Investigation of monopolar multi-channel surface EMG of the external anal sphincter muscle

Cescon, Corrado and Mesin, Luca and Nowakowski, Michal and Merletti, Roberto (2010) Investigation of monopolar multi-channel surface EMG of the external anal sphincter muscle. In: Proc. of the XVIII Congress of the International Society of Electrophysiology and Kinesiology, 16-19 June 2010, Aalborg, Denmark.

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AIM: This study proposes a new method to investigate the geometry of fibres of the external anal sphincter (EAS) muscle using high density surface electromyogram (EMG). METHODS: Monopolar surface EMG signals recorded using an array of electrodes placed in circular direction have no common mode (CM) components if the muscle fibres are circular, with constant depth within the muscle and parallel to the detection array. In this case monopolar signals can be estimated from SD signals and compared to the experimental ones. Thus, the presence of CM signals of physiological origin was used to provide indications about the geometry of muscle fibres of EAS. Surface EMG signals were recorded from EAS of 12 subjects (7 females and 5 males) using an anal probe carrying three arrays of 16 electrodes. Single motor unit (MU) discharge times were identified from EMG signals obtained with these arrays and were used as triggers to compute the average action potential of each MU. The amount of CM was estimated from each MU action potential (MUAP) and was analyzed with respect to the radial depth of the MU (estimated by the CoV of the MUAP amplitude in the three arrays) and to the MU position along the anal canal (indicated by the array with higher potential amplitude). The estimation error of each monopolar MUAP was computed as the difference between the original monopolar signal and the reconstructed signal (ARVe) normalized w.r.t. the original monopolar signal (ARVm). RESULTS: The contribution of CM components to single MUAPs is lower for MUs located more superficially in the muscle and at a lower depth within the anal canal. CONCLUSION: Presence of low CM voltages on MUAPs is related to the geometry of the fibres that, as confirmed by anatomical studies, are circular and parallel to the detection array only at low radial depth within the muscle and along the anal canal. Large CM components are present on MUAPs from MUs located deeply in the muscle or more internally along the anal canal. Moreover, large CM signals are also present in the interference EMG, suggesting that there is crosstalk from far and large co-contracting muscles (e.g. puborectalis, glutei) or deeper MUs with non-cylindrical geometry. ACKNOWLEDGEMENT: This work was sponsored by Projects TASI (Else Kroner-Fresenius-Stiftung, Compagnia di San Paolo), and TIFNI (Regione Piemonte, Italy).

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