Locking MTP and Subtalar Joints

[Lisa MacFadden]

Does anyone know why when running CMC in the gait2354 model, the MTP and subtalar joints need to be locked? I am trying to run CMC with a feline model of the hindlimb that I developed and the MTP joint is an important joint in the sit-to-stand transition that I am trying to simulate, so I am trying to understand why these two joints are locked in the human model.

Cheers!
Lisa

[Joseph Alston]

I might be completely off, but I thought it was because in the example there was no IK information about those joints. Therefore for ease of calculation, those joints were removed. At least that is the reasoning I'm using for my model, and it seems to be working.

Joseph

[Lisa MacFadden]

Joseph,

Thanks that makes sense!

Cheers
Lisa

[Sam Hamner]

Hi Lisa,

I think the MTP and subtalar joints are locked in this example due to the fact that the muscles in the model crossing those joints are generally not strong enough to match the accelerations of those segments if they were unlocked. Also, in general, these are segments with small mass with large forces and moments being applied (i.e., the ground reactions), also making them harder to control. Therefore to simplify the example, the joints were locked.

It is possible to generate a simulation in CMC with these joints unlocked, but it becomes more difficult (or requires larger reserve actuator moments) with faster movements (i.e., higher accelerations) or movements with larger ground reaction forces.

Hope that helps clear things up.

Cheers,
Sam