Computer simulation of field distribution and excitation of denervated muscle fibers caused by surface electrodes.

In the course of this study, 2 submodels have been developed and combined, the 2-D finite element modeling of the electrical potential distribution in the human thigh and a Hodgkin and Huxley (HH) type model to calculate fiber excitation and action potential propagation. To determine the excitation of the target muscle fiber with the help of the activating function, the fiber's orientation within the muscle has to be known. The electric field along the fiber has to be calculated as a function of the applied electric current and the potential at the electrodes, respectively. The excitement of the muscle fibers varies across a wide range depending on the active and passive membrane parameters and the intracellular and extracellular mediums. Persisting denervation leads to a decay of muscle cells, and a partial substitution by fibroblasts occurs. The electrical activation of these tissues is more difficult, and biphasic stimulation pulses up to 200 ms in duration and 60-100 V in amplitude are needed to cause a contraction of the denervated muscle. An example shows the field distribution and the simulated activity in one representative muscle fiber of a well trained m. rectus femoris.