Unexpected bushing forces during SO

[Ryan Byrne]

Hi everyone,

I have been running into an issue when trying to implement the BushingForce element at intervertebral joints. Currently, the bushing forces are located at the same location and orientation as the joint center between two vertebrae. When only flexion-extension is prescribed (other DOF locked) and static optimization is run, the resulting bushing moment (Mz) is exactly as expected and the forces/moments along the other DOF are zero. However, when only translation is prescribed along the X direction (Tx), bushing forces appear in the X,Y direction and a moment Mz also appears. To me, the alignment of the bushing and joint center should result in bushing forces only along the X direction when only Tx is prescribed. Any suggestions or explanations to why this may be happening would be greatly appreciated. I have attached the model along with a motion file and CoordinateActuator file for SO (

LumbSpine_withBushings.zip

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Ryan Byrne

[Ryan Byrne]

t appears that the disc forces and moments are reported with respect to the ground XYZ coordinates, so the forces in the X and Y direction are effected by the overall orientation of the spine. However, I am still struggling to understand why a flexion extension bushing moment is appearing due to pure Tx translation at a joint. This value is not effected by the overall orientation of position of the spine in the ground frame.

[Ryan Byrne]

After some more reading, it appears that this is likely do to our center of masses being located at the vertebral centers, as opposed to at the joint!

[Thomas Uchida]

After some more reading, it appears that this is likely do to our center of masses being located at the vertebral centers, as opposed to at the joint!

Ah, that sounds plausible. If you haven't already, I would suggest drawing a picture of all the bodies, joints, and frames in your model. Might be useful for verifying the results as well.