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For complicated reasons not worth explaining, is there a way to restrict the movement of an end effector-- say the foot-- to a certain plane? Basically, I want to test how activation of different muscles contribute to moving the foot in the x-y plane, and thus restricting any movement in the z plane. Is there an "easy" way of doing this? Thanks!

I'm assuming you mean the local coordinate system for the foot, not the global.

The easiest method to restrict motion to only the x-y plane will be to lock the ankle rotation coordinate in your model through the OpenSim GUI. Just click on the coordinates pane and scroll down to the ankle coordinates. Specify the angle you want for rotation (or whatever else you want to prevent motion in) and click on the lock button to the left of the angle slider bar. This will make it so you only have ankle dorsi/plantar flexion.

Hi, thanks for your reply.

Unfortunately, what I desire is more complicated than this: I want to keep the end effector (the foot) constrained to move in a plane (with some room for error) while all the joints can move as long as it's within their constraints (will use coordinate limiting forces). Does that make sense? To clarify an image in your head: I am not using a human limb model, but instead a rodent model.

Thanks!

Is it possible to do this with a custom joint perhaps? Make a joint for the end-effector(foot) and limit it's movement to a whole plane?