Filtering data

[Herman van Werkhoven]

Hi there,

I have a question regarding filtering of the input data. Should the kinematic marker data (as *.c3d, *.trc files) be filtered (low-pass) before input into AddBiomechanics, or does filtering occur within the AddBiomechanics environment?

Regards,
Herman

[Nicholas Bianco]

Hi Herman,

Filtering is applied after the kinematics step (e.g., marker fitting) to smooth joint kinematics before dynamics fitting, but we do not apply any filters to the input data. Whether or not you filter the input data, it is most important that the input marker data is "clean" (i.e., no frames with missing markers, no swapped marker). The same is true for the input ground reaction forces if you have them: try to pre-process/filter forces so that, for example, the center of pressure estimates are consistent with the foot markers.

We do our best to correct this inconsistencies in AddBiomechanics, but the cleaner your input data, the better chance you have a good result.

Best,
Nick

[Herman van Werkhoven]

Hi Nick,

Thank you for your response. I just wanted to get more details related to the filtering. We are comparing results from OpenCap to AddBiomechanics generated joint kinematics and the filtering details are important. I would really appreciate it if you can answer a few more questions:

• Is it suggested to filter input marker data? I've always thought that filtering should occur as early in the process as possible, but maybe not?

• You wrote 'Filtering is applied after the kinematics step (e.g., marker fitting) to smooth joint kinematics'. Does this mean that the joint angle kinematics outputs (in the *.mot) files are filtered, or does the filtering only occur after this step, and really only affects dynamics results?

• If the joint angle outputs are filtered, where can I find details of the filter method (e.g., type of filter, cut-off frequency etc.)?

Thank you,
Herman