Run static optimization using free joints

[Margarida Machado]

Hi,

I'm trying to perform a static optimization in a knee joint model. My model has 3 bodies (femur, tibia and patella). The femur is fixed to the ground (weld joint) and the tibiofemoral and patellofemoral joints are modeled as free joints (6DOFs for each). The model includes 14 muscles (biceps_fem_lh, biceps_fem_sh, gastroc_lat, gastroc_med, semimemb, semitend, tens_fasc, vast_med, vast_inter, vast_lat, rectus_fem, pat_lig_1, pat_lig_2, pat_lig_3). Tibiofemoral contact forces and Patellofemoral contact force are also considered in this model.
When I run the static optimization, I always get constraint violation problems.
Anyone can tell me how can I solve this problem?
I tried to learn from the OpenSim examples, but all the models use pin joints, and with pin joints my model works fine. Although, my goal is to evaluate the medial and lateral contact forces and, so I need free joints.

Thank you,
Margarida Machado

[Ayman Habib]

Hi Margarida,

Can you post the error messages you get? If your model does not have actuators to produce the accelerations computed from the motion you provide, then Static Optimization with fail.

You'll need to add "residuals/reserves" at those degrees of freedom that can not be driven by the muscles so that the acceleration constraints are satisfied.

Hope this helps,
-Ayman

[Margarida Machado]

Hi Ayman,

I didn't include any residuals/reserves in my model, so that could be the problem.
I only have the markers file from the gait lab. How can I get these residuals/reserves values? There is anyway to get them from OpenSim?

Thanks again for your help.
Margarida