Evaluation of the criteria to identify single-fibre potentials in human muscle fibres

Abstract

The criterion normally used to identify a potential generated by a single muscle fibre (SFAP) is that it must have identical shape at consecutive discharges. Technical problems accompanying the recording of single-fibre electromyographic (SFEMG) potentials introduce certain variability in the shape of these potentials, thereby compromising the ability to detect pure SFAPs. This study aims to determine the conditions necessary for two fibres to generate a compound potential that fulfils the single-fibre criterion. This has been done by analysing the alterations in the waveform of compound spikes formed by the summation of two SFAPs whose relative weight in the composite potential can differ considerably. Several factors responsible for this shape variability, with a geometrical, physiological or accidental origin, have been included in our study. It has been shown that a distant interfering component will be hardly detected in the composite potential if it is smaller than approximately 15% of the main component. For this interfering component to generate a notch in the rising phase of the compound potential, it must be greater than about 30% of the main component. A compound potential will fulfil the single-fibre criterion if the time dispersion between the individual components is less than 80-120μs. These results permit the estimation of the amplitude of interfering potentials so they could be useful in fibre density studies. The article also emphasises the inherent variability of SFEMG potentials and the impact of this variability on jitter estimation.