Basic EMG Signal Pick-Up
There are three electrodes used for EMG guidance procedures:
- a surface reference electrode
- a surface ground electrode
- a hypodermic needle electrode
This is the basis of what is required to measure a usable biosignal using a differential amplifier to improve the quality of the signal by subtracting out the common mode noise.
Think about it like this: The hypodermic needle electrode signal is subtracted from the reference electrode signal to yield the signal that is outside of the noise that is common to both. Ideally, this will leave a sparkling EMG signal devoid of the common noise.
Short Electronics Tutorial
The common mode rejection ratio (CMRR) of a differential amplifier is a metric used to quantify the ability of the device to reject common mode signals, i.e. those that appear simultaneously and in-phase on both inputs (hypodermic needle electrode and reference electrodes provide the signals).
A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI).
Myoguide employs high quality instrumentation amplifiers in the front-end circuitry, to not only act as a buffer, but to also impedance match a wide range of needle electrode situations, that might otherwise degrade results.
As we know, EMG signals have very low amplitudes. In all cases, the common mode noise will affect the measurement accuracy. Due to the effect of common mode signals, the bioelectric amplifiers cannot distinguish the artifact (noise signal) from the original signal. Electrical signals are electrical signals, despite where they originate!
The term “common mode rejection” refers to a differential amplifier’s ability to remove signals common to both the signal and reference electrodes. In the case of EMG signal acquisition, this is NOISE!Both signals are based upon the ground electrode. Think about the ground electrode as the anchor for this signal saga. The outcome of this subtraction presents a reasonably clean EMG signal.
This is also the reason why better results occur when the reference electrode is located close to the needle electrode. There is generally less noise measured over shorter distances.
The ground electrode is needed as the base from which the overall signals arise. In the case of recording signals, it’s nice to have the ground in the vicinity, but more or less, central to both the needle and reference electrodes.
Reference electrodes can also be central to multiple areas being injected, if its position is reasonably equidistant to those sites. This can save time and improve signal quality by not having to reposition the surface electrodes.
Where the position of the ground electrode becomes very important is when the electrodes are used to stimulate. While not the focus of this topic, keep in mind that the ground is also part of the stimulation pathway, and the shorter this path is, the less work the stimulator has to do, to deliver the selected current based stimulation pulse through the needle electrode.
In summary, try to keep electrodes in the general and immediate area of concern. There is no benefit locating the ground electrode too far away from the action.
Ancient notions of locating the ground electrode on a boney protuberance is just that. Electrode placement strategies should be in place to minimize moving surface electrodes for multiple injection procedures. Save time and improve overall signal quality.
Signal quality is further improved as Myoguide uses a method of active input guarding to further protect the signal from stray electrical noise, and movement artefact, that would otherwise reduce the quality of the EMG signal.
Myoguide’s input cable was designed with input guarding protection to further protect against common mode noise.
The process sends an active signal on the input cable’s braided shield to further protect the “high impedance” signal path from stray noise pick-up. Think about this as an active noise protection system for the very low EMG signal, BEFORE it has entered the amplifier. This protection is for the “high impedance” pathway, which is more subject to stray noise pick-up. Once the signal is amplified, it is far less subject to noise pick-up.
Input guarding can be considered a force field protecting the EMG signal from noise, before it can be amplified.
Unshielded cables, such as electrode wires, are noise antennas! This is one of the reasons we have advocated for shorter unshielded electrode leads. You are far better off taking advantage of existing stray noise protection already built into Myoguide, rather than supporting methods that generate more noise.
We have responded to calls for a longer Myoguide input cable (Intronix Model 8008-CT15), which is now available exclusively from the Myoguide Store. This new input cable takes advantage of Myoguide’s active shielding, for those who prefer more distance between the patient and the Myoguide device. We encourage clinicians to avoid long, unshielded, electrode leads, wherever possible.
The closer the electrodes are to Myoguide’s actively shielded cable the better the EMG signal quality!
Ultimately, tab electrodes clipped to Myoguide’s alligator adaptors are the shortest path. However, good quality leaded electrodes are fine, as well, providing you use the shortest leads possible.
What Does EMG Look Like?
We can look at EMG signals in both time and frequency domains to define bandwidth. The overall electrical signal varies according to muscle excitation.
Typical EMG signal is defined by muscle fiber population electrical activity picked up by surface and needle electrodes. The closer the electrodes are to the target, the larger the signal will be.
The more active the area is, the more fibers will be evoked, and the larger the signal and EMG audio will be.
The characteristic of muscle signals is defined by the capability of the muscle fibers to generate an electrical event.
Typical muscle signal characteristics span a bandwidth of 10-500 Hz. EMG signal strengths below 10 and above 500 Hz are very small.
One can say that measurements of activities outside of the peak range is considerably less descriptive, and although hearing anything requires quite a bit more amplification, there is less useful information here.
Myoguide has been designed to operate well within the EMG bandwidth, within the 10-700 Hz range. Myoguide’s amplifier has 9 gain settings, to ensure the best chance to hear proper EMG audio characteristics.
EMG signals are amplified through a high-quality audio amplifier capable of 20-700 Hz bandwidth for best audio alignment with the EMG signal characteristics. Myoguide employs a large waterproof speaker that also aligns with the EMG signal frequency range.
Silent procedures are supported with the ability to audio mute, should the need arise. We find that silent mode is quite handy for working with children and spasmodic dysphonia treatments. This is where having the Myoguide signal display becomes an essential part of your procedures. Audio mute is also useful between patient encounters, which saves battery life, while keeping Myoguide at the ready.
Myoguide can be used to improve patient outcomes by supporting improved injection accuracy.
Ordering direct fromThe Myoguide Store , provides you access to the best deals on Myoguide, an upgraded 2-year warranty, great prices on accessories and electrodes, and responsive support.