Large Residual Force in Y direction (FY)

[Ali Khalilianmotamed Bonab]

Dear Opensim experts,
I added some bodies to my model lower extremities, and I am trying to perform RRA and CMC using this model with the bodies added to the model. However, I am getting large FY residual force which on average is 22 N, and in peak, it is 50 N. I followed the provided troubleshooting instructions to reduce this residual force, but I am still getting this large force (ascending from small force to large force).
1) I checked if my added bodies have similar properties and mirror-image symmetry in both left and right legs.
2) I decreased the Lumbar Actuators weights to increase its penalization. It worked and reduced my residual force max from 75 to 50 but decreasing more did not help.
3) I decreased the lumbar joint position weights, but it did not make any change on the residual force.
4) I am just getting this significant error on FY and the other forces and moments are in the good and acceptable region.
It should be noted that the bodies have considerable inertias and masses.
The model can be provided upon the request to check any possible error.

[Dimitar Stanev]

Hi,

If you are getting on average 22N, this might be ok. Just to clarify.

You have the kinematics and ground reaction forces of a subject. Then in your model you add some extra bodies. In that case, the model + extra bodies would not have to match the experimentally measured ground reaction forces. Therefore, you will have residual errors in the pelvis.

[Ali Khalilianmotamed Bonab]

Dear Dr. Dimitar Stanev,
Thanks for your reply,
I noticed that my added bodies were not properly scaled, after scaling them, this drift on the residual force has been solved.
Sincerely,
Ali.