JRA joint location

[pauline neerpelt]

Hello everyone,

This is probably a very easy question but I would like to be sure I understand it correctly.
I'm using a FD simulation with a JRA. I used the following settings to check whether the forces on two adjacent bones are equal and opposite:

settings - Copy.JPG (23.25 KiB) Viewed 574 times

This is the case, however the points in which the forces are expressed (PX,PY,PZ) are identical (see image A). Does this mean that P=C (see image B) for this joint?

Image A:

points.JPG (43.87 KiB) Viewed 574 times

Image B:

reference - Copy.JPG (35.34 KiB) Viewed 574 times

Kind regards,
Pauline

[Thomas Uchida]

Does this mean that P=C (see image B) for this joint?

Yes. This is true for most joints, a notable exception being the knee in models like gait2392 (which is defined as a CustomJoint with nonzero translations between the origins of the parent and child frames). It might be helpful to visualize the parent and child frames. Please see the "Object-Specific Commands" page of the documentation, under the heading "Displaying Coordinate Axes for Body and Joint Nodes" (https://opensimconfluence.atlassian.net ... c+Commands), which describes how to toggle on/off the parent and child frames.