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Hi,
I'm trying to get a joint reaction force on the knee (on tibial frame) without muscle force during a jump task. The following is what I did and related problem.

Removing muscle from model file
Joint reaction force was too small than my exception, it's around 80 N.

Applying 0 N force with modified static optimization file
Showed same results as above.

ID with 6 DOF knee joint
I could get reasonable value but the results should be on the parent coordinate system (800 N of axial force and 600 N of shear force on femur), then I cannot get joint reaction force on tibail frame.

If someone can solve this issue, please let me know.
Thank you for your help!

You should included muscle forces from static optimization.

When you unlock the joint (free joint) then you don't expect any reaction force (they are zero) because the movement in every direction is not restricted.

Hi Dimitar,
Thank you for your quick response!
I did 3 ways separately, so it' not free joint when I run the SO or JR. I used 6 DOF knee only for ID. Sorry for confusing.

I also tried SO and JR with only coordinate actuator without any muscle force that described in the following post:
viewtopicPhpbb.php?f=91&t=5512&p=0&start=0&view=
The results of knee joint reaction force was around 150 N and still very small because the vertical ground reaction force is about 1200 N.

Is there any other way to calculate joint reaction force without muscle force? My goal is to calculate the net joint force on tibial frame to see the effect of ground reaction force.

Thank you,
Ryo

Hi Ryo Ueno,
Have you solved this problem? I also encountered the same solution and need your help.
Looking forward to your reply.