calculation of center of pressure using Hunt and Crossley contact model

[Park SangJun]

Hi expers,

i have a question using Hunt-Crossley contact model to calculate center of pressure applying on foot.
I use opensim model(Bianco et al. 2023, Simulating the effect of ankle exoskeleton torques on walking kinematics, https://simtk.org/projects/balance-exo-sim) that contains six Hunt-Crossley contact element on foot.
When i give the input motion kinematics(.trc) in this model, it works well, reporting 3-axis force and moment of each contact elements.
but this model doesn't provide COP trajectory of each simulation frame.
So, i think i have to calculate the COP by myself.
The method to calculate the COP i guess is using the location of each six element.
I wanna know that this contact element's force applying point is equal to the center of element.
if not, how can i calculate the COP?
need help, thanks!

[Nicholas Bianco]

Hi Park,

You can use MocoUtilities::createExternalLoadsTableForGait() for this. I should be using it somewhere in my balance-exo-sim project.

Note that there are a few issues with this utility function that we're currently working on addressing (see https://github.com/opensim-org/opensim-core/pull/3918). But in its current form it should give you the COPs you're looking for.

Best,
Nick

[Park SangJun]

Thank you for your answer!
My experimental action is sit to stand.
I created a .mot file through Scale -> IK, and I performed force analyze to get the three-dimensional reaction force and moment values of the six Hunt-crossley elements on the foot.
Is this the right way to get reaction and moment values?

[Nicholas Bianco]

Hi Park,

You will be able to calculate forces that way, but I don't think the forces from a foot-ground contact model computed directly with IK kinematics will be very accurate. The IK solution will not capture the fine kinematic between the foot and the ground that are important for good force estimates. If you only have kinematics, then you will probably need some sort of optimization approach (e.g., Moco) to estimate forces reasonably well.

Best,
Nick