Functional electrical stimulation of denervated skeletal muscles: a modeling study

Finite element and finite difference models of the human thigh are used to analyze the excitation process in the fibers of denervated skeletal muscles which are stimulated via surface electrodes. The MATLAB research tool FES-FIELD was developed to simulate the stationary field for real human 3D geometry and FES ANALYSE for a first estimate of supra-threshold regions. Action potential initiation is simulated with a muscle fiber model of the Hodgkin Huxley type and with a generalized form of the activating function. At the endings of a target fiber the activating function is proportional to the first derivative of the extracellular voltage, whereas the second derivative is the driving element for the central part. The analysis demonstrates that it is generally easier to initiate action potentials at the fiber endings especially for fibers in deeper regions.