Electromyogram.
What it is?

An electromyogram (EMG) - (myo - muscle, graphically - I write) is a record of the electrical signals of a muscle or muscle group. Registration of electrical signals is carried out using special sensors located on the skin surface or injected intramuscularly (needle electrodes).

Muscles that are at rest do not exhibit electrical activity, but when doing any work, their activity manifests itself in the form of a change in the electric potential (both on the muscle itself and on the skin surface), and the value of this potential can be recorded.

The elementary unit of muscle is muscle fiber, or muscle cell. When a muscle is activated, muscle fibers are activated in groups, since each motor neuron forms a connection with several muscle fibers at once. Such a combination of a motor neuron and a group of muscle cells is called a motor unit (MU) [1].
Motor units operate on the principle of "all or nothing", or they are activated and produce a strictly dosed effort, or MU remains inactive. The effort produced by the MU is regulated by adjusting the frequency of their contraction, which averages 8-15 pulses per second (pul/s) with fluctuations from 5 to several tens of pul/s.
The formation of an electrical signal in the process of muscle work is as follows: after the arrival of a nerve impulse to the MU, the membrane of the muscle fiber is depolarized (within 0.5 ms). Membrane depolarization is due to inward movement of Na + ions. The depolarization process spreads to both sides of the muscle cell at a speed of 2-6 m/s.
The electrodes located in this field record the voltage (electric potential) of muscle fibers. If the electrodes are placed along the muscle fibers, the electrical signal of the fibers will be bipolar in shape and the sign of the first wave will depend on the location of the electrode with respect to the fibers.

The image of the myogram from the MU group is shown in Figure 2.

Depending on the situation, the EMG can vary greatly in amplitude and frequency. Consider the simplest situations (taken from [2]):

1. The state of complete rest of the muscle. In this case, the EMG amplitude is 4-8 μV and reflects the activity of the end plates of the muscles (the place of attachment of the motor neuron to the muscle fiber).

2. Maintaining the posture (lying, sitting, standing). In this case, the amplitude is 20-110 μV.

3. With a static force. In this case, the amplitude is 15–140 μV, and the frequency is 30–85 Hz. It can reach 1.5 mV and frequencies of 160 Hz.

Electrodes for surface myography are of the following types:

• Standard Disposable Gel-Based Electrodes

This type of electrode is convenient in that it adheres securely and provides good contact with the skin surface. The disadvantage of these electrodes is that they are disposable and occupy a large area of the skin surface. They are difficult to use in household devices.

• Electrodes requiring the use of a medical gel.

They are metal plates / suction cups coated with Ag/AgCl which, before installation on the body, are lubricated with a special gel to ensure good conductivity.

The quality of contact with the skin is not inferior to gel electrodes, but at the same time they create great discomfort for the user due to the large size, the need to use gel, as well as traces remaining after use.

• Dry electrodes

Dry electrodes are made from hypoallergenic materials and require only dry contact with the skin. This type of electrode can be inferior in quality of contact with the skin, but it is easily leveled by a measuring circuit. This type of electrode is very convenient when creating everyday devices.