MOTOR UNIT POTENTIAL CONTRIBUTION TO SURFACE ELECTROMYOGRAPHY

The background of the bioelectric activity of muscle recorded from the surface of the skin (surface electromyography) in terms of the repres entation of single motor units of the underlying muscle(s) is not very well documented or understood. An insight into the composition of an electromyogram is essential for the proper interpretation of one of th e most widely applied electrophysiological techniques. In the present paper, a study of the contribution of single motor unit potentials to; the surface electromyogram is presented. To this end, the decline of d ifferent components of the motor unit potential with depth of the moto r unit is quantified. Experimentally, the action potentials from motor units at several positions in the muscle were recorded by 30 skin sur face electrodes. Simultaneous use of scanning electromyography provide d information about the actual position and size of the motor unit. Ob served linear log-log relationships between motor unit potential magni tudes and distance indicated the usefulness of a power function to des cribe the motor unit potential's dependence on recording distance. It is shown that different specific surface motor unit potential characte ristics fall off differently with depth. The magnitude-distance relati onship is shown to be dependent on the recording configuration (unipol ar vs. bipolar recording, including the inter-electrode distance) and the chosen motor unit potential parameter (negative peak amplitude, po sitive peak amplitude and area).