simtk.org

Hello,
using my own model, I would like to perform a static optimization (using v.1.8) to compute the time histories of muscles forces. Both the IK and ID work well. I've tried to use as input the IK result or the ID states result (motion files), but after a warning "the optimizer could not find a solution at time= x.xx", the simulation doesn't go ahead. Any suggestions on how to use this tool correctly?
Thanks.

Hi Giordano,

Static optimization runs ID and uses the results to solve the distribution problem. The fact that ID worked suggests to me that this is a problem with distribution of forces/torques among muscles. In the past we had similar issues when the model was too weak.

Very likely, increasing "Max Isometric Force" from muscles will solve your problem.
You maybe able to figure out which muscles need strengthening by looking at ID results if you want to make this change as minimal as possible.

Hope this helps,
-Ayman

Hi Ayman,

I strengthened the muscles and used my "InverseDynamics_force.sto" file (ID results) to run static optimization, without entering ground reaction forces. What happens is that the computing stops at a certain analysis percentage (depending in the time range), the model doesn't follow the gait trajectories and I receive a warning for every frame, concerning not finding a solution, constraint violations and optimization failed...
Could it depend just on muscles parameters?
Thanks for your help.

Giordano

Hi Giordano,

If you're doing gait (which appears to be the case) you also need to apply the pelvis residuals as well since muscles will not produce enough force to match the accelerations computed by ID.

-Ayman

Hi Giordano,

Could be also the tendons slack length that you use as input in your model! Have you measured that or estimated through an optimization technique?

Dimitris

Hi,
simplifying my model (using the pelvis like ground and having the same number of muscles and dof), and using muscles parameters from gait2392, I'm able to run static optimization. Anyway I still receive messages about acceleration constraint violations (model unsuitable for optimization). In addition, plotting the results (muscles forces vs time), the curves appears like noise, with many high value spots. What is the best way to get acceptable results solving the distribution problem?
Thanks.

Hi Giordano,

Would you please tell me which files you used as inputs for the analysis? I have tried it a couple of times and still have not been able to get any results.

Thanks.

The inputs for the Static Optimization tool are: the current model, the file containing the time history of the generalized coordinates (computed by IK) and the external loads (if any).
Anyway I still have troubles using this tool.

Cheers.

I was able to run the analysis through the Analyze tool. Try building an extra .xml file with reserve actuators. I had problems in the plevis with MX, MY, MZ and FX, FY, FZ; so by addding actuators in each coordinate, the model was able to run perfectly.