Modeling Needle Stimulation of Denervated Muscle Fibers: Voltage–Distance Relations and Fiber Polarization Effects

Abstract

A finite-element model of the human thigh was coupled with a 1-D compartment model to simulate the excitation of denervated muscle fibers with a needle electrode. For short electrode-fiber distances, the specific characteristics of the needle geometry determined the areas of lowest threshold values. With increasing distance, these areas shifted toward the needle's center of charge. Comparison of the 1-D model with a 3-D fiber model showed that the assumption of rotational symmetry underlying the 1-D model leads to an overestimation of thresholds. For a 40- micro-diameter fiber stimulated with 50 micross pulses at electrode-fiber distances between 50 microm and 1 mm, the 1-D/3-D threshold ratios were between 1.14 and 1.35 for the muscle fiber model, and between 1.11 and 1.17 for Hodgkin-Huxley membrane properties at 20 degrees C. For both membrane models, the deviation was more pronounced for large fiber diameters and short stimulation pulses. Qualitative results of the 1-D model like voltage-distance relations and predictions of spike initiation sites were correct.

Muscle Activation During Peripheral Nerve Field Stimulation Occurs Due to Recruitment of Efferent Nerve Fibers, Not Direct Muscle Activation

“…The study showed that direct muscle activation requires stimulation intensities outside the range of PNFS, this finding is in agreement with other studies investigating direct muscle activation , where it is was found that only muscle fibers located within 0.2 mm of the electrode could be activated with intensities within the range of PNFS (<10 V).…”