[Lowlimbmodel09-news] Scaling Coordinate Coupler Constraints

[Edith Arnold]

Hello LL2009 Subscribers,

I want to bring your attention to a bug I've encountered that will
affect your work if you are scaling the OpenSim versions of this
model.

The model uses coordinate coupler constraints to define the motion of
the patella as a function of the coordinate knee_angle.  When the
patella is scaled, the scale factors in these constraints appear to be
double counted so the patella kinematics, and thus the attachments for
the quadriceps, are incorrect. This problem is easy to observe if you
manually scale the model a large, uniform amount (try 2). The patella
moves away from the femur and tibia and the motion is very different
from the unscaled version.

I've reported this bug to the OpenSim team and have been told that it
is fixed in OpenSim 2.1, but if you don't want to change versions,
here's a work-around.

If the patella is scaled in your scaling setup:
Save the scaled model as a .osim file*
Open up the .osim file in an xml editor such as Notepad++.
Find this section:
   <ConstraintSet name="">
        <objects>
             <CoordinateCouplerConstraint ...
Find this line for each coupler constraint:
    <scale_factor> theresanumberhere </scale_factor>

Replace theresanumberhere with the value 1.0
"Save" or "Save As..." the .osim file.
Open it up in the GUI and check that the kinematics look the same as
the unscaled version.

*If you are scaling in the command line instead of the GUI, skip this
step. You already have a .osim file output from scale.exe.

Please let me know if you have any questions.

Best Regards,
Edith Arnold, MS
PhD Candidate
Neuromuscular Biomechanics Lab
Stanford University