A finite element model of the electrically stimulated human thigh: changes due to denervation and training

The complete denervation of muscles leads to changes in the muscle fibers as well as in the surrounding tissue. Concerning excitability the most important changes are reductions in fiber diameter, in muscle cross-sectional area, and in electrical conductivity of the muscle tissue. These changes can be partially reversed by intensive electrical stimulation. Evaluation of a 3D finite element axial symmetric model of the human thigh shows that the training leads to a reduction in threshold values between 17 and 51 percent, depending on the position of the fiber in the thigh. Single parameter variation clarifies the influence of each of the different factors. The electrode position was found to be most effective with the electrodes as far apart from each other as possible. Due to (i) comparatively higher changes in potentials at the distal electrode; and (ii) variations in sodium channel dynamics, lowest threshold values can be reached with a hyperpolarizing first phase of the biphasic impulse at the distal electrode. The tissue of the denervated muscle is known to be highly inhomogeneous. Simulations demonstrate that the related irregularities in the field can actually initiate fiber activation. 3D finite element simulations show the overall positive effects of FES on muscle tissue, especially an improved excitability of the muscle fibers. Furthermore the method gives an insight into the relations between potential distribution, electrode position, geometric effects, and muscle fiber activation that cannot be obtained by measurements.