simtk.org

Hi,

I am interested in doing a type of stability/perturbation analysis, but cannot quite figure out how to go about it in OpenSim. I have an OpenSim model of mine with mocap and kinetics while standing. I would like to take this model and apply some kind of additional external force to a specific point on the body (for example, the sternum) with the model's feet held fixed so that in response to the force, the model would essentially sway backwards in reaction to this force and then use its muscles to return back to equilibrium/standing straight up.

Has anyone done something similar to this before? So far I have tried adding the force in the "external forces" file of Forward Dynamics but this has not been successful and when using this approach I cannot specify exactly where on a body the force is applied, the amount of force that is applied, etc.

Any help or ideas would be greatly appreciated! Thanks!

Best,
Margaret

Hi Margaret,
To do this type of analysis you dont need mocap or kinetics data. If you had collected experimental perturbations (applied a known force to a subject during standing) then you would be able to to estimate subject specific muscle recruitment and recovery strategies.

As the original data hasn't got that, you would run a forward simulation on a simple model while standing. You could set constraints between the feet and the ground, set an objective function for the COM position, and then apply a force to a body of the trunk.

The force file allows you to specify which body (femur_r, pelvis, calcanus_l) you wish to apply a force. The direction of that force is dependent on the three dimensional force vector you assign.

Cheers,
-james