simtk.org

During this analysis Opensim computes the min of a weighted least squares problem; my question is: what are the units for marker and for generalized coordinates? meters and degrees?
thanks,
bruno

Hi Bruno,

The length units are the model's length units (default is meters I believe) for rotational degrees of freedom, the units would be radians (all the computation is done internally in radians).

-Ayman

Are you sure that length units are meters?
because the input file .trc with coordinates for makers is in mm...

Bruno,

It's true that the trc file can be of any supported units, but we internally convert it to meters and the error messages you see in the log window refer to the errors in meters.

Best regards,
-Ayman

ok, thanks Ayman

Hi,
I have one comment on this. I spent the whole day trying to run my scaling process … in the end I went through (again) the Tutorial 3 (Scaling, Inverse Kinematics, and Inverse Dynamics; Gait2354) and I realized, that in the model, the positions of the markers are in meters, e.g. [0.605; 1.425; 0.02], but in the static.trc file, the marker’s position are in millimeters(?!), e.g. [605; 1425; 20]. When I adjusted this in my files, then “suddenly” everything went all right.

What surprised/confused me was, that in the User’s Guide (page 157, chapter15-Inverse Kinematics) is written, that “The units used by IK are the model’s units, which are meters and radians”. That’s why I kept meters everywhere .

Hopefully, the IK will run successfully.

Michala

One more note – the units “problem” was also discussed in the following threads

• Inverse kinematics
  https://simtk.org/forums/viewtopic.php? ... nits#p6236
  
  preview data display scaling vs model scaling
  https://simtk.org/forums/viewtopic.php? ... nits#p4682
  
  Upper extremity model and markers ?
  https://simtk.org/forums/viewtopic.php? ... nits#p2473

Hi Michala,

Thanks for pointing out this potential pitfall. Please file a bug report detailing what you ran into and we'll try to improve the documentation in the next release to address this issue.

Best regards,
-Ayman

Hi,
Maybe one more comment here. The model is by default in meters, and if you have the coordinates in the .trc file in millimeters, and it is specified in the header, then it will be coupled with no problems.

Michala