ABSTRACT
The purpose of this study was to noninvasively extract information about the size and muscle fiber density of muscular units through the inverse analysis of surface electromyograms. Surface motor unit action potentials (MUAPs) were recorded with a multi-channel electrode array arranged along the circumference direction of the biceps brachii. The depth and intensity of equivalent current dipoles were estimated through the inverse analysis of surface MUAPs. The simulation of inverse analysis of surface potentials generated by the muscular unit models showed that the relationship between the depth and the intensity depends on the muscular unit size and muscle fiber density.<BR>In the simulation, we systematically varied the model parameters including distance from the skin, radius, and fiber density and used the inverse analysis to estimate the depth and intensity of current dipoles. And, our method to estimate the radius and fiber density of muscular units using estimated depth and intensity is demonstrated. Mean values (± SD) estimated from the surface MUAPs were 3.0±1.8mm for depth and 13.8±32.0nAm for intensity. The estimated distance ranged from slightly less than 1 mm to slightly more than 2 mm. The estimated radius ranged from 1.8 to 4.6 mm and fiber density from 0.7 to 5.4 fibers/mm<SUP>2</SUP>.