coordinate systems

[Richard Heinz]

Hi! I have some questions about the coordinate systems described in the 2009 study (A model of the lower limb for analysis of Human Movement): Usually, the transepicondylar line is used as with the midpoint therof being the origin of the coordinate system. In your paper, you define the origin of the tibial coordinate system with the midpoint of the femoral condyles. Is that a typo? Or how do you determine that point as the condyles are an extended feature?
Also: Are those coordinate systems the same as used in Delp, et al., "Surgery Simulation: A computer graphics system to analyze and design musculoskeletal reconstruction of the lower limb" and Brand, et al. "A model of lower extremety muscular anatomy"?
Thanks for your help!

[Edith Arnold]

Hi Richard,
The coordinate systems are the same as in Delp et al. except for the pelvis coordinate system. The two pelvis segments were reoriented and the "neutral position" was defined such that the ASISs and pubic tubercle lie in the frontal plane.
The tibia coordinate system is the same as in the Delp model, but there are additional coordinate systems that match the convention by Walker et al. to make implementing those kinematics more straightforward.
If you'd like to look at the reference axes in Opensim expand Bodies, right click on a body, and select Show Axes.
I hope this helps. Let me know if you need more details.

[Richard Heinz]

Thanks for the answer!
I have another question touching a completely different topic: In the model all the origins and insertions are included with coordinates. Are these coordinates somehow scaled to the bone size or absolute values (in m?)?
Thanks a lot again.
Richard

[Edith Arnold]

The coordinates are absolute for the segment coordinate system. If you scale the model, then compare the new geometry points to the old ones you'll see they change accordingly.