Demodulation of e.m.g.s. of pathological tremours. Development and testing of a demodulator for clinical use

In clinical tremour recordings, e.m. g. signals allow highly localised recordings to be made. This is in contrast to mechanical recordings which have been used extensively in the past. E.m.g. recordings make it possible to distinguish between tremours in agonist and antagonist muscle groups. E.m.g. signals, however, have poor signal-to-noise characteristics. In this study the e.m.g. of pathological tremours is considered as amplitude-modulated noise. The design of a demodulator and some tests of its performance are described. The visual detectability of an undemodulated tremour e.m.g., when written on a polygraphic recorder, is expressed as a modulation depth, and is about 60%. This improves after demodulation to a level of 20%. The placement of the electrodes and filtering of the e.m.g. signal will influence the shape of the power spectrum of the e.m.g. itself. However, these factors do not critically influence the demodulated tremour e.m.g. For this reason also a demodulator is a useful instrument for recording clinical tremours. © 1983 IFMBE.