How to position a joint coordinate using marker data?

[adamkewley]

This question was ported from [email protected]. Some details are REDACTED because the initial email was private.

Following on from #848 , I successfully translated the coordinate system on my mesh files, and the skate is now performing the "klap motion" as I wanted (see REDACTED video). I do have one follow-up question regarding the positioning of the skate for which I could use your expertise.

The skate comprises two parts: the bridge and the blade. The bridge, housing the measuring equipment, is standardized for all participants, while each participant uses their own blade. When substituting the skater's original bridge with the measuring bridge, there's a possibility that the blade ends up in a slightly different position for each participant – either rotated around the y-axis (with y as the upward axis) or shifted laterally, positioned slightly more towards the small toe or big toe, depending on the personal preference of the participant.

I have marker data available (one on the tip and one at the end of the blade, as shown in the attached photo), and I utilize this in the scale tool to ensure the correct blade length for each participant.

However, I am trying to find a way to efficiently adjust the blade position for each participant. I think that the way to go about this is to utilize the relative experimental marker positions (e.g distance between ankle joint markers and blade markers) to correct for the blade position relative to the foot. However, as far as I can discern, OSC doesn't have the option to create an offset frame with respect to a marker (only with respect to another body).

I believe there is a possibility to scale the model in OpenSim, relocate the experimental markers to their virtual positions, determine the relative marker positions, and convert them into the foot's reference frame. Subsequently, opening the model in OSC and inputting these values into the offset frame from blade to foot. However, this appears to be a somewhat cumbersome process in my view.

[adamkewley]

This is a little bit out of my area of familiarity (I'm mostly just a developer for OSC), but I think the trick I have seen for adjusting the position etc. of your blades is something like:

• Initially model the blade with something that's less-constrained and capable of rotating in the way that individuals adjust the blades (e.g. use a UniversalJoint or BallJoint). This will mean that you have additional coordinates that can be used to adjust the model to match the person
• Use the inverse kinematics (IK) tool in OpenSim GUI (not Creator - as of 0.5.11, I haven't written a feature like this yet) to combine your known marker data with the "free-er" model. The IK tool (iirc) will adjust the coordinates in your model to minimize marker error
• Your blades will then be positioned to minimize marker error
• Lock (padlock symbol on the coordinate editor) the coordinates that you don't think should actually be allowed to freely move during simulation (i.e. lock all the "extra" ones you added for IK, effectively giving you a PinJoint back)

I've seen people do this to (e.g.) position muscles or joint frames etc. - it's a bit of a hack but the IK solver in OpenSim is effectively doing what you want (minimizing marker-to-model differences).

If you're feeling a bit gross about using not-PinJoints in the final (adjusted) model, there might be a little hack to deal with that:

• OSC has a "rezero joint" feature. It appears as an option if right-clicking a joint in the UI. The way it works is that it will adjust the offset frames of the joint such that the "zero point" of joint (i.e. where coordinates == 0) is the current position of the joint (which may have nonzero coordinates after IK etc)
• So do your IK with UJs, BallJoint, etc.
• Then "rezero" the joint, giving you 0,0,0 for all of the coordinates (because OSC has math-ed its way around repositioning your joint frames accordingly)
• Then "Change Joint Type" --> PinJoint, which should do nothing (if you didn't rezero it, it'll snap around, because your IK will have wobbled it)
• Tah dah: model with PinJoint but the blades wobbled around via IK
• (although no guarantees that this all works: I haven't used the rezeroing or 'change joint' feature in a couple of years