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All rows below this line contain the corresponding data in each column. Columns 2-4 (ground_force_vx, ground_force_vy, ground_force_vz) represent the x, y, and z components of the right foot's ground reaction force vector in a specific body coordinate system (ground for the model coordinate system). Columns 5-7 represent the x, y, and z components of the right foot's center of pressure (i.e., the point at which the ground reaction force is applied to the right foot). Similarly, columns 8-13 represent the ground reaction force vector and center of pressure for the left foot. Columns 14-16 represent the x, y, and z components of the right foot's ground reaction moment vector (typically in the model coordinate system). The last three columns represent the analogous quantities for the left foot.

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Nans in your force file

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 zero. 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 spliningNaNs are not compatible with any dynamics analysis and must be removed from mot files. Force data is splined during computation and occurrences of NaNs cause the entire force, moment, or COP column to become NaN.  NaNs often occur when computing COP, more details can be found in the C3D files documentation. 

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