The ability of MUP parameters to discriminate between normal and neurogenic MUPs in concentric EMG: analysis of the MUP "thickness" and the proposal of "size index".

The ability of a motor unit potential (MUP) parameter or a set of parameters to discriminate between control MUPs and MUPs from muscles with severe to moderate inactive neurogenic changes was investigated using discriminant analysis. As a single parameter, area gave rather good discrimination, while amplitude, duration and area/amplitude did not give good discrimination. Only 15-30% of MUPs from neurogenic muscles were judged to be abnormal by duration alone. The combination of amplitude and the area/amplitude ratio (area/amp; MUP thickness) improved the discrimination considerably and around 70% of MUPs in the neurogenic group were judged to be abnormal. A simple assigned discriminant function, 2 x log 10(amp) + area/amp, gave similar good results for both biceps brachii and tibialis anterior muscles, and this value was named as "size index." The experiment of manual scanning of an MUP revealed that area/amp increases, that is, the MUP becomes thicker, as the amplitude decreases for each MUP with increasing recording distance, while the size index remains almost constant for each MUP irrespective of the electrode position. The new parameter, size index, is promising since it is more stable and gives better discrimination results than duration.