Non-invasive detection of the single motor unit action potential by averaging the spatial potential distribution triggered on a spatially filtered motor unit action potential

For research as well as diagnostic applications the non-invasive detection of the activity of single motor units is of interest. The most direct infor mation is expected to be found in monopolarly recorded data. But when an ar ray of surface electrodes is used for the monopolar recordings of the poten tial distribution on the skin, in most cases an additional invasive needle electrode is utilized to detect the exact points in time when a certain mot or unit is firing. With this supplementary information, an averaging of the monopolar EMG tracings can be performed. In this paper, a completely non-i nvasive methodology is presented which replaces the invasive needle by a sp atial filtering procedure. The EMG signals from the m. biceps brachii are r ecorded monopolarly with an electrode array. Afterwards, a spatial filterin g procedure, called normal double differentiating filter, is applied to the data. The EMG signals obtained are investigated by means of an amplitude t hreshold to distinguish the activity of different motor units. The point of the maximum amplitude of the selected peaks then is used as trigger point to average the monopolar EMG data. The time courses of the motor unit actio n potential signals found after applying the described procedure show simil ar shapes, while two different components are to be identified: correspondi ng to the spread of the excitation, one is referring to stationary, the oth er to travelling events. These results justify the possibility to replace t he needle electrode to obtain a trigger event in the future by the non-inva sive spatial filtering procedure. (C) 1999 Elsevier Science Ltd. All rights reserved.