simtk.org

Hi,

I have 3 questions concerning the estimation of ground reaction force.

1. The paper of OpenCap illustrates that the GRFs are estimated based on a contact model. What I know is that the GRF calculation part is in mainOpenSimAD.py, but cannot find any detail about the calculation part and the modeling of the contact. Could you please tell me where I can find these functions?

2. I wonder if there is only one contact model supported in OpenCap. If not, is it possible for me to use other contact models to replace the original one and how can I switch it using code?

3. Is the model for estimating GRFs supportive for different kinds of movement, without changing its parameters?

Looking forward to the reply. Kind regards!

Hi Stephen,

1. You can see the parameters for the contact spheres here: https://github.com/stanfordnmbl/opencap ... ng.py#L529. If you are interested in the actual equations, they are described in this paper: https://nmbl.stanford.edu/publications/ ... rm2011.pdf. The API calls for the spheres is here: https://simtk.org/api_docs/opensim/api_ ... phere.html and the half space is here: https://simtk.org/api_docs/opensim/api_ ... Space.html

2. Yes, currently our code is built to only support the Hunt-Crossley contact spheres with a half space. If you want to use a different contact model, you would need to re-engineer the code to add it in to the optimization.

3. The spheres that we currently have work for most movements, but they can always be tweaked. If you look into the literature, there are many different approaches to modeling foot ground contact, from walking to sprinting.

Hope this helps,
Matt