If you record a sEMG signal, the question is: Where does the signal come from? For example, if you stick the electrodes over a whole group of muscles, the signal can come from all these muscles. This reduces the fractional amount of cable artifacts caused by movement of the sensor cables to a minimum, since the amplification is performed before the artifacts are generated.

Modern sEMG technology is also equipped with a first stage of amplification directly in the sensor. Where v are the MUAPs and n is the ambient noise.

With a simple differential calculation, the EMG signal can now be calculated, and the noise gets cancelled out: Both electrodes now record the basic noise as well as the MUAPs with a time delay (time delay because the potentials spread along the muscle fibres and thus reach first the first and then the second electrode). Here, a sEMG sensor consists of two electrodes, which are attached to the skin at a certain distance (usually 10mm) along the muscle fibre. This can be significantly reduced by so-called differential electrodes. These emanate from electromagnetic radiation, which is omnipresent in technical environments. The energetic distribution of the EMG signal is essentially in the frequency range from 0 to 500 Hz, with the dominant components lying in the range 50-150 Hz.Īmbient noise includes both power line noise (about 50-60Hz) and cable artifacts. Because of this low amplitude, it is necessary to amplify the signal by amplifiers with a range of 1000 to 10000. The amplitude of the sEMG signal is in a range from µV to low mV. The resulting EMG signal is bipolar with a symmetrical distribution of positive and negative values, which means that the average value is zero. However, with electromyography you are not able to view and record all MUAPs in a differentiated manner and therefore, the signals are superimposed (Fig. This is because usually, several motor units “fire” and thereby cause the muscle to twitch. However, under normal conditions, it is extremely rare for a single motor unit (which consists of a motor end plate and one or more muscle fibers) to be innervated individually. If an electrode is now placed above the muscle to be examined, depolarization and subsequent repolarization is measured in the form of a bipolar voltage change. After the original excitation, this zone shifts as a Motor Unit Action Potential (MUAP) at a speed of 2-6 m/s along the muscle fibre. The size of the resulting depolarization zone is described in the literature as approximately 1-3mm². This signal is the so-called action potential (AP), which is conducted from the nerve cells via motor end plates to the muscle. As described above, we use EMG to record the electrical signal that initiates muscle contraction.

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