Noninvasive approach to motor unit characterization: Muscle structure, membrane dynamics and neuronal control

The standard surface EMG reflects the compound activity of a high number of motor units which is finally due to its low spatial resolution in the detection of the potential distribution on the skin surface. Therefore, detailed information about the structural and functional characteristics of the muscle consisting of populations of motor units, like the functional anatomy, the excitation spread or the innervation pattern cannot be obtained from the standard surface EMG. A novel noninvasive EMG-procedure with high spatial resolution (HSR-EMG) allows in contrast to the standard surface EMG even the detection of the single motor unit activity. In this way, the noninvasive determination of detailed information about the muscle structure, the membrane dynamics and the neuronal control becomes possible. First applications of the HSR-EMG have shown that especially the noninvasively measured conduction velocity of the excitation is highly affected by physiological details, like the muscle temperature, the relative muscle fibre diameter or inhomogeneities in the connective tissue forming part of the volume conductor around the muscle. From the results of the HSR-EMG investigations it can be concluded that the information about the structural and functional characteristics of the muscle as well as a deeper insight in the active state of the muscle is essential for a correct interpretation of the standard surface EMG.