Detection volume of simulated electrode systems for recording sphincter muscle electromyogram.

BACKGROUND Recording surface electromyogram (EMG) signals from sphincter muscles has become of increasing interest due to potential applications in diagnosis or investigation of the mechanisms of incontinence. Recently developed probes allow high-resolution detection of EMG signal from the external anal sphincter. One of the main issues in the interpretation of experimental signals is the detection volume of the recording electrodes. METHODS An analytical model of generation of EMG from the anal sphincter was applied to simulate single fiber action potentials varying the fiber length, conductivity and thickness of the mucosa, shape of the electrodes, and detection system (monopolar, bipolar, and double differential). RESULTS The new probe is more selective than classical probes with big electrodes and large interelectrode distance. The decay of the recorded surface EMG action potentials with distance of the fiber was faster for increasing depth than axial distance (this is due to the geometry of the volume conductor, as indicated by comparison with simulations with a planar volume conductor). Point electrodes led to smaller detection volumes than rectangular electrodes, and the double differential system was the most selective recording configuration. The relative decrease of the detected potential generated by more distant fibers was lower for increasing thickness and conductivity of the mucosa. End-of-fiber components are enhanced by the circular geometry of sphincter muscles with respect to the case of a planar volume conductor. CONCLUSIONS The detection volume of EMG recording systems from sphincter muscles extends more in the axial than in the depth direction and is affected by many parameters that cannot be estimated in practical situations, thus introducing a rather large variability in the muscle portion investigated among subjects. More selective information can be obtained using the recently developed multi-channel probe with respect to classical probes. Selectivity is increased with small electrodes and double differential recordings.