EMG: the signal that gives you an hand!

Even for non-professionals, the trend of an electrocardiographic (ECG) trace is not a big news. We all know very well that the heart, by contracting, generates an electrical signal which, taken through appropriate electrodes and devices, is represented with the classic wave pattern recognizable by the Q-R-S complex.

Image: researchgate.net

How does EMG signal work?

The electromyographic signal (EMG) is the electrical signal generated by the muscles, but it is very different from the ECG! Why? It’s easy! You can’t rule your heart, but you can do it with other muscles!

In particular, while the heart is an involuntary muscle and contracts without direct conscious control, skeletal muscles (also called striated muscles) contract voluntarily.

As a consequence, the EMG, is simply the signal, of very low extence, generated during muscle activity. It does not have the immediately recognizable form of the ECG, but it has an irregular and jagged pattern.

Image: intechopen.com

This happens because, unlike the heart muscle, the skeletal muscles don’t contract synchronously, but activate their functional portions, the so-called motor units, in a number proportional to the intensity of the force exerted: it means that the greater the force that you intend to exercise, the greater the portion of muscle involved and, therefore, the greater the amplitude of the recorded EMG signal.

EMG electrods 

In electromyography, the electrodes that are used are divided into two main types: needle electrodes and surface electrodes.

Needle electrodes

Image: technomed.nl

The needle electrodes allow to obtain a very clean signal, ensuring what is called a high spatial resolution, that is the ability to “listen” to the EMG signal also coming from tiny portions of muscle.

Needle electromyography, being an invasive technique, is used only in a clinical context for diagnostic investigations of the neuromuscular system, and it’s certainly not indicated for prolonged use or in dynamic conditions.

Surface electromyography

Surface electromyography uses both disposable electrodes and reusable electrodes (“plates” in metallic material). A major problem of surface electromyography is maintaining a suitable electrode-skin coupling: dry, poorly hydrated skin and the presence of scars or hair can cause a high impedance of the electrode-skin contact which will “dirty” the EMG signal, compromising its use.

Furthermore, the fixing of the electrode to the skin is also important, since the surface EMG signal is particularly corrupted by the movement artifact, an electrical noise that perfectly overlaps the EMG signal and it’s also due to the smallest relative displacement between electrode and skin.

EMG signals of Adam’s Hand

The myoelectric prostheses, such as Adam’s Hand, work by exploiting the surface EMG signal recorded with reusable electrodes. These are designed to be light (to minimize the movement artifact) and to have a good electrical coupling with the skin.

Currently, our team of electronic and biomedical engineers is developing an algorithm of artificial intelligence on the basis of these signals, which can correctly record EMG signal, even in particularly complex conditions, where muscle integrity is not fully guaranteed, as often happens after amputations due to trauma or accidents.

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