Electromyography

Surface EMG signals can be detected with electrodes of various shapes and sizes positioned on the skin.

Surface EMG signals can be detected with electrodes of various shapes and sizes positioned on the skin above the muscle of interest. The monopolar arrangement is the most basic configuration. It requires one electrode to be placed over the muscle that detects the surface EMG signal with respect to a second reference electrode. This arrangement is sometimes used in clinical EMG biofeedback devices and is subject to a variety of contaminations from ambient noise and other sources. The bipolar or single differential arrangement uses two electrodes, generally placed 1 to 2 cm apart along the orientation of the muscle fibres, to measure the EMG signal with respect to a third reference electrode. Surface electrodes can be either passive or active. In the passive configuration, the electrode consists of a detection surface that senses the current on the skin through the skin—electrode interface. The signal is then transmitted to the amplifier for conditioning. In the active configuration, the amplifier stage is at the sensor site, decreasing the path length between detection area and signal conditioning. This characteristic greatly increases the input impedance of the electrodes, reduc-ing noise and artefacts and ensuring high-quality signal recording. Electrodes are either of solid reusable construction with pure silver bar electrodes. or disposable silver/silver chloride (Ag/AgC1) electrodes. The shape and size of the electrodes and their interelectrode distance determine the area of muscle tissue being examined and, therefore, the number of motor units that may be detected within this area.
. The greater the area and the inter-electrode distance, the greater the electrode pick-up area and the number of detected motor units. However, if the electrodes are too large or if they are placed too far apart, they are more susceptible to detecting signals from surrounding muscles in addition to the target muscle. Interference from electrical signals originated by nearby muscles are referred to as crosstalk signals and can mislead the identification of the activity of the target muscle.

Surface EMG electrodes are exclusively used to pick electrical signals from superficial muscles located close to the skin. Signals from deeper muscles can not be recorded reliably. Even for superficial muscles, a thick layer of fatty tissue between the electrode and the muscle acts as an insulator and can affect both the magnitude and frequency response of the EMG signal recorded. In these cases, there are no options other than using intramuscular EMG recording techniques. Grids of surface electrodes are used when the spatial properties, rather than temporal properties, of a muscle are under investigation.