In our setup, the accelerometers and the force plates have analog outputs, and they all go into the same AD board. So there should not be a synchronization problem. But now I am starting to question that assumption...

If there is a time delay between channels on the same AD board, it should be small enough that it does not matter, when considering the bandwidth of the signals.

I am curious what AD board you use and how large the time delay between channels is.