Moving a joint center in OpenSim

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David John Saxby
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Joined: Mon May 09, 2011 8:39 pm

Moving a joint center in OpenSim

Post by David John Saxby » Thu Sep 15, 2011 12:30 am

Hi All,

We are trying to determine the compartmental reaction force during gait. We have estimates of muscle force from EMG-driven models, and we have experimental torques from ID. To resolve the compartment reaction forces we must balance the frontal plane torques with the muscle force torques. The issue is that we must solve two equations of quasi-static equilibrium: 1) To solve for MEDIAL compartment loads we balance the torques about the LATERAL compartment, and 2) to solve the LATERAL compartment reaction force we balance the torques about the MEDIAL compartment. Now to solve the muscle torques we need the moment arms for each muscle with respect NOT to the normal joint center, but to one joint center located in the medial and one located in the lateral compartments.
My issue is this: in the .osim xml file when we apply an offset to the joint location (child) or a translation constant it moves the whole bone and the muscle attachments as well. Naturally this will totally disturb our ability to do ID for the frontal plane moments and the MTU moment arms.

Is there a way to just change the joint center location without disturbing all the surrounding geometry?

PS. I am using OpenSim 2.2, but as I am having separate issues with the residual reduction, I will probably move to 2.4.

Thanks so much for any and all help.

David Saxby

Musculoskeletal Research Program, Griffith Health Institute, Griffith University, Australia.

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