We propose a model for surface electromyography (EMG) signal generation with cylindrical description of the volume conductor. The model is more general and complete with respect to previous approaches. The volume conductor is described as a multilayered cylinder in which the source can be located either along the longitudinal or the angular direction, in any of the layers. The source is represented as a spatio-temporal function which describes the generation, propagation, and extinction of the intracellular action potential at the end-plate, along the fiber, and at the tendons, respectively. The layers are anisotropic. The volume conductor effect is described as a two-dimensional spatial filtering. Electrodes of any shape or dimension are simulated, forming structures which are described as spatial filters. The analytical derivation which leads to the signal in the temporal domain is performed in the spatial and temporal frequency domains. Numerical issues related to the frequency-based approach are discussed. The descriptions of the volume conductor and of the source are applied to the cases of signal generation from a limb and a sphincter muscle. Representative simulations of both cases are provided. The resultant model is based on analytical derivations and constitutes a step forward in surface EMG signal modeling, including features not described in any other analytical approach.

Background/Aims: This work focuses on recording, processing and interpretation of multichannel surface EMG detected from the external anal sphincter muscle. The aim is to describe the information that can be extracted from signals recorded with such a technique. Methods: The recording of many signals from different locations on a muscle allows the extraction of additional information on muscle physiology and anatomy with respect to that obtained by classic bipolar recordings. Multichannel EMG methods have been recently developed for the assessment of the external anal sphincter. An anal probe was used in this study to record signals at different depths within the anal canal during contractions at different effort levels. The plug is 150 mm in length and 14 mm in diameter, holding a circumferential array of 16 equally spaced silver bar electrodes, located at a distance of 20 mm from the probe tip and aligned with the probe axis. Results: Information about localization of the innervation zone, fiber length, EMG amplitude, muscle fiber conduction velocity and single motor unit analysis can be obtained from the signals recorded with the circumferential array by means of innovative signal processing techniques. Conclusions: The type of information extracted from multichannel surface EMG signals cannot be obtained with other currently available techniques. The technological innovation described in this work is promising for a further insight into the investigation of pelvic floor pathologies and rehabilitation treatments.

Surface electromyographic (EMG) signal modelling is important for signal interpretation, testing of processing algorithms, detection system design and didactic purposes. Various surface EMG signal models have been proposed in the literature. This study focuses on the proposal of a method for modelling surface EMG signals, using either analytical or numerical descriptions of the volume conductor for space-invariant systems, and on the development of advanced models of the volume conductor by numerical approaches, accurately describing the volume conductor geometry and the conductivity, as mainly done in the past, but also the conductivity tensor of the muscle tissue. For volume conductors that are space-invariant in the direction of source propagation, the surface potentials generated by any source can be computed by one-dimensional convolutions, once the volume conductor transfer function has been derived (analytically or numerically). Conversely, more complex volume conductors require a complete numerical approach. In a numerical approach, the conductivity tensor of the muscle tissue should be matched with the fibre orientation. In some cases (e.g. multi-pinnate muscles), accurate description of the conductivity tensor can be very complex. A method for relating the conductivity tensor of the muscle tissue, to be used in a numerical approach, to the curve describing the muscle fibres is presented and applied to investigate representatively a bi-pinnate muscle with rectilinear and curvilinear fibres. The study thus proposes an approach for surface EMG signal simulation in space invariant systems, as well as new models of the volume conductor using numerical methods.

Objectives: The objective of this work was to investigate the distribution of the innervation zones of the motor units that make up the external anal sphincter (EAS) in healthy males and females. Methods: A cylindrical probe carrying a circumferential array of 16 electrodes was used to detect the generation, propagation and extinction of individual motor unit action potentials (MUAPs) at 1, 2, and 3 cm depth from the orifice of the anal canal during maximal voluntary contractions of the EAS. Fifteen healthy males and 37 healthy nulliparous females were investigated. Results: IZs could be detected in all males and in 34 out of 37 females. In the males, the IZs are scattered in the right and left hemisphincter at each of the three levels and their distribution is not affected by depth. In the females, the distribution is also concentrated in the right and left hemisphincter at depth 1 cm but is more uniform at depth 2 cm and more concentrated in the dorsal and ventral regions at depth 3 cm. ANOVA shows a statistically significant dependence of the IZ distribution on depth only in females and not in males. Conclusions: It is concluded that (a) IZs of the EAS can indeed be detected with a circumferential array placed at different depths along the anal canal; (b) large individual variability is observed, and (c) IZs show similar distribution at the three depth levels in males and different distributions in females.

Spatial filters are used for increasing selectivity in surface EMG signal detection. The study investigated the importance of the description of the volume conductor to the inference of conclusions on comparing filter selectivity from simulation analyses. A cylindrical multi-layer description of the volume conductor was used for the simulation analysis. Different anatomies were analysed with this model, and results on filter selectivity were compared. The longitudinal single (LSD), double (LDD) and normal double differential (Laplacian, NDD) filters were investigated. Largely different conclusions could be drawn when comparing filter selectivity resulting from simulations with different volume conductor models. A filter that performed best with a particular anatomy could be the poorest with another anatomy. With a bone-muscle model and superficial fibres, the ratio between peak-to-peak values of the propagating and non-propagating signal components was approximately 220% for LDD and LSD and lower than for NDD (approximately 290%). With a bone-muscle-fat-skin model, LSD performed significantly worse (150%) than both LDD and NDD, which showed similar performances (approximately 300%). Similarly, if the lateral distance of the recording was increased by 10 degrees, the signal amplitude was reduced to 2% with LSD and LDD and to 4% with NDD. With another anatomy, LSD and LDD reduced signal amplitude to 20-25%, and NDD reduced it to 4%. Similar considerations could be drawn for other selectivity indexes. Thus, modelling should be used carefully to infer conclusions on spatial selectivity and to indicate particular choices of spatial filters.