CMC with ULB model failed

[Gary Lu]

Dear all, I am now working on the simplified version of ULB model. I use the simulation data from this project in SimTK

https://simtk.org/frs/?group_id=899

. The model goes well on IK,ID,RRA, but failed in CMC. I always got the error below:

Exception:
Model::equilibrateMuscles() Unable to compute equilibrium for this muscle.
Please verify that the initial activation is valid and that the length of
the musculotendon actuator doesn't produce a pennation angle of 90 degrees
or a negative fiber length.
Solver for force-velocity multiplier failed to converge.
In Object 'glut_med1_r' of type Thelen2003Muscle.
Thrown at thelen2003muscle.cpp:1704 in calcfvInv().
In Object 'glut_med1_r' of type Thelen2003Muscle.
Thrown at thelen2003muscle.cpp:350 in computeInitialFiberEquilibrium().
file= c:\projects\opensim-build-releases\opensim-core\opensim\simulation\model\model.cpp
line= 1193

Since I have already checked the properties of muscle glut_med1_r and it seems to be normal.
Can anyone tell me how should I do to solve this problem? Thank you.

[Thomas Uchida]

The exception message indicates that no solution can be found in which the muscle and tendon are in equilibrium. During initialization, the algorithm may be using the default_fiber_length and default_activation properties as initial guesses. As suggested by the exception message, you could try adjusting these properties. You might also try starting the simulation at a different time point to determine whether there might be a more general issue.

[Gary Lu]

Thanks a lot, Thomas. I'll try your suggestions to see if the result improve.