Considerations for surface EMG: Factors influencing signal quality
EMG allows you to record electrical activity produced by muscles. Several factors can influence the signal quality while recording EMG signals. The quality of an EMG measurement largely depends on skin preparation, electrode positioning, and the quality of the EMG equipment. This blog explains why these factors are important and how you can obtain good quality surface EMG signals.
How should you prepare the skin?
The main goal of skin preparation is to achieve stable electrode contact and low skin impedance. How thorough the preparation needs to be depends on the recording site (on the body) and the level of required movement that is expected. Body parts with many hairs, sweat, and dirt require more attention. Moreover, for dynamic movements, a great quality of connection is important.
Preparation usually involves the following steps:
- Shaving to remove hair. Removing hair leads to improved adhesion of the electrodes.1,2
- Scrubbing the skin with an abrasive gel. This step removes the dead skin cells, dirt, and sweat from the skin.1,2 Alternatively, fine sand paper or 70% alcohol rubbing can be used.
The participant should not experience discomfort while preparing the skin. However, the skin might turn red after preparation. This is usually an indication that the connection will be good.
Note: Sometimes, the hygienic behavior of the participant can influence the electrode-skin connection. Using lotions or creams before the measurement, can lead to higher impedances. Moreover, not showering for a long time can lead to an excess of oil on the body.
What is the best electrode placement on the muscle?
Electrodes should be placed over the muscle that you want to record. This muscle should be superficial and ideally easily palpated. Generally, the SENIAM guidelines are used to locate the correct recording site.3 These guidelines described how to use anatomical landmarks to determine the appropriate electrode location.
Starting from the motor end-plates in the innervation zone, the action potential travels along the muscle fiber in both directions (read more about EMG here). Therefore, it is important to align the electrodes with the muscle fibers. Ideally, the electrodes should be placed in the middle between the most distal motor endplate zone and the distal tendon and be located away from major blood vessels and nerve trunks. For muscles that shorten during contraction, it is important to check if the electrodes are still placed over the muscle during muscle shortening.
The distance between electrodes affects the signal that is recorded. For larger inter-electrode distances (IED), the recorded signal has a higher amplitude. However, there is an increased risk of cross-talk from neighbouring muscles and MUAP extinction at the tendon.4 For bipolar EMG recordings, SENIAM recommends an IED of 2 cm for most muscles, because a maximal EMG signal amplitude is expected with this IED.3
For high-density EMG (HD-EMG), grids with multiple electrodes are used to record EMG signals. Typically, the IED for HD-EMG is between 5 and 10 mm, depending on the research question.5 It is important that the grid is well connected to the skin to prevent artifacts caused by the electrodes losing contact.
What is the best EMG equipment?
The quality of the recording equipment greatly influences the quality of the EMG recordings. In the next sections, the most optimal properties of electrode materials and amplifiers are discussed.
What is the best electrode material?
Electrodes are the interface between the skin and the amplifier. In the body, current is carried by ions. In the electrode and lead wire this current is carried by electrons. Therefore, at the skin-electrode interface the current carriage method changes from ions to electrons. Chemical reactions at the site of the electrode and the gel (electrolyte) allow this transition. However, this chemical reaction changes the ionic concentrations locally around the electrode, resulting in a potential difference (half-cell potential) with the rest of the solution.
The most commonly used electrode material is Ag/AgCl.3 Electrodes made of Ag/AgCl are almost perfectly nonpolarizable, which means that current can freely pass across the electrode – electrolyte interface.6 As a result, the previously mentioned half-cell potential hardly changes over time, making Ag/AgCl electrodes very stable, which reduces power line interference and the occurrence of artifacts related to body movements.7
What determines an amplifier's quality?
The function of an amplifier is to take a weak electric signal of the body and increase its amplitude so that it is compatible with recording and displaying devices. A high-quality EMG amplifier has the following characteristics:
- A high input impedance, so as not to distort the signal coming in.6
- A bandwidth in the frequency band of EMG: between 6 and 500 Hz.1
- No hardware filters, so the signal information is not altered.1
- A high common-mode rejection ratio (CMRR). This suppresses all signals ‘common’ to all recorded electrodes. One such example is 50 or 60 Hz powerline noise. Typically, the common mode voltage is much larger than the EMG signal, so a high CMRR is needed to distinguish the EMG signals.1
This blog discussed multiple factors that influence EMG signal quality. The skin should be well-prepared, the electrodes should be placed on the body with care, and lastly, high-quality recording equipment is important.
At TMSi, we make high-quality amplifiers and sensors for your EMG research. Find out more about our products here.
References1. Konrad P. The ABC of EMG. A practical introduction to kinesiological electromyography. 2005.
2. Tankisi H, Burke D, Cui L et al. Standards of instrumentation of EMG. Clinical Neurophysiology. 2020;131(1):243-258. doi:10.1016/j.clinph.2019.07.025
3. Stegeman D, Hermens H. Standards for surface electromyography: The European project Surface EMG for non-invasive assessment of muscles (SENIAM). 2007;1.
4. Campanini I, Merlo A, Disselhorst-Klug C, Mesin L, Muceli S, Merletti R. Fundamental Concepts of Bipolar and High-Density Surface EMG Understanding and Teaching for Clinical, Occupational, and Sport Applications: Origin, Detection, and Main Errors. Sensors. 2022;22(11):4150. doi:10.3390/s22114150
5. Merletti R, Muceli S. Tutorial. Surface EMG detection in space and time: Best practices. Journal of Electromyography and Kinesiology. 2019;49:102363. doi:10.1016/j.jelekin.2019.102363
6. Webster J, Nimunkar A, Clark J. Medical instrumentation. 4th ed. 2010.
7. Geddes L. Electrodes And The Measurement Of Bioelectric Events. new york: Wiley, John & Sons; 1972.