Estimation of scaling factors in electrically evoked myoelectric signals

During a sustained contraction the surface myoelectric signal undergoes amplitude scaling (by a factor h), due to reduction or loss of motor unit action potentials, and time scaling (by a factor k) due to reduction of muscle fiber CV and other factors. These changes are particularly evident in the responses evoked by electrical stimulation (M-waves) when stimulation rate is above 30 or 35 Hz. The time expansion factor corresponds to a frequency compression factor of the signal's power spectral density (PSD) function and in previous research it has been estimated from the progressive decrease of the mean and median frequencies (MNF and MDF) of the PSD function.