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

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- mum-diameter fiber stimulated with 50 mus pulses at electrode-fiber distances between 50 mum 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 ^degC. 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.