Modifica di Electromyography (sezione)

===Geometry and Placement of Electrodes===
Throughout the history of electromyographic recordings, the shape and placement of the electrode surface have never received much attention. This is likely because a qualitative evaluation of the signal was given more emphasis, and this approach has persisted even in the approximate methodologies that have been used. To this day, the study of the sEMG signal has not achieved a stable "quantitative" reputation in the sense of being widely accepted. This is even more curious when considering that another specific "sEMG signal," that of the cardiac muscle or electrocardiogram (ECG), has long been established as an examination of undeniable clinical importance.

Moreover, signal processing through computer methods now presents significant challenges in terms of quantifying or at least objectifying electromyographic measurement.

====Distance Between Electrodes====
The distance between the electrodes greatly influences the bandwidth, amplitude, and phase of the sEMG signal. This means that the distance between the electrodes significantly affects the signal's shape, thus producing a kind of distortion. The fact that it also influences the phase tells us that time measurements (delays, latencies, periods) derived from the sEMG signal in reference to external stimulation events also depend on this. Ultimately, the distance between electrodes, although often underestimated in practice, is a fundamental parameter for performing quantitative sEMG measurements, i.e., reproducible and therefore comparable.<ref>A C MettingVanRijn 1, A Peper, C A Grimbergen. [https://pubmed.ncbi.nlm.nih.gov/7934255/ Amplifiers for bioelectric events: a design with a minimal number of parts.] Med Biol Eng Comput1994 May;32(3):305-10. doi: 10.1007/BF02512527.<br /></ref>

It is therefore clear that it would be highly preferable for the set of electrodes to be mounted on a rigid support so that the arrangement of the electrodes cannot vary in subsequent installations on the same subject or on different subjects (under comparable anatomical conditions). The distance between electrodes also depends on their size and the need to make measurements on small muscles without interference from sEMG signals from nearby muscles. A minimum distance of 1 cm is often considered adequate, but there are applications where the distance is even smaller.<ref>E M Spinelli 1, N H Martínez, M A Mayosky. [https://pubmed.ncbi.nlm.nih.gov/10612904/ A transconductance driven-right-leg circuit] . IEEE Trans Biomed Eng1999 Dec;46(12):1466-70. doi: 10.1109/10.804574.<br /></ref>

Small distances are generally avoided because it is believed that the signals may be altered by local conditions. Sweat is considered a hazard in these cases because it tends to "short-circuit" the electrodes on the skin. This is a controversial issue and is not considered valid by the author. In fact, beneath the skin, there is a natural "short-circuit" made up of the extracellular fluids of the subcutaneous tissue and dermis. An external "short-circuit," with an impedance similar to that of the interior, should therefore not alter the measurement. Some argue that constructing amplifiers with extremely high input impedance would be pointless in this way. However, the "short-circuit" would occur between the electrodes, not between the wires leading from the electrode to the amplifier, and the high input impedance of the amplifier continues to be relevant in counteracting the electrode impedance itself. Some also argue that, for the same reason, it would not be possible to make sEMG measurements in water, while the author has regularly developed radiotransmitting electromyographic systems for swimmers. Moreover, no one has ever questioned biopotential measurements, such as sECG, taken in "humid" environments, such as intraesophageal ECG or even invasive biopotential measurements.<ref>Palla´s-Areny R, Webster JG. AC amplifiers. In: ''Analog signal processing''. (Wiley, New York, 1999:97–109).</ref>

====Size and Shape of Electrodes====
It is certain that the larger the size of the electrode, the higher the level of the recorded signal and the lower the noise. However, a large electrode has the disadvantage of acquiring signals from different muscles or from parts of the muscle that are not of interest; specifically, spatial selectivity is lost. Therefore, an electrode is needed that captures the maximum number of muscle fibers from a restricted area with low noise. It is evident that these requirements are in conflict, and some compromise must be reached.

In addition to the conventional circular shape, other configurations such as array or bar electrodes are now being used, each with relative advantages and disadvantages. The "correct" shape remains an achievement reached through more or less heuristic attempts and depends on the operator.

====Localization and Positioning of Electrodes====
Electrodes should be placed between a motor point of muscle innervation and the tendon or between two motor points, and oriented along the muscle belly's longitudinal median line. Thus, the longitudinal axis of the electrodes should be aligned parallel to the length of the muscle fibers.

Electrodes should not be placed near the tendon. In such locations, muscle fibers are thin and sparse, and there is also the risk of "picking up" sEMG signals from other muscles (e.g., agonists).

Similarly, electrodes should not be placed on the motor point, although this is a difficult preconception to overcome. The motor point is the point on the muscle (and its equivalent projection on the skin) where the injection of a minimal current causes a well-defined contraction of the muscle itself. Usually, but not always, this point corresponds to the part of the muscle where innervation occurs and where the highest density of neurons is found.

However, from the standpoint of signal stability, measuring with two electrodes near the motor point is the worst situation to be in. From this region, the activation potentials of the muscle fibers propagate proximally and distally, and the relative positive and negative phases either sum or cancel out on the electrodes, producing a very distorted signal characterized by sharp, sudden spikes due to the random situation. Stability is particularly compromised here because it is evident that small movements of the electrode will cause huge variations in the trace and its frequency and phase characteristics.

It is also not advisable to place electrodes at the muscle's extremes (one on the origin and one on the insertion). In this case, too large a volume of tissue is under observation, and signals from muscles that are not of interest are easily captured.

====Orientation Relative to Muscle Fibers====
It is therefore clear that the longitudinal axis of the electrode configuration should be parallel to the muscle fibers. In this way, most of the fibers present in that area will be recorded along with the signal's spectral characteristics. This is important because the independence of the signal's spectrum from any trigonometric factors will prevent erroneous estimation of conduction velocity. For similar reasons, delay, period, and latency measurements will be more accurate and repeatable.