Modifica di Electromyography (sezione)

====Randomness of the EMG Signal====
The last and less obvious source of noise in the sEMG signal is the quasi-random nature of the sEMG signal itself. This occurs mainly in the 0 to 20 Hz range of the spectrum and is due to the random frequency of motor unit discharges. Motor units, in fact, have an activation frequency precisely in the 0 to 20 Hz range. The unstable nature of these components of the signal should lead them to be considered noise and, therefore, filtered out. This is normally done. Unfortunately, the reader unfamiliar with signal theory may not fully understand this point. Filtering signals in the 0 to 20 Hz band, where the motor unit firing frequencies are present, might seem counterintuitive, leading to the removal of any informative content from the signal.

But a common-life example might help. Imagine listening to loud rock music from the car stereo of a nearby vehicle, both waiting at a traffic light. What you hear is just a rhythmic succession of drumbeats. But if the driver of the nearby car rolls down the window, you can immediately hear the music. Before the window was lowered, you could perceive the rhythm (low frequency) better than the other sounds (which also followed the rhythm of the music but had a higher frequency content), which the rhythm itself prevented you from understanding. In this example, the window had to be lowered (to allow the music through), whereas in the case of sEMG, the rhythm is filtered out (to better see the signal).