simtk.org

Hello,
while I was working on one of the model model provided by the software, I wasn't able find anywhere a sort of protocol that tells us where each reference frame of each bony segment is placed. Can anybody tell me where could I find it?

Thank you so much for you help

Carlo

Hi,

The geometry files are transformed based on 3 rotations and 3 translations shown as Transform () in the property editor window when the geometry is selected. Some geometry files have offsets built into them, but to see the geometric origin of a body/segment you can select the corresponding body-node in the GUI navigator and select "Show Axes" and that will display a coordinate frame at the geometric origin of the corresponding body/segment.

Hope this helps,
-Ayman

Than you for you reply Ayman. My question comes because reading some papers and articles by Cappozzo and Davis I find out that usually the origin of the femur reference frame is placed in the centre of the knee, halfway between the epicondiles, while in OpenSim I see that it is placed into the femural head. Not a big problem at all, in the end, just a matter of coordinates transformation.

Thanks for your great work

Carlo

Hello,

My problem is very similar to Carlo's. I have muscle attachment sites and muscle force vectors during a gait cycle defined in what I believe to be the individual segment reference frames (center of the femoral head for the femur, and at the midpoint of the femoral epicondyles for the tibia). I am hoping to apply these vectors as boundary conditions in a finite element model at various stages of the gait cycle. The FE model I have produced is very similar to that offered by the Open Knee project, available here on SimTK for download. As such, I should have no problem implementing muscle force vectors appropriately where they attach to the tibia. However, due to the fact that my model includes a femur truncated near the distal end of the body, I have no way of defining this segment coordinate system within the femoral head.

Per the suggestions in this thread, I have located the axes used for each body segment, and confirmed that both the femoral and tibial reference frames seem to be oriented in the same directions for the case of zero knee flexion. Is there a way to determine a vector that connects the origin on the femoral segment reference frame to the origin of the tibial segment reference frame within OpenSim (such that I could transform the attachment coordinates and muscle force vectors defined in the femoral frame to the tibial frame assuming a rigid body rotation during gait)? If this is not possible, is there an easy way to change the output reference frame of all computed muscle attachment sites and force vectors to be the tibial segment frame?

Thank you very much for your help.

Spencer

Hi Spencer,

I'm not sure which model of the lower limb you are using, but many models include a knee joint that is composed of n rotational dof's and two splines (ant/post and superior/inferior) coupled to the flexion or first rotation of the knee. The combination of free rotation and coupled spline motion models the complex behaviour of the knee. Now if you evaluate the superior/inferior spline at zero knee flexion angle node (you can look at the properties in the GUI or in the "model".osim file), you will find the generic (non-scaled) vertical translation from the parent (femur origin). This value will change with a scaled model, but you can find it the same way. It think this is your vector.

If you don't like that you could put a virtual marker at the origin of the two bodies and use the Analyze tool with Point Kinematics for the relevant points and see what you get. The vertical translation will change (couple to flexion), so maybe this approach is better than the single zero flexion angle vertical translation value listed above.

Best,

David

Hello,

I have a question in relation to the reference frame and translation function of the knee joint. Is there a underlying thought why the reference frame of the knee joint is placed in the femoral head or is it possible to place it at the origin of the tibia as well and alter the translation function in relation to this new location? In this way changing the orientation and location of the knee joint will more intuitive (without coordinate tranformation).
Thank you in advance for your response.

Elisah