Locking subtalar and mtp joints

[Ruoli Wang]

Hi,

I read from some documents said we need lock subtalar and mtp joints in RRA and CMC, which means that we are not able to track subtalar and mtp kinematics. I want to do a perturbation analysis on subjects with abnomral gait pattern, especially in ankle/subtalar and mtp joints. I wonder any other Opensim users have successfully perfrom RRA, CMC without locking subtalar and mtp joints? Is it any possible to do it in practical? Any suggestions will be very appreciated.

Thank you in advance.

[Andrew Lewis]

Hi Ruoli,
I've been doing RRA/CMC on the gait2392 and LLM2010 models and haven't locked any of the coordinates. The resulting foot kinematics seems to match reasonably well with my original kinematics from our Vicon motion capture system. My vicon model has a solid foot section so only produces ankle dorsi/plantarflexion and foot-progression angles which I have compared to OpenSim ankle_angle and subtalar_angle respectively. This is a bit of a fudge but I used it only to check that OpenSim was producing something sensible. I didn't have any MTP angles to compare to so I don't know how well OpenSim tracked these.

You shouldn't have any problems generating foot kinematics in OpenSim as long as you are using a complex enough marker set. I believe the problem with CMC arises because a lot of the models (eg gait2392 and LLM10) don't have enough musculature in the foot to allow the model to generate the appropriate joint torques.

Give it a go and see what happens!

Hope that helps,
Andy

[Ruoli Wang]

Thanks Andrew,

Just wondering how did the residual look like in your simulations without locking any coordinates? I heard some rumors saying that since very small inertials of the bones in the foot makes very high residual and crashes in CMC.