Location of the motor unit electrical axis and determination of the muscle anisotropy ratio - A model approach

A method to localize the electrical axis of a motor unit (MU) and to simultaneously determine the anisotropy ratio (the ratio between the transverse and the longitudinal extracellular conductivity) of the muscle is proposed. The method is based on the use of Laplace's equation. Particular attention is paid to the theoretical consideration of an asymmetric MU. The method is tested utilizing simulation data on the extracellular potentials of a MU in which the propagation of two depolarized zones along a participant muscle fiber is accounted for. It is shown that the method is reliable and the calculation accurate mostly for the moments of time associated with small changes in the MU potential.