Peculiarities of extracellular potentials produced by deep muscles. Part 2: motor unit potentials.

The potential fields generated by single fibres far from the sources are non-propagating. This suggests that there should be differences in the features of surface motor unit (MU) potentials (MUPs) detected from deep and superficial muscles. We explored the features using a simulation approach. We have shown that the non-propagating character and similar shapes among surface MUPs recorded over a wide area above deep muscles with monopolar or longitudinal single differential (LSD) electrodes are natural. Contrary to close distances, at large radial distances single differentiation did not emphasize MUP main phase relative weight. The position of the end plate area could be estimated with LSD detections only for MUs with long (123 mm) almost symmetric fibres. With short fibres, the LSD main phase was masked by the outlined terminal phases. This could be misleading in MUP analysis since the terminal phases reflect standing sources. The highly asymmetric fibres could yield peculiar MUP shapes resembling MUPs of two distinct MUs. We have shown that the relative weight of terminal phases at large fibre-electrode distance decreases under abnormal peripheral conditions. However, the changes in membrane depolarization due to fatigue or pathology could be assessed non-invasively also from deep muscles.