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I have a model of the spine with 3 DOFs, but 15 constrained coordinates (i.e. the DOF is flexion extension of the spine, and each of the lumbar joints have a coordinate that is constrained to move a fixed percentage of the net flexion-extension, etc), and I've been able to run Static Optimization without any problems on it.
However, the output file for the muscle activation says that all the muscles in my model (i have 4) are are activated at the minimum activation level possible (=0.02) throughout the motion. this doesn't seem to make sense since i have the model flexing-extending from -10 to 10 degrees, and I was wondering what the cause is. The force file has values that change with time, so I'm not quite sure what's going on.

I've also been getting the same result with another model that I have (which has 15 unconstrained coordinates, and bushing forces in between each vertebra to mimic the disc-stiffness and prevent the spine from falling apart) -- for some reason, i have never get the muscle activation to be anything but the minimum activation values...

thanks,
adila

p.s. I'm not sure if this will help figure out what's going on, but in running my constrained model for static optimization, the execution finishes, but I also keep getting this message:Constraint violation = -1.#IND
what does that mean?

I've also tried changing the maximum isometric force of the muscles (from 2500N to 30N) and thres still no difference in the activation levels....
any help would be appreciated!

Hi Adila,

I'm having these same issues with the lumbar-238 model, ie: minimum muscle activation for 40 degrees of forward flexion. Did you ever find a solution?

Thanks,
Alex