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Hello Everyone,

Currently I am trying to extract GRF solution from squad2stand prediction study. To do so, I combined exampleSquad2Stand MATLAB script and GRF obtaining part of example2DWalking's GRF prediction part.
But when I've run my script, even if "contactForces" are exist in model( I can see them in OpenSim, under Forceset. Like in 2D_Gait.osim file), I had an error and it says "contactForces" could not found.

How can I solve this error and What is the reason of this error ?

Regards, Burak.

Hi Burak,

The SquatToStand model's foot is welded to the floor, there are no contact forces in this model. You could compute the reaction forces between the foot and the ground as an alternative to get a rough approximation of what the contact forces would have been.

-Nick

Hi Nick,

I found the results from joint reaction force calculation were not the same as that from contact forces calculation. You said the former is a rough approximation of the latter. I wonder the reason why they are different.

Thanks ,
San

Hi San,

How did you calculate the contact forces? As I said before, there are no contact elements in the model.

To be more specific, the weld constraint between the ground and the foot is a decent approximation of contact forces only if the foot is not being pulled upwards, which creates a reaction force that is not possible (unless your model stepped on some chewing gum ). Obviously, this is something the model can exploit during the optimization, so it's possible that you won't find a good match to contact forces from an experimental squat to stand motion.

-Nick