Changing Weld Joints to Track Kinematics and Actuation around that Joint

[Adam Park]

Hello Moco Forum,

I am currently collecting a dataset with the current example model from the opensim core for 3D walking, and have been able to produce accurate (low marker error) inverse kinematic solutions using this model scaled to my subject.
My question is;
Would moco inverse still be capable of using the current model to produce actuator force estimates about an added joint that was previously a Weld joint? For example, if the 1st metatarsophalangeal (MTP) was changed into a Pin Joint.

For just some further context, I have collected and partially processed a dataset including the same template marks PLUS additional markers that were part of an extended foot model able to track movement about the MTP joint (e.g. Oxford foot model, Amsterdam foot model, etc.). Would it be possible to use these additional markers to track the kinematics about this MTP joint if it were changed from a Weld joint to a Pin joint? Furthermore, assuming that the IK solutions would be of good enough quality, would I be able to use these joint trajectories - and this now adjusted model - to have Mocoinverse track the prescribed movement about this joint and estimate the required muscle forces?

Any assistance is much appreciated, please let me know if any further clarification is required.

Adam

[Nicholas Bianco]

Hi Adam,

As long the actuators (including muscles, if relevant) are strong enough to produce the joint motion for the MTP joint, you should be able to get your problem to solve. However, we usually apply ground reaction forces to the model when using MocoInverse, and currently there is no good way to apply forces across two bodies (e.g., with ExternalLoads). In other words, it is not possible to apply the GRFs to either the calcaneus or toes based on the location of the COP. Therefore, we usually apply the loads to the calcaneus and lock the toes.

If you do add in the MTP joint, the problem might still solve, but the MTP joint moments will probably be unrealistic since no forces will be applied through them.

Best,
Nick

[Adam Park]

Thanks a ton for your response Nick,

Just a quick couple of follow-up questions then regarding the same issue;

i) Am I correct in assuming then, that by selecting the "calcaneus" body to apply the loads to, the forces would be applied to the same point of the calcaneus throughout the gait cycle?
The only reason I ask is that I am attempting to conduct a study design similar to the one published by [url]Yong et al. in 2020 [https://simtk.org/projects/rfs-ffs-pfs/url], and would like to use MocoInverse to explore how different running conditions would affect loading mechanics and energy storage in tendons.

ii) Given the respective actuators would be strong enough and I have a valid markerset to track the movement, would it also be possible to track movement (i.e. inversion/eversion) around an unlocked Pin Subtalar joint?
Given that I would like to estimate the forces of the triceps surae, I would like to see how this would affect the joint kinetics and simulated muscle activation.

Your insight's appreciated,
Adam