Peculiarities of extracellular potentials produced by deep muscles. Part 1: single fibre potential fields

The similarity among surface electromyography (EMG) signals recorded by the poles of electrode arrays above deep muscles like erector spinae is a substantial obstacle in determining major muscle characteristics. What makes EMG signals so different when detected at various distances from the fibres? To answer this question, we simulated and analyzed extracellular potential fields produced by a single muscle fibre. We considered the fields at a few specific time instants. They corresponded to the origination of two depolarized zones at the end-plate, their propagation along both semi-fibres, and extinction at the fibre-ends. We used intracellular action potentials and muscle fibre propagation velocities typical for non-fatigued or fatigued muscle fibres. We have shown that at relatively small distances from the fibre, the strong potential fields are concentrated mainly near the sources. The interaction between potential fields is weak and the propagation of the fields and EMG signals in relatively long fibres is clearly apparent. At large distances, the potential fields are wide and the interaction between the fields produced by the two depolarized zones is strong. The total potential field could remain non-propagating during the entire main phase. As a result, the propagation will be obscured also in EMG signals.