Mathematical programming applied to the decomposition of EMG signals

The relevant parameters for this study are the recruitment of motor units and them activation frequencies, called firing frequency. The combination of these terms determines the force that changes from muscle to muscle. The electrical activity of a muscle depends on the same terms. Both electrical activity and force depend on recruitment and frequency, so we can say that a relation between them exists. However, we have to remember that there are a lot of factors that influence the EMG signal trend like: muscular fatigue, energy metabolism and oxygen availability. We can observe that: to an increase of the number of motor units corresponds the increase of the spike amplitude and to an increase of the activation frequencies corresponds an increase of the characteristic frequencies of the analysed signal.
The electrodes are real transducers that transform the ionic currents generated in the muscles into an electronic current that can be manipulated with electronic circuits and stored in analog or digital form as voltage potential. The electrodes are classified according to several criteria, such as: the detection site (depth and surface electrodes) and the type of configuration (monopolar and bipolar electrodes). Electrodes include some common materials like platinum, stainless steel and silver-silver chloride; the electrodes differ from the needles they use.

The first distinction is done to the depth of application of the electrodes.
Depth electrodes are inserted into the muscle tissue. They allow the study of deep muscles. They are able to record a reduced number of motor units because they have a reduced surface that detects the signal. So, they are selective. Therefore, these kinds of electrodes permit to investigate different areas of the same muscle. The main disadvantage is their invasiveness, their little tolerability by the patient and their costs. Depth electrodes could be classified in needle electrodes and wire electrodes. Inside needle electrodes there are one or more conducting wires. The terminal parts of conducting wire form sensible aureoles. The diameter of these aureoles is of the order of tenth of a millimeter. The number of aureoles depends from the configuration of the electrode: monopolar electrodes have only one aureole and bipolar electrodes have two aureoles.

Wire electrodes are the evolution of the needle electrodes. They have flexible wires inside a hollow needle. The wire diameter is from 25 to 100µm. Wires are inserted into the muscle through a needle and they are hooked to the muscle. The advantage of these electrodes is their flexibility, they don’t break during muscle contraction. They could be used for motor analysis. Surface electrodes are applied to the skin of the patient. They usually have the form of a suction cup and they are applied through a bulb. We are referred to the Figure 3.7. A disadvantage is the attenuation of the signal due to the passage through the adipose tissue. Sometimes electrolyte gel can be added to improve the connection. In fact, having low contact impedance permits to reduce the noise of the EMG signal.

The second distinction is due to the configuration of electrodes. Standard EMG signals recording requires a minimum of two electrodes. The measurement is done by the electrodes and the measures obtained are compared. There are at least two kinds of recordings, the first is the mono-polar one characterized by the presence of two electrodes, the first one is positioned close to the interested fibre, while the second is positioned in a region in which the contribution of the active fibres is minimal. They have a stainlesssteel core that is coated with Teflon except for an exposed surface from 1 to 5mm that acts as the active electrode. Mono-polar electrodes measure include the signal, but also biological and ambiental signals. It is difficult to eliminate these components of the signal. The solution is provided by the second kind of configuration. Bipolar recording implies that the electrodes are both placed closed to the active fibres. There is also a reference electrode placed in an electrically inert zone. With this kind of configuration, we limit the overlapping of other muscles signals to the desiderata. This problem is named crosstalk one. Moreover, having electrodes nearby allows us to hypothesize that they are affected by the same noise. With the measurement of the difference of voltage, we eliminate the disturb components. EMG signal is placed at the amplifier input. The general purpose of the amplifier is to make the signal viewable. Amplification usually occurs in two steps, the first one decreases the interconnection error with electrodes. In the second step there is the introduction of a differential gain that usually has low values to avoid amplification of the error. Instrumentation amplifier is composed by three operational amplifiers.
After amplification it is usually applied filter elements (analogical or digital) to decrease noise.