*2.2.1. Electromyography (EMG)*

Free EMG (or fEMG) is the recording of the bioelectrical activity from muscles. Usually, fEMG is recorded by a pair of intramuscular stainless steel electrodes that are inserted into the muscle separated by a distance of 2 cm.

Although we may be interested in the state of the muscle, the most common use of the fEMG signal is to identify changes in the CN or peripheral nerves. In fact, we used nerve-induced electrical activity of the muscle as a measure of the state of that nerve. In this way, we could be aware of irritative activity originating in the nerve as a consequence of traction, torsion, compression, or heat injury, among others. In addition, the patient should not be curarized during most of the surgery. It is important to distinguish between mechanical activity induced by surgical activity, which is usually transient, and neurotonic discharges, which are contin‐ uous and persist after the surgeon leaves the surgical field.

Settings that are used to record fEMG depend on the muscle being recorded, the size of the electrodes and the objective. However, it is quite common to use a 10–3000 Hz bandwidth, with either the notch on or off (in this case, the high pass filter should be increased up to 50 Hz), a gain always greater than 30 μV/div (the most common is 50 μV/div), and a time base between 100 ms/div (for small periods) and 750 ms/div (for longer periods). In the last case, we are aware that the shape of the muscle activity can be distorted (depending on the screen resolution) and could be misinterpreted as an artefact.
