Lumbar Joint Reaction Forces


I need to report the compression force for L5/S1 joint in the endplate frame of scrum. I read in one of the topics (Is proximal-distal force equivalent to disk compression force??) that the proximal-distal force are the same as compression forces on the endplate of sacrum.
I also read in another topic (The coordinate system of intervertebral disc forces) that:

"The joint is made up of two reference frames, and the joint reaction forces are expressed in the coordinate system of the first reference frame inserted in the joint definition. If you look at JointsLumbar.any file, that will typically be the reference node on the proximal segment in the joint."

I ran a model using a bvh file for a static trial. Then, I tried to draw the reference node on the proximal segment (SacrumL5JntNode) and measured the rotation angles between the global frame and the first frame of the joint for standing posture. The result shows that the first frame for the joint is almost paralleled with the horizontal line; on the other hand, it seems that the model did not consider sacral slope for measuring joint reaction forces.

Could you please advise if there was an error in my approach? Alternatively, if my understanding is correct, could you assist me in measuring joint reaction forces by considering the sacral slope?

Specifically, I am wondering if it's feasible to define a frame with the sacral slope on the sacrum and report the joint reaction forces on that frame.

Hi Mohamad,

Your observation is correct.

The SelectedOutputs that you saw with proximal-distal trunk joint reaction force were not aligned with the bone geometry.

This has been fixed in AMMR 3.0. Please see the changelog here. Now, you will find that the selected outputs have been renamed to compression force instead of proximal-distal force as the compression force outputs are measured in reference frames that are aligned with the proximal bone surface in the joint.

This way, you will see the joint reaction force considering the sacral slope.

Best regards,