THE MOTOR UNIT POTENTIAL DISTRIBUTION OVER THE SKIN SURFACE AND ITS USE IN ESTIMATING THE MOTOR UNIT LOCATION

The amplitude of a surface electromyogram is dependent on the number o f active motor units, their size and the relative position of the reco rding electrode. it is not possible to interpret the surface electromy ogram quantitatively without disentangling these different aspects. In this study the decline of different components of the motor unit pote ntial with increasing radial distance from the motor unit is quantifie d. Fifty-two motor units in the biceps brachii muscle were studied usi ng 36-channel surface electromyography combined with intramuscular sca nning electromyography. Scanning electromyography was used to locate p recisely the motor unit. The dependence of the surface motor unit pote ntial magnitude on the radial distance between the motor unit and the recording electrodes can be described fairly well by an inverse power function. The steepness of this function depends on the chosen motor u nit potential parameter and the interelectrode distance, but also vari es between motor units. The change of the negative peak amplitude of t he motor unit potential over the skin surface can be used to give a fa irly accurate estimate of the location of the motor unit under the ski n surface. We found that for all practical purposes the depth of a mot or unit in the biceps brachii muscle can be estimated as 20% of the di stance over the skin surface where motor unit potentials can be record ed with higher amplitudes than 50% of the maximal amplitude recorded a t the skin surface caused by activity of the same motor unit.