Muscle fiber action potential changes and surface EMG: A simulation study.

The muscle fiber membrane electrophysiology may change under fatiguing circumstances. In a simulation study, the influence of muscle fiber membrane electrophysiological changes on two major characteristics of the surface electromyogram interference pattern (SEMG) is quantified [median frequency (F(med)) and SEMG amplitude (RMS)]. Frequency and amplitude parameters are calculated from a model of the motor unit action potential in which the mean muscle fiber conduction velocity (U) and the intracellular action potential duration (T) characterize the state of the sarcolemma. F(med) appears to be proportionally related to U, while T does not play a significant role in F(med). The simulations further reveal that the RMS amplitude is proportional to T and the square root of U. The simple dependencies of SEMG parameters on U and T can be disturbed by a different sensitivity of thick and thin muscle fibers to factors that decrease the membrane excitability. It is stressed that the results of this study concern bipolarly recorded interference EMGs. It is concluded that volume conductor parameters and interelectrode distances, although important with respect to the absolute values of F(med) and RMS, do not interfere with the influences of T and U when relative SEMG changes are considered.