Modifica di Electromyography (sezione)

==Electrical Characteristics of Amplifiers==
The design of the amplifier is the most critical part of the electronic devices used to record the sEMG signal. The fidelity of the sEMG signal detected by the electrodes and the amplifier influences all subsequent processing and presentation stages, and nothing can be done to restore a signal that has been incorrectly or distortedly acquired. A number of characteristics are important for this purpose; they are often advertised by manufacturers of the equipment, but much more rarely understood by the users.

===Differential Amplification and CMRR===
[[File:Opamppinouts.png|thumb|'''Figure 2:''' Operational amplifier symbol. The inverting and non-inverting inputs are distinguished by "−" and "+" placed in the amplifier triangle. V<sub>s+</sub> and V<sub>s−</sub> are the power-supply voltages; they are often omitted from the diagram for simplicity but must be present in the actual circuit.]]

As mentioned earlier, the issue of 50 Hz noise is potentially a rather difficult inconvenience to eliminate. The universally adopted technique to address this starts from the concept that such noise should be the same at all points on the body, while the bioelectric signal to be acquired in the same points must be different. Therefore, a differential amplifier is used. This can be thought of as consisting of two identical amplifiers whose output signals are subtracted from each other by an appropriate subtraction module. If the disturbing signal is the same at both inputs, it will be canceled at the output by the subtractor, while the useful signal, which is different at both inputs, will be amplified in a so-called differential manner. The disturbing signal that is the same at both inputs is also called a "common-mode" signal. Any signal generated far from the body has a high chance of being seen as a common-mode signal, while all signals generated near or inside the body will be "differential" signals. Therefore, noise generated by electromagnetic induction from power lines at 50 Hz will be actively canceled from the final recording of the sEMG signal. Clearly, this explanation requires the availability of highly accurate subtractors, since the common-mode signal can be thousands of times larger than the differential signal. In practice, perfect subtraction can never be achieved, only approximated to varying degrees of quality. The accuracy with which the subtractor performs the difference of the signals from the two inputs can be numerically expressed by the CMRR parameter of the amplifier. The CMRR is the "common-mode rejection ratio" and represents the ratio between the amplification of the differential signal and the amplification of the common-mode signal (which is very low and tends to zero due to the subtractor). Therefore, a perfect, ideal subtractor will have a CMRR equal to infinity. In practice, CMRR values range from 90 to 120 dB (the measurement is expressed in dB as 20 times the base-10 logarithm of the above ratio).<ref>Wang Yang. [https://iopscience.iop.org/article/10.1088/1742-6596/1846/1/012034/pdf A New Type of Right-leg-drive Circuit ECG Amplifier Using New Operational Amplifier.] Journal of Physics: Conference Series '''1846''' (2021) 012034 doi:10.1088/1742-6596/1846/1/012034</ref><ref>Bruce B. Winter; John G. Webster. [https://ieeexplore.ieee.org/document/4121504/authors#authors Driven-right-leg circuit design]. Journals & Magazines IEEE Transactions on Biomedic..Volume: BME-30 Issue: 1</ref>

As strange as it may seem, there are at least three reasons why it is not practical to have a very high CMRR: the first is that amplifiers with extremely high CMRR tend to be excessively expensive; the second is that such amplifiers are increasingly less stable and reliable in the long term as the CMRR value increases; and the third is that common-mode signals are not necessarily common-mode in an absolute sense, as they may have small phase or amplitude variations that undermine the best CMRR. In addition, alterations or asymmetries in the electrodes can have dramatic effects in lowering the overall CMRR of an amplifier that is otherwise of good quality. (Fig. 2)

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====Input Impedance====
The impedance of a circuit in which alternating currents flow (i.e., currents that do not always have the same direction and intensity over time) is the equivalent of resistance for direct current circuits. The difference is that impedance varies with the frequency of the currents, and thus a filtering effect is generally obtained, whereby certain signals at a particular frequency may be recorded with higher or lower intensity depending on the impedance of the circuits (typically electrodes and cables) through which they pass.

Specifically, to avoid unwanted attenuation and distortion, the impedance of the skin and electrode must be as low as possible, while the input impedance of the amplifier must be as high as possible, so that the current drawn from the biological generator that flows through the external circuits is minimized. Modern electronic circuits allow the creation of amplifiers with input impedances reaching up to 10<sup>15</sup> ohms, with input capacitance on the order of a few picofarads. Considering that the voltage from the surface sEMG signal is on the order of 10 mV, with an impedance of 10<sup>15</sup> ohms, the current in the electrodes and amplifier is minuscule, amounting to only a few thousand electrons (!) per second. However, it is not just the absolute value of the input current into the amplifier that matters: the balance of the currents in all the electrode circuits is also highly important. This requires not only careful amplifier design but also precise measurement techniques.

====Design and Use of "Active Electrodes"====
The requirement for a very high input impedance of amplifiers introduces a problem known as capacitive coupling at the inputs.

Indeed, with a very high input impedance, even the small capacitance between the electrode cables and the electrical distribution wires of the power grid can no longer be ignored. The solution to this issue involves reducing the length of the electrode cables or moving the amplifier as close as possible to the electrodes. So close, in fact, that it is incorporated into the electrode itself, thus creating an "active electrode." The already pre-amplified signal is then sent to the instrument through low-impedance cables, completely immune to the problem mentioned above.

====Filtering====
Even with the aforementioned considerations and the most scrupulous methods, the sEMG signal can still be contaminated by unwanted signals that can be eliminated using various filtering techniques. These techniques are based on circuits (or software programs in the case of digital filters) that allow the useful signals to pass through almost unchanged while strongly attenuating noise or other unwanted signals. For the sEMG signal, filters can pass signals in the band from 20 to 500 Hz, with out-of-band attenuation decreasing by 12 dB/octave, meaning a 12 dB reduction for every doubling or halving of the frequency beyond the minimum and maximum limits.

''Stability of the Electrodes''

Electrode stability refers to mechanical, electrical, and electrochemical stability. Mechanical stability has already been discussed. Electrical and electrochemical stability are related to the progress of the redox reaction that occurs at the electrode's contact with the skin and the skin's electrical characteristics. Problems can usually arise from abnormal changes in the hydration state of the electrode, such as drying out or excessive moisture due to sweat, for example.<ref>J.V. Basmajian and C.J. De Luca, ''Muscles Alive. Their Functions Revealed by Electromyography'', fifth edition (Williams and Wilkins, Baltimore, 1985).</ref>

===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.

===The "Mysterious" Reference Electrode===
The main issue with the reference electrode is that in most electromyographic equipment, it is called "ground" or "earth." The operator, usually unfamiliar with electronic or bioelectric aspects, perceives it as something related to patient safety or noise reduction (e.g., 50 Hz noise that would be "discharged" to "ground," as one would do with a household appliance). This is absolutely false and leads to great failures and wasted time.

The need for and importance of using a differential amplifier to record bioelectric signals has already been explained. It was said that a differential amplifier is essentially composed of two amplifiers that amplify the potential at two points, and the difference is taken instant by instant. Each amplifier will have two electrodes between which the potential difference is measured. Consider placing one electrode near the right temporalis muscle of the patient in Figure 1 and another electrode somewhere else on the skull. A recording of the potential difference between the muscle and the reference electrode will be obtained. If a second amplifier is used, with the electrodes placed between another area of the muscle and the same reference electrode, or another reference placed on the ear tragus, as in Figure 1, another recording of the potential difference between the masseter and the tragus will be obtained. The difference between the two (i.e., the difference of the two potential differences) will be the potential difference between the two muscle areas! It sounds like a tongue twister, but let’s do the math to clarify the concept.

Let <math>V_a</math> be the potential difference between electrode <math>A</math> and the reference electrode <math>C</math>; similarly, <math>V_b</math> will be for the other electrode. The value of <math>V_a</math> will be the sum of two components: the biological potential difference in that area (<math>V_{ac}</math>) plus the common-mode signal, for example, the 50 Hz signal (<math>V_n</math>). Similarly for <math>V_b</math>. In formula:

<math>V_a=V_{ac}+V_n</math>

<math>V_b=V_{bc}+V_n</math>

We know that the differential amplifier amplifies the difference between the inputs, and thus the output <math>V_o</math> of the differential amplifier, after an amplification of 1 (for simplicity), will be:

<math>V_o=V_a-V_b=V_{ac}-V_n-(V_{bc}-V_n)</math>

Simplifying the algebra:

<math>V_o=V_a-V_b=V_{ac}-V_{bc}</math>

The same exact procedure applies to the masseter muscle (D, E, R).

This is precisely the potential difference between the two muscle areas under electrodes A and B. As can be seen from the formula, the common-mode signal has disappeared in the final equation, meaning it could have been anything, assuming that the common-mode voltage between either of the two electrodes A or B and the reference electrode is equal.

[[File:Riferimento 2.jpg|left|thumb|'''Figure 3:''' Electrode placement as discussed in the text]]

Indeed, because of the difference between the signals of the two amplifiers in the differential amplifier, it is not necessary to place the third electrode exactly on the leg. It could be placed anywhere. Not surprisingly, this electrode is often called the "indifferent" electrode because it can be "indifferently" placed anywhere on the body surface. It is also called "ground" or "earth" or "reference," but in the sense of being the reference for the differential amplifier. It is more of a technical, electronic issue than a bioelectric one. In electrocardiographic (ECG) recordings, the indifferent electrode is the "right leg" electrode.<ref>M J Burke 1, D T Gleeson. [https://pubmed.ncbi.nlm.nih.gov/10721622/ A micropower dry-electrode ECG preamplifier] . IEEE Trans Biomed Eng. 2000 Feb;47(2):155-62. doi: 10.1109/10.821734.<br /></ref>

In practice, the indifferent electrode should be placed far from the recording site. An area where it can be well connected with low impedance contact, perhaps over a bony prominence (in electroencephalography, the mastoid process is used). For the same reason, it should preferably be a large electrode.

It is important to remember that it is not a "ground" electrode in the electrician's sense. It is often also identified as "isolated ground" to indicate that it is a reference for the amplifier, not the safety or shielding ground of the machine or machines in the recording area. Otherwise, the patient would be at risk of electrocution, as the patient must always remain isolated from everything to ensure safety, much like a pigeon on a high-voltage wire.

====Electrical Safety of Equipment====
A failure in a device powered by electricity that has direct galvanic contact with the patient's skin can pose a health risk, as a potentially dangerous current could flow through the subject, who typically cannot defend themselves.

This problem is usually non-existent in battery-powered low-voltage equipment (from 3 to 15 V), but it becomes important in mains-powered equipment. While absolute safety cannot be achieved in all possible cases, isolation between the circuits connected to the patient (low-voltage powered) and the remaining parts of the device is usually considered adequate. This can be achieved through magnetic coupling (isolation transformer) or optical coupling (optoisolator or photocoupler). The isolation transformer is generally the simplest method from a technical perspective, but it can also be the source of the most problems regarding recording fidelity. In both cases, isolating the patient from the rest of the circuit also minimizes induced 50 Hz noise.

The safety levels of sEMG equipment are regulated by specific harmonized standards at the European level, which are used to assess the quality of the instruments. A "minimum" level of safety must be present in the equipment according to various European directives. Only if this minimum level of safety is met can the equipment be marked with the CE (Conformité Européenne) mark, allowing its commercial circulation within all EU states.

===Processing of sEMG Signals===
For a long time, the most common form of processing the sEMG signal was to integrate the rectified waveform. This is done by rectifying the signal, i.e., making the negative deflections of the trace positive using appropriate electronic circuits. The resulting signal is then integrated, meaning it is passed through a low-pass filter that outputs a much smoother signal, averaging all the peaks of the original rectified signal instant by instant.

This type of processing was particularly popular because it was easy to implement with simple electronic circuits long before the advent of computers and digital signal processing.

Today, more appropriately, especially thanks to the use of digital signal processing, the root mean square (RMS) value of the signal is used.<ref>E M Spinelli 1, N H Martinez, M A Mayosky. [https://pubmed.ncbi.nlm.nih.gov/11410389/ A single supply biopotential amplifier.] Med Eng Phys. 2001 Apr;23(3):235-8. doi: 10.1016/s1350-4533(01)00040-6.<br /></ref> In this case, each signal value is squared and then averaged over time. In this way, the negative values of the signal become positive since squaring a negative value gives a positive result. Another type of processing is the one that provides the mean rectified value. This, along with integrated rectification, is an approximate measure of the area under the sEMG signal, but neither has a precise physical, physiological, or clinical meaning. The RMS value, on the other hand, is a measure of signal power and therefore has a more relevant clinical meaning. For this reason, it is increasingly used today.

In addition to these amplitude-related measurements, it is essential to remember the time measurements related to the onset of various sEMG signals. These times can be correlated with an external mechanical or electrical stimulus, as in the study of reflexes, or with movements or forces applied or exerted by one or more skeletal segments. These measurements are of interest in biomechanical studies.


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