Help Needed to Downsample Force Plate Data Upsampled by Qualisys During C3D Conversion

[Annajean]

Hi everyone,

I'm encountering an issue with force plate data sampled at 100 Hz, which was automatically upsampled to match EMG data sampled at 2000 Hz when exporting to C3D format in Qualisys. I'm trying to downsample the force plate data back to its original 100 Hz rate but haven’t had any success so far.

I've tried using the "Import Data from MAT-File" and "Evaluate Expression" pipeline features, but neither seems to work for downsampling. I’d really appreciate any advice or suggestions to help troubleshoot this issue. Thanks so much for your help!

[Andrey Aksenov]

Hello Anna,
The force plate data was upsampled to match EMG as there is no way to store them both with different sample rate in ANALOG folder in c3d files.
I checked how to downsample data in Visual3D via script and used has-motion wiki Downsample Data [HAS-Motion Software Documentation]
It worked without any problems, just created a script in notepad and ran it. Force data matched the marker data, however EMG signals were downsampled as well.
Please, contact Qualisys support https://www.qualisys.com/support/ and team can assist you.

[Krystyna Gielo-Perczak]

Andrey,
Thank you very much for your immediate and informative response to Anna's problem. Witnessing your action is very uplifting.

Best,
Krystyna Gielo-Perczak, Biomch-L Co-moderator

[Scott Selbie]

Hi Anna,

The common use of the C3D file format restricts the ANALOG data rate to one value. This means that all signals that have been declared ANALOG must have the same sampling rate.

The choice for many software applications is to upsample all signals to the highest sampling rate. While this is wasteful of disk space, rarely would there be any information lost in the process.

In other words there would be nothing gained by downsampling the force data as it would not impact any analyses in a meaningful way.

On the other hand, downsampling the EMG data would likely be disastrous from an analysis perspective.

Scott