Signal Versus Noise Characteristics of Filtered EMG Used as a Control Source

A common method of initially processing surface-detected electromographic (EMG) activity is to differentially amplify, rectify, and then smooth (using a low-pass filter) the rectified activity. The output of the filter exhibits a mean value slightly obscured by ripple. Both the mean value and ripple increase with increasing muscle contraction. The mean value can be called the signal while the ripple, undesirable when only the signal is desired, can be called noise. The SNR depends, at least, upon the contraction level, type of smoothing filter, and the amount of smoothing for the particular filter. This defined SNR is important in signal communication problems of both a design and a theoretical nature.