Global and detailed features of motor unit potential in myogenic and neurogenic disorders

Motor unit action potential (MUAP) may be characterized by two complementary features: global and detailed. The global feature is represented by the envelope of the curve while the detailed feature describes the fluctuations around the envelope. The global feature represents the magnitude of the signal and is related to the number of fibers contributing to the MUAP. The detailed feature indicates the single fiber potential's synchronization. The global feature is characterized by amplitude and duration and may be quantified by the following indexes: area/amplitude or size index, which is interpreted as the potential "thickness". The detailed feature is characterized by the number of phases and turns. The irregularity coefficient, which is related to the "length" of the curve, combines the number of phases and turns with their amplitudes. The aim of this study was to analyze the relationship between the global and detailed features of MUAPs in various neuromuscular disorders. In neurogenic cases, with the exception of the correlation between the duration and number of turns (r = 0.58), no correlation between any other parameters related to global and detailed features were significant (r < or = 0.4). In myogenic cases, a significant correlation between the irregularity coefficient and duration (r = 0.71), irregularity coefficient and area/amplitude (r = 0.54) rather than that between the irregularity coefficient and area (r = 0.32), has been found. The correlations between parameters were more significant in more chronic processes.