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When computing Center of Pressure (COP), often the numerical value will be computed as NaN (Not A Number). NaN values occur when the force values become very, very smallzero. Typically, most COP computation systems include some cut-off value that zeros all forces below a threshold, which stops these NaNs from occurring. If you have NaNs in your force file, and you use them during Inverse Dynamics analysis, you will get all NaN values for the output moments. This is because OpenSim splines the force and COP data before computing joint moments. The OpenSim splines don't know how to deal with NaN values and the entire force and COP values become NaNs. If you find these NaNs in your COP, you should try to pre-process and solve for these values, either by zeroing all values below a threshold value or by performing your own data splining.
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