Fundamentals of power spectra of extracellular potentials produced by a skeletal muscle fibre of finite length. Part II: Effect of parameters altering with functional state.

The reasons for dependence of the power spectra of extracellular potentials (EPs) produced by a skeletal muscle fibre of finite length, on parameters altering with functional state was analysed. The sensitivity of the EP power spectra to alterations in the parameters depends on the distance of the observation point from the fibre. At large distances the sensitivity can change with longitudinal position as well. The differences in the sensitivity are due to the changes in the inter-relations between the power spectra of the input signal (the first temporal derivative of the intracellular action potential) and of the impulse response (IR) of the fibre of finite length as a linear system of EP generation. It was shown that not only the parameters affecting the IR (propagation velocity of the waves of depolarisation), but also the parameters determining the input signal (intracellular action potential duration and after-potential) can affect the characteristic frequencies of the EP power spectra.