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Dear experts,

We have done a hopping experiment in which subjects were hopping on a force plate. We couldn't use two different force plates for each leg as we needed to apply a perturbation to our subjects by changing the height of the force plate. So now we have just one force vector and one center of pressure in the middle of two feet. In Opensim we need to assign force to each leg to calculate the Inverse dynamics. Are there any suggestions on how we should do the ID in Opensim?

Best regards,
Vahid

Vahid,

I would suggest using a 2-dimensional model.

A 2-D model for hopping could assume symmetry and have only one leg (with mass properties and muscle strength values doubled).

If you use a 2-legged model, it can also be done by dividing the force plate data equally between the two legs.

You should not use a 3-D model because you have no information about the medio-lateral position of the center of pressure.

If you want to turn a 3-D model into a 2-D model, lock the pelvis translation perpendicular to the sagittal plane, and lock all joint rotations except flexion-extension. Also all flexion-extension axes should be perpendicular to the sagittal plane. This is the case in many OpenSim models, but you should check to make sure.

Ton van den Bogert