What is the EMG signal?
The electromyographic (EMG) signal is generated by the electrical activity of the muscle fibers during a contraction.
The EMG signal is a representation of the electric potential field generated by the depolarization of the outer muscle fiber membrane. Its detection involves the use of intramuscular or surface electrodes which are placed at a certain distance from the sources.
EMG can be detected by intramuscular electrodes or by surface electrodes. The surface electrodes are placed on the skin whereas the intramuscular electrodes are an invasive measurement system. Indeed, the insertion of electrodes directly into the muscle allows the detection of electric potentials very close to the source. For this reason, the action potentials of the different motor units (MU) can be identified more easily than the surface electrodes.
With the EMG sensors integrated into our new WaveX system, you can accurately detect EMG signals using surface electrodes.
For precise EMG signal detection
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How to acquire a good EMG signal?
EMG signal features depend on several anatomical, physical, and detection system parameters. The most important factors are:
- The thickness of the subcutaneous tissue layer (only for surface recordings)
- The depth of the sources within the muscle (for surface recordings) and the distance from the source to the electrodes (for intramuscular recordings)
- The inclination of the detection system with respect to the muscle fiber orientation (mainly for surface recordings)
- The length of the fibers (mainly for surface recordings)
- The location of the electrodes over the muscle (or within the muscle in case of intramuscular recordings)
- The inter-electrode distance used, as it constitutes a spatial filter during signal detection
- The electrode size and shape (for surface recordings)
- Crosstalk among nearby muscles (for surface recordings)
The traditional modality for surface EMG (sEMG) signal detection is mostly based on a pair of individual electrodes placed on the skin in the region above the muscle. The EMG signal and its features are strongly influenced by inter-electrode distance, electrode size, and placement.
Conventional electrodes, either wet or dry, behave like transducers converting ionic current (in tissue and gel) into a flow of electrons in the metal. These electrical sensors require careful skin preparation to reduce the impedance and noise associated with this interface.
Commonly used skin preparations are:
- Rubbing the skin with ethyl alcohol or other solvents to remove oily substances
- Rubbing the skin with abrasive conductive paste and then cleaning it with a wet cloth
- Stripping with adhesive tape
Therefore, to acquire an EMG signal is important to clean the skin and place correctly the electrodes on the muscle of interest.
If these steps are not performed correctly, it is possible to have movement artifacts, crosstalk, and noise.
The movement artifacts are generally caused by:
- Poor electrode-skin adhesion,
- Mobile contact,
- Adhesive tape on the electrode,
- Not prepared skin,
- Rigid cable.
Crosstalk
Crosstalk refers to the signal that is detected over a certain muscle, but is generated by another, mostly nearby muscle. This phenomenon happens in surface recordings when the distance between the detection points and the sources in different muscles is of the same order of magnitude. Crosstalk is one of the most important sources of error in interpreting surface EMG signals. This problem becomes relevant in particular applications such as gait analysis, where the onset detection in different muscles is crucial. To eliminate these issues, it is possible to apply a high pass filter.
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