A “wandering baseline” effect in the EMG signal can be present when there are no high-pass filters applied to the raw signal.

This is the expected behavior of an EMG waveform with no high-pass filters applied. The drift is related to the behavior of the electrodes—There is a small DC voltage created by the two electrodes in contact with the body. It is typically on the order of millivolts, and changes to this small voltage over time will appear as a drift in the baseline of the EMG waveform. It looks pretty strange if you’re unfamiliar with this phenomenon! Below is an example of what it looks like (the baseline drifts around “0”.)

Normally, the high-pass filter applied as the default for Ultium sensors (e.g. 10 Hz) eliminates the low-frequency content of the signal which contains the offset caused by the voltage difference between two electrodes. With the DC offset removed, a flat baseline is achieved. Other EMG manufacturers have built-in hardware filters on their surface EMG sensors because a bandpass filter is a basic requirement for measuring surface EMG. In the case of Ultium, the “No filter” option is necessary for other kinds of (non-EMG) signals.

Note that the digital filters selectable from the Hardware Setup are not shown in the processing history. Unless it is changed, the default is a 10 Hz highpass filter and 500 Hz lowpass filter.