Low-pass filter effect in the measurement of surface EMG

Many publications and articles describe the electrical properties of muscular tissue. In these articles the terms `low-pass filter' and `purely resistive medium' often occur in parallel. From the viewpoint of electrical engineering (namely from a network theoretic viewpoint) one would not expect that a purely resistive medium behaves as a low-pass filter. Instead the existence of some capacitive and/or inductive elements would be expected. In this paper we present a network theoretic description of the low-pass filter effect of muscular tissue. We will use a descretized and simplified tissue model, e.g. a network of resistors. For this model the low-pass characteristic of muscular tissue will be verified. The fundamental reason for the low-pass characteristic of muscular tissue is the fact that electrical activity travels with some constant velocity along a given path (e.g. the muscle fibres) while the measurement of this activity is done at some fixed point. The independent variables `time' and `space' can be mapped onto each other. The low-pass filter effect occurs in space domain.