The calculated influence of a metal electrode shaft in needle EMG recordings

Two factors that are expected to have a large influence on needle recorded EMG signals are the electrical double layer at the metal-electrolyte interface and the presence of an oedemeous layer around the needle electrode. Solutions for this problem can be obtained in a cylinder symmetrical potential field problem, described by Laplace´s equation. Analytical expressions have been derived for simulating single muscle fiber-action potentials (SFAPs) recorded by a typical single-fiber needle electrode. Results indicate that, due to the electrical double layer, the SFAP amplitudes decrease or increase, depending on whether the muscle fiber is located at the opposite side or at the same side, respectively, of the electrode as the lead-off surface. The oedemeous layer causes a general decrease in amplitude, the magnitude of which depends, for example, on the thickness of the layer. No important SFAP waveform changes due to the presence of the electrode shaft are found