Magnetic stimulation-induced modulations of motor unit firings extracted from multi-channel surface EMG

The noninvasive assessment of motor unit (MU) firing patterns on the basis of topographical information from 128-channel high-density surface electromyography (SEMG) is reported. First, multi-channel MU action potential (MUAP) templates are obtained by clustering detected firing events according to the surface topography of the MUAP. Second, a template-matching algorithm is used to find all firings of a MU, including the superimpositions of MUAPs. From a single recording, the firing pattern of up to five MUs could be derived. The modulation of MU firing by transcranial magnetic stimulation was analyzed in peri-stimulus time histograms. The results are similar to previous results of transcranial magnetic stimulation (TMS) obtained by needle electromyographic (EMG) recordings. The method can be used to investigate MU firing patterns in patients with central motor disorders. An additional advantage of the technique, apart from its noninvasiveness, is the structural and functional information that it provides on the MUs, which is not obtained by needle EMG.