issue with updating Simm Spline at the knee

[David John Saxby]

Hi All,

I have been playing around with the 2392 model, moving the yamaguchi knee to different locations to test a separate knee mechanism we are using. I have found that if I change the location of the joint medial or lateral I don't have any issues.

Pseudo Code:
1) get model
2) instantiate
3) get joint set
4) index to 'knee_r'
5) apply vector (0 0 -0.02) to parent and child locations
6) print

So far so good, the spline translations at the knee function exactly as specified in 2392 as a function of flexion angle.

Then as a separate test I am modifying the the vertical position of the joint as well.
Pseudo Code:
Same as above, except
5) apply vector (0 -0.035 -0.02) for example. (The second entry is different from above)

Now when I print what I get is the joint in the correct spot, however when I toggle the knee flexion in the coordinate pane the knee only appears to rotate and there no corresponding translations, neither in the x or y directions.

This is very odd to me because I have not touched the transform axes or functions, used only the API to construct and print the model, used a generic model from the version release which I have tested and works.

My conclusion: for some reason the vertical offset I've applied is the issue. Why?

Has anyone experienced this issue at the knee or other joints?

Best,

David

[jimmy d]

Is the actual simmspline in the new (printed) model different from the original?

[David John Saxby]

Hi James,

No the new spline is exactly as the original. This was my first check to see if for some reason the modifications to the joint def were affecting the spline values.

Very odd. I solved my issue with the knee mechanism another way but I would definitely like to know why this is happening.

Best,

David

[jimmy d]

When I change the parent-child locations (0 -0.035 -0.02) of the joint I still see the translations. How are you verifying that the translations are not occurring? Just interested because I havent been able to create the problem.

-j

[David John Saxby]

Hey James,

Thanks for answering this. At this stage I'm hoping this becomes an interesting solution and not something foolish on my behalf.

To verify I ran 3 checks and I'm attaching the data to this thread if you want to take a look. Attached are the model file, mot and PK setup.

Checks:

1) Compared the the modified knee osim xml with 2392. Splines exactly the same, double checked with Notepad++ Compare plugin, no differences in spatial transforms at all.

2) Visual check: I load a knee range of motion .mot and you can see the difference in the tibia position comparing left and right knees.

3) Point kinematics check: ran PK analysis on the tibia origin expressed in both the femur and global on the left and right sides. I get different translations, see screen shots of the plot of the results.

Best,

David