Influence of the fiber length on the power spectra of single muscle fiber extracellular potentials.

Influence of the fiber length on the power spectra of the single muscle fiber extracellular action potentials (SMFEAPs) as well as on the spectral changes under alterations in the propagation velocity (v) and intracellular action potential duration (Tin) was studied theoretically for a fiber-electrode distance typical of surface recordings. It was shown that the magnitude and distribution of the SMFEAP spectral power depend on the fiber length. The shorter the fiber, the wider the frequency region in which the total spectral power is distributed and the smaller the spectrum max amplitude. The fiber length affects the spectrum changes that are due to alterations in v or Tin, and, consequently, the spectrum characteristic frequencies--Fmax, Fmed and Fmean, as well as the total spectral amplitude (TSA) and total power (TP). Whereas for a relatively long fiber Fmax depends linearly on v and Fmed and Fmean increase with v increasing, for a short fiber the linearity of the relation between Fmax and v can be disturbed, and Fmed and Fmean can decrease with v increasing. Under the condition of a relatively short fiber, a change in Tin can cause changes not only in Fmed and Fmean, but in Fmax as well. It was shown, that as a result of the finite length of the muscle fibers as well as of the asymmetry of the end-plate location in relation to the fiber ends, dips can be observed in the spectrum even in a case of monopolar recording. Positions of the dips in the power spectra depend on propagation velocity.