What does the EMG envelope algorithm in Polygon do?

The envelope algorithm in Polygon is intended to produce a curve which gives an idea of the shape of the underlying raw EMG. It is based on a running average algorithm, but has been modified to give better response to the peaks in the raw EMG data (a simple running average will produce an envelope curve which fits nowhere near the peaks of the raw data).

The envelope algorithm takes a single parameter which is the width of the envelope as it passes through the raw data. What this means is that if you have entered, say, 10 ms for the envelope width parameter, any given sample in the time series will be affected by the sample within a 10ms envelope either side of it. If this sounds too technical suffice to say that the lower the value the more “tight fitting” the envelope will be.

Furthermore, increasing the value will “smooth” the curve. There’s no way to determine a “perfect” value, so the best strategy is to experiment a bit - try to overlay the enveloped EMG using different parameter values (for example 10,20,30,40 and 50) and the raw EMG to get an idea of what the algorithm produces given different parameter values.

The default value is 25

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