Inquiry to the methodology used to develop the model

[Obidah Alawneh]

Hello,

I was running static optimization on the model, with a motion file that has a 40 degree pelvis tilt (sagittal plane), and after looking at the forces in the muscles I noticed that the left and right muscle forces are very different, for example levator scapulae Right was way higher than left, and keep in mind that no external forces or actuators were applied.

Even after trying to add actuator files and external forces the differences were big and some of the results did not make sense.

Am I doing something wrong or was the model developed unsymmetrical?

The images attached:
The one of the plot has actuators applied
The one of the model is without anything applied (other than the motion file)

Thanks,
Obidah Alawneh

[Jacob J. Banks]

The model is/should be symmetrical. You can always double check this by looking at the model specifics in opensim or via a source code editor. Therefore, I hypothesize 1 of 2 things:
1. Your 'motion' isn't as symmetrical as you think.
2. Your actuator(s) on the pelvis are not large enough to fight gravity (so the model is falling). So increase their max/min force/moment magnitudes (for Fy and then if you are tilting forward you would need to increase Mz).
However, I cannot be certain without seeing your files etc.
Jake Banks

[Obidah Alawneh]

Thanks for the quick reply, so if I increase the optimal force of all the actuators to 1000 for example would that solve the problem?

I know the motion file is symmetric because I put 0 for everything except pelvis tilt.

Best Regards,
Obidah

[Jacob J. Banks]

Not for all (you want the muscles to do things)!!!! Just the 6 Pelvis (3x Forces and 3x Moments, make min and max's ~1000 in magnitude... huge overkill for moments) ones so that there is a reason for the model to be staying at 0,0,0 and not falling.
I suspect this is your issue... otherwise I'll need more info.
Jake Banks

[Obidah Alawneh]

Ill try to see how it goes with changing the actuators optimal force!
Thank you for your help and sorry for the inconvenience!