

Hi Laurence,

Inverse Dynamics does not perform a simulation, instead it cycles through the frames of the input motion and computes the required joint torques/forces to reproduce this motion. Please double check the motion/states file provided by loading it into the GUI as a motion to see what it looks like. What version of OpenSim are you using?

Best,
-Ayman

Hi Ayman,

Thanks for your reply.

The motion looks fine when I load it in the GUI. I'm using the OpenSim 2.2.1.

It may be an issue with the modification I made to the Gait2354 model. The torso has been split into more bodies but for testing purposes, I have welded them together so that there are still 3 DOFs for the torso (lumbar_extension, lumbar_bending, lumbar_rotation). This was done to satisfy the Gait2354 example motion data. Could this be the problem?

Regards,
Laurence

Hi Laurence,

I'd revisit the model modifications to see what broke in the model (since if it's the exact model with same degrees of freedom names it works). Potential issues:
- Some rotational degree of freedom being treated as translational.
- Changes in names of coordinates or having coordinates unspecified in the motion file.
- Order of coordinates shouldn't matter but just in case double check.
Please let me know how that goes or you think there's a bug somewhere.

Cheers,
-Ayman