weld constraint and ground reaction forces

[george rose]

Hi. With the help of Mr. Habib we have been able to input our recorded joint angles from a single markered leg when squatting into opensim. We have also created (and attached) another mot file with both legs moving (we copied and pasted the text again and changed the co-ordinates where appropriate to see this would solve our issue). We are still unable to scale the model or to input our ground reaction forces (taken from a force plate under both feet, and also attached) using the external load section of the inverse dynamics tool.When we click 'run' in the GUI and error message pops up in the bottom right corner saying expected error, the matrix is singular. Our second is that we have been unable to use the weld constrain to fix the feet to the floor (at the moment the hips and fixed and feet move upwards). We attempted to this by following the instructions on weld constraints in the user guide.

We have attached two mot files, single leg and both legs, a trc file, ground reaction forces.

The aim of this project to test whether changes in pelvic tilt starting angle have any effect on the joint torque at the ankle, knee and hip when squatting. Thank you for taking the time to help us to resolve these issues. Please contact me if you need any more info.

[jimmy d]

Hi George,

It seems that GRF file has been incorrectly determined for OpenSim. The GRF vector and COP have the incorrect coordinate system and units, respectively. You will have to assume that the GRF is equal for both feet and the COP position (under each foot). This is so the external loads can be applied to each foot directly.

This page will give you a description of the correct data format for OpenSim;
http://simtk-confluence.stanford.edu:80 ... +Utilities

If you have a pelvis (x,y,z, tilt, list, rotation) positions you can apply them such that the model does a squat, rather than an apparent leg lift.

cheers
-james

[nicolas velasquez]

Hi,

Did you guys ever update the model ?
We are working on a project to study squatting motion and I was wondering if you fixed the "squatting" versus "leg raise" in the model ?

Thanks

Nico