simtk.org

Hello,

I have seen people performing simulations and adding a coordinate limit force to a specified joint in order to simulate an assistive device such as an exoskeleton. I have been trying to perform the opposite and apply a resistive force that would prevent a model from reaching the specified target. Has there been a proven method of applying this previously? The simulation runs on for infinity as it never reaches the specified target, which makes sense, but also classifies the simulation as failed although it technically would be successful based on what I am trying to do. A restrictive brace also would increase the more the joint moves past a threshold, like the Coordinate Limit Force, but I get an error where the joint shoots up from 0 degrees to the joint angle it gets stuck at, and a difference of -0.5 in the stiffness changes from instant change violating the joint speed restraints to a normal joint movement with no apparent changes. Any advice on how to pursue this endeavor?

-Jayson

Hi Jayson,

We used Moco in a study where we simulated increases in passive forces around the shoulder in response to surgery based on joint position. You can find it here: https://pubmed.ncbi.nlm.nih.gov/33241584/

The method to simulate the resistive forces was similar to what you described, just that we adapted the ExpressionBasedCoordinateForce to use two (instead of one) coordinates in the function. The restrictive forces weren't over the top in this study, so it didn't limit the model from reaching the target joint positions - but there was a definite increase in the muscle force/activity required to do so.

Aaron