Surface EMG and muscle fatigue: multi-channel approaches to the study of myoelectric manifestations of muscle fatigue

In a broad view, fatigue is used to indicate a degree of weariness. On a muscular level, fatigue posits the reduced capacity of muscle fibres to produce force, even in the presence of motor neuron excitation via either spinal mechanisms or electric pulses applied externally. Prior to decreased force, when sustaining physically demanding tasks, alterations in the muscle electrical properties take place. These alterations, termed myoelectric manifestation of fatigue, can be assessed non-invasively with a pair of surface electrodes positioned appropriately on the target muscle; traditional approach. A relatively more recent approach consists of the use of multiple electrodes. This multi-channel approach provides access to a set of physiologically relevant variables on the global muscle level or on the level of single motor units, opening new fronts for the study of muscle fatigue; it allows for: (i) a more precise quantification of the propagation velocity, a physiological variable of marked interest to the study of fatigue; (ii) the assessment of regional, myoelectric manifestations of fatigue; (iii) the analysis of single motor units, with the possibility to obtain information about motor unit control and fibre membrane changes. This review provides a methodological account on the multi-channel approach for the study of myoelectric manifestation of fatigue and on the experimental conditions to which it applies, as well as examples of their current applications

Sensitivity of surface EMG-based conduction velocity estimates to local tissue in-homogeneities – influence of the number of channels and inter-channel distance

The aim of this simulation study was to investigate the influence of local tissue in-homogeneities on the estimates of muscle fiber conduction velocity (CV) from surface EMG signals. A recently developed analytical surface EMG model was used to generate simulated surface EMG signals from a planar layered volume conductor, comprised of the muscle tissue and fat layer, with spheres (1 mm radius) in the fat layer of conductivity different from the surrounding tissue. CV was estimated with a maximum likelihood multi-channel approach, varying the number of channels and the inter-channel distance used for the estimate. The action potentials detected along the muscle fiber direction changed shape due to the presence of the in-homogeneities, thus affecting CV estimates. CV estimates were influenced by the location of the in-homogeneities with respect to the fiber and detection electrodes. The maximum percent variation of CV estimates due to the presence of in-homogeneities decreased with increasing number of channels and inter-channel distance: 19.6% (2 channels), 12.1% (3 channels), 6.4% (4 channels), for 5 mm inter-channel distance, and 12.0% (2 channels), 5.2% (3 channels), 2.4% (4 channels), for 10 mm inter-channel distance (for double differential detection). The results were in agreement and explained previous experimental findings. It was concluded that multi-channel methods for CV estimation significantly reduce the sensitivity of CV estimates to tissue in-homogeneities

Analysis of 2D Spatial Filtering of Simulated Muscle Action Potential Using Grid Arrays

Surface grid electrode is a noninvasive technique, which can be utilized for topographic analysis of EMG signals. In this study, an innovative spatial filtering technique is proposed in the form of grid electrodes to enhance the selectivity of surface EMG signal considering the effect of intermediate tissue layers between source and recording electrode. A simulation algorithm is developed to generate complete profile of single fiber action potential (SFAP) using previously derived mathematical model and published clinical data. A multiple-layer model is investigated in order to determine the potential distribution at the skin, fat and muscle surface based on the solution of the Poisson equation in spatial frequency domain. The arbitrary constants of the solution are determined by imposing the boundary conditions. The characteristics of subcutaneous fat and skin tissues are incorporated in the SFAP model to develop a systematic approach to select an appropriate inter-electrode distance of two dimensional grid arrays in order to eliminate spatial aliasing and distortion. The minimum grid spacing is determined by satisfying the Nyquist criterion for spatial sampling. The subcutaneous tissue layers reduce the frequency contents and attenuate the amplitude of the potential distribution at muscle surface. A two dimensional spatial filter is designed by manipulating the inverse of transfer function of fat and skin in order to compensate their spatial widening effect. The inverse transfer function is approximated to represent it in the form of filter mask for a discrete grid array. This spatial filtering technique is also investigated to eliminate the effect of a particular thick anisotropic medium inside muscle

Analysis of electrode arrays for multichannel surface electromyography

Nowadays medical evaluation requires in each further step improvements in all the areas concerning engineering and materials. This applies to the application of electromyography and in specific to surface electromyography which is being used every day to evaluate muscular and neuromuscular diseases. In this field electrodes are used as interfaces to acquire the biopotential signals generated by muscles. The materials usually used in these electrodes are expensive. This fact can be a limitation factor for the applications that use a considerable number of them such as High Density surface electromyography. This thesis aims to evaluate using tests, parameters extraction, results analysis and performance comparison a set of different electrode arrays in order to evaluate the suitability of low cost material specifically stainless steel to perform the same tasks as other materials used in medical applications such silver or silver chloride electrodes