Modifica di Electromyography (sezione)

====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>