Influence of tissue inhomogeneities on noninvasive muscle fiber conduction velocity measurements--investigated by physical and numerical modeling.

The determination of conduction velocity in the muscle fibers of single motor units from noninvasive recordings of single motor unit action potentials can be improved by the method of spatially filtering multielectrode EMG. The use of this conduction velocity as a diagnostic tool requires a high reliability of the detected values. However, experiments did reveal that the measured conduction velocity values showed remarkably high fluctuations depending on the recording site along the muscle fibers which could not be attributed to the influence of the endplate and tendon region. The present work examines the hypothesis that the observed fluctuations in propagation velocity were caused by electrically inhomogeneous tissue, regions of different electrical conductivity which are located between the excited muscle fibers and the recording electrodes and which cause a deformation of the extracellular electric current field. The investigation was performed by means of a physical model as well as by finite element model calculations. In both models single, simple shaped (cylindrical) inhomogeneity regions with a conductivity of 0.1 to 10 times that of the surrounding medium and diameters ranging between 1.6 and 2.7 mm were placed between excitation sources and recording site. The results indicate that the observed conduction velocity fluctuations of up to some 10% can be well attributed to inhomogeneity effects of the tissue conductivity. Based on these results, one may look for signal processing methods to cut down such fluctuations in conduction velocity measurements.