Prescribe rotation using spline

[Alexandra Yurova]

Hi,

can anyone explain, how one defines rotation in tibiofemoral or patellofemoral joints using spline?
E.g. in Arnold et al 2 Legs model it is defined like that:

<TransformAxis name="rotation2">
<function>
<NaturalCubicSpline name="">
<x> 0.00000000 0.17453293 0.34906585 0.52359878 0.69813170 0.87266463 1.04719755 1.22173048 1.39626340 1.57079633 1.74532925 1.91986218 2.09439510 </x>
<y> 0.00000000 0.01268093 0.02269692 0.02960539 0.03320492 0.03353540 0.03087791 0.02575478 0.01892953 0.01140698 0.00443314 -0.00050475 -0.00167820 </y>
</NaturalCubicSpline>
</function>
<coordinates> knee_angle_r </coordinates>
<axis> 0.00000000 0.00000000 1.00000000 </axis>
</TransformAxis>

As far as I understand, x is an angle of knee flexion in radians. Isn't it enough for knee rotation?
What is y in this case? How one can compute it?

Regards,
Alexandra

[Thomas Uchida]

The knee joint has only one degree of freedom, but it also translates and rotates about the other two axes as it flexes and extends. More information can be found on the "Gait 2392 and 2354 Models" page in the documentation (https://simtk-confluence.stanford.edu:8 ... 354+Models) and the papers listed in the "Associated Publications" section at the bottom of that page.

[Alexandra Yurova]

Thank you for response.
I still can't get, how one can compute <y> variable?

[Thomas Uchida]

I still can't get, how one can compute <y> variable?

It depends what data you have. One way would be to implant bone pins in the femur and tibia, track the motions of the bones, compute the transformation from one reference frame to the other over the entire range of motion, and then express this relationship as a spline for each of the dependent degrees of freedom.