Time is not corresponding between IK result and GRF data

[Chiho Ng]

Dear all:

I have a question about ID(Inverse Dynamic) calculation: if my input data, IK(Inverse Kinematic) result and GRF(Ground Reaction Force) data do not have the same time stamp, what would OpenSim do during ID?
I have come up with 2 guesses:
1.Find the corresponding point between IK and GRF data by comparing the time stamp and interpolation, and then use these data to calculate implement ID, if the sampling rate of GRF data is smaller than IK result, fill the gaps of GRF with 0;
2.Just use the IK and GRF data line by line and ignore the time stamp.
Does anyone know the real process of ID in this situation?
Appreciate any answers! Thank you so much!

Chiho Ng
2022.9.5

[John Davis]

OpenSim will use spline interpolation to get the GRF at each timepoint from the IK result if the timestamps don't match up exactly. You'll get an ID results file with one frame of ID results for each frame of IK, and the ID timestamps will exactly match the IK timestamps. There won't be any zeros in between. It works great, and is one nice little advantage of OpenSim--you don't need your GRF to be sampled at an integer multiple of your marker data like you do in some other software.

[Chiho Ng]

Thank you so much for your reply Mr. Davis! It is really helpful!