Modeling the excitability of the denervated human thigh for the stimulation with surface and needle electrodes

3D models of the human thigh based on CT data or with simplified geometries are combined with a muscle fiber compartment model for analyzing how excitation depends on electrode position, stimulus strength and rehabilitation status. Assuming idealized conditions, excitation at a fiber ending obeys a quadratic current distance law whereas it is cubic elsewhere, resulting in easier central spike initiation for electrode-fiber distances up to the fiber’s length constant (~1/3 mm). Realistic conditions like fiber tapering, reduced sodium channel density towards the fiber end or inhomogenities in extracellular conductances (e.g. fat-muscle mix) keep up central excitability.