Modifica di Electromyography (sezione)

===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.