Change isometric force of muscles

[Pia Stefanek]

Hi,

I implemented a path spring parallel to the rectus femoris muscle in the Gait10dof18musc model. Now I want to weaken the isometric force of the rectus femoris muscle to see the effect of the spring. In the model the rectus femoris has a maximum isometric force of 1.169N. I changed it to 292.25N (25%) and 876.75N (75%) and performed CMC. For 25% of the the maximum isometric force the simulation showed no error. For 75% of the maximum isometric force the simulation crashed and I got the following exception:

SimTK Exception thrown at interiorpointoptimizer.cpp:264:
Optimizer failed: Ipopt: Maximum iterations exceeded (status -1)
OPTIMIZATION FAILED...


CMC.computeControls: ERROR- Optimizer could not find a solution.
Unable to find a feasible solution at time = 1.51.
Model cannot generate the forces necessary to achieve the target acceleration.
Possible issues: 1. not all model degrees-of-freedom are actuated,
2. there are tracking tasks for locked coordinates, and/or
3. there are unnecessary control constraints on reserve/residual actuators.

I don't understand why the model cannot generate sufficient Forces when I set the maximum isometric force of rectus femoris to 75%, but can generate enough when I set it to 25%. Could somebody explain this to me?
Further, when I add control constraints to CMC and set the isometric force to 75% everything works. I don't understand why the control constraints help with this problem.

Thanks in advance.

[Thomas Uchida]

CMC is a tracking algorithm; depending on the model, the experimental data being tracked, the simulation parameters, etc., there may be different amounts of error (i.e., you may be solving a different optimization problem) at 1.51 s when Fmax is set to 25% vs. 75%. Do the kinematics diverge? Do you have the appropriate reserve actuators in your model? There are some additional tips in the "Best Practices and Troubleshooting Tips" section on the "Getting Started with CMC" page here: https://simtk-confluence.stanford.edu/d ... ootingTips.