EMG Standards of instrumentation

Introduction

Standardization of Electromyography (EMG) instrumentation is of particular importance to ensure high quality recordings.  Standardisation of instrumentation and recording will enable comparison of Electromyography (EMG) and nerve conduction studies (NCS) results within and between laboratories.  Such standards and guidelines will allow a uniform practice and improve the selection of patients for research studies.

Instrumentation

Contemporary EMG machines have a dedicated hardware unit with amplifiers, stimulators, and control panel. and a separate computer.  In the EMG hardware, the amplifiers and stimulators are the most important parts.  An important aspect of any machine is its ergonomics and control panel functions, including the foot switch and hand controls.  When setting up a machine, it should be tuned optimally, preferably by a team including representatives from the users (physicians, technicians, local engineers) and manufacturers.

When considering which machine to purchase, these details must be considered, since they influence medical quality, operability, and in the long term, cost efficiency.

Computer hardware:

Analysis software:

The primary function of the electrodiagnostic system is to faithfully record and analyze various biological signals.  It is important to have an optimal ‘signal to noise ratio’, i.e. amplify the neurophysiological signal voltage while attenuating background noise.  This is done using analog hardware and digital signal processing techniques.  The signal and noise are recorded by surface or needle electrodes.  It is carried to the amplifier input via electrode leads or cable.  These components behave like an antenna and may add more noise.  A differential amplifier magnifies the signal while attenuating the unwanted noise, aided by analog filters.  The amplified signal is measured using an analog-to-digital converter (ADC) and the voltage values are stored as an array of numbers.  This digitized signal allows further computerized analysis.  Some algorithms reduce the noise, e.g., digital filters, averaging, smoothing, etc., and others make measurements such as latency, amplitude, and area in NCS.  More sophisticated algorithms can detect MUPs in needle EMG.  Signal characteristics are also assessed from the signal sound, generated either with analog hardware or using digital technology.  

An EMG machine also offers stimulation devices to excite nerves and muscles.  These may generate electrical, visual, or auditory stimuli.  External devices providing other forms of stimulation, e.g. magnetic field, contact heat, reflex hammer, etc. can be interfaced to provide timing signals through so-called ‘triggers’. To achieve this, some instruments pass the digitized signals through a ‘digital-to-analog converter (DAC)’, to convert digital signals (with much less noise) into analog form.  This can be used for research where the investigator wants to re-sample the signals and develop algorithms for their own analysis.

The EMG machine displays signals, measurements, and the settings of the amplifier and stimulator. The latter can be changed using a dedicated control panel or using software commands via a mouse or computer keyboard.  Data collection can be initiated using the foot switch.  The software is responsible for signal processing and for generating reports.  Databases can be created, and remote reviews used for second opinions or to help with interpretation.  Constant changes in operating systems and in regulations governing patient information protection can make this a challenging task.  The electrodiagnostic systems can also record non-neurophysiological events.  Temperature, for instance, should be measured and recorded during NCS.  A ‘patient response’ unit may be used to record the number of times the test subject acknowledges different types of stimuli in cognitive function assessments.  Video cameras may be integrated into the system to observe the patient’s behavior during a study, e.g., focussing on the checkerboard pattern in a visual evoked potential investigation.  Recently we have seen the addition of ultrasound imaging probes to the device.  The handling of these inputs is very different from that of the neurophysiological potentials, and is outside the scope of the current discussion.

Digital instrumentation

EMG equipment uses digital computers for data sampling, storage and signal processing.  After the analog signal has been amplified, the analog-to-digital (AD) converter discretizes the signal in both time and amplitude, and assigns a digital value to the amplitude at defined time points.  This assignment of the amplitude to a digital value is performed by using a finite number of digital amplitude values.  In this conversion process, two important criteria must be satisfied.  First, the sampling frequency should be sufficiently high to reliably represent the original analog signal.  Second, the digitization of the amplitude should be sufficiently fine to accurately represent the amplitude of the original signal in the digital domain.  The digitizing an analogue signal, consisting of a sine wave with frequency 1 Hz and amplitude 1 mV using different sampling rates and AD converters.  The phenomenon is known as ‘‘aliasing”: if the sampling frequency, fs, is lower than the highest frequency present in the signal of interest, fmax, the frequency obtained has an erroneous value, in this example 0.1 Hz.  Using appropriate  algorithms, it is possible to reconstruct the waveform in detail if the sampling rate is more than twice the highest-frequency component of the waveform (Nyquist theorem).  In practice, the sampling frequency used is typically 2–5 times the highest frequency component in the signal of interest.   In order to  guarantee that the maximum frequency in the signal is known, an analog ‘‘anti-aliasing” filter is used before the signal is digitized. 

Amplifiers

The amplifier is perhaps the most critical component for the quality of the electrodiagnostic system.  Selective amplification of a neurophysiological potential while attenuating background noise can be accomplished using a ‘differential’ amplifier (DA).  DA requires inputs or connections from three electrodes.  In past the electrodes were called ‘G1 , ‘G2 and ‘ground’.  The terms G1 and G2 refer to the grids of vacuum tubes used in old amplifiers which are no longer available. Later these inputs were called ‘active’, ‘reference’ and ‘ground’.  The term ‘ground’ is confusing.  The ‘ground’ in electrodiagnostic recording refers to a point on the amplifier circuit that is used as a point of reference for voltage measurement.  Outside electrodiagnostics, it is also used to describe one of the connections in the power supply and wall outlets.  The ‘reference’ electrode is presumed to be electrically silent but does record large volume-conducted potentials, such as the electrocardiogram (ECG).  Given the confusion of terms and their origins, the terms ‘E1 , ‘E2 and ‘E0 are recommended for the three connections to the amplifier.  On most systems these inputs are colour-coded as black (E1), red (E2) and green (E0).

The amplifier does not amplify the voltage at E1 or E2 inputs.  It magnifies their difference, and hence it is called a differential amplifier.  E1 electrode (a monopolar needle) recording a 50 uV fibrillation potential.  The E2 is a surface electrode placed on the skin surface and for simplicity it is assumed that it records no electrical activity, i.e. 0 uV.  Their difference is amplified and the fibrillation potential is seen as a 50,000 uV signal.  This amplification by 1,000 is the ‘differential’ gain of the amplifier.  The ambient noise is also recorded by both electrodes and their cables.  Here the ‘common’ noise is 1,000 uV, but the difference between signals at E1 and E2 is zero, and the noise will not be seen at the amplifier output.  Similarly, the very large ECG potential can be eliminated by differential amplification.  This enables the selective amplification of the small neurophysiologic signal in the presence of high-amplitude noise.  The example reflects an ‘ideal’ DA.  In practice the ‘common signal’ at E1 and E2 inputs is also amplified, but much less.   The noise voltage at the output is 500 uV.  The ratio of output to input noise voltage, produces a ‘common mode gain’ of 0.5 for the amplifier.  In this example, the signal-to-noise ratio at the amplifier input is 0.05 and would make it difficult to recognize the fibrillation potential.  However, at the output of the amplifier, the signal-to-noise ratio is 100, and this would allow it to be recognized quite easily.   A DA should have a high differential gain and low common mode gain.  These properties are defined in a single characteristic called the common mode rejection ratio (CMRR).  It is reported in units of decibels and calculated as CMRR (dB) = 20  x Log (Differential Gain/Common mode gain.

In our schematic amplifier, the CMRR is 66 dB.  Modern electrodiagnostic systems have amplifiers with CMRR exceeding 100 dB.  It is important to note that the CMRR decreases at higher frequencies.  Most vendors specify the value at 50 or 60 Hz (i.e., power line frequency).  

Another characteristic of the amplifier is the input voltage range.  As example, if the range is 50 to + 50 uV, then signals with amplitudes between these ranges can be handled without distortion.  If the signal amplitude is outside the range, it will saturate the amplifier and the true signal amplitude cannot be measured. This is recognized from the ‘clipped’ peaks of the signals on the display.  So, the amplifier range should be set higher than the amplitude of signals recorded in the test.  In sensory NCS, the stimulus artifact can be much bigger than the nerve response.  If the amplifier saturates, it can generate a long-duration artifact that interferes with the sensory potential to be recorded.  The range is controlled in the software to avoid such distortion.  The electronic components of the amplifier also produce some noise This is addressed by the E1, E2, and E0 electrodes being connected together and the amplifier output measured.  The peak-peak amplitude or root mean square (RMS) value of the signal is reported. RMS is usually less than 1 uV, but also depends on the amplifier range and the filter settings.  A high amplifier range (i.e. low gain setting) and short band width will give lower noise.  The amplifier is also characterized by its ‘input impedance’.  As the amplifier needs a tiny amount of current to measure a voltage, the input impedance needs to be several orders of magnitude larger than the input impedance of the generator of the voltage, i.e. muscle, nerve, and body fluids.  Essentially, the voltage measured (Vm) is given by: Vm = Vsource*(Ropamp)/(Rsource + Ropamp).  With Rsource the ‘internal impedance’ is meant.  If Ropamp = Rsource, then Vm = 0.5*Vsource.  Without going into details of circuit analysis, the amplifier impedance should be high.  Low impedance makes the system more sensitive to environmental noise.  It may also underestimate signal amplitude.  Fortunately, modern systems report impedances in excess of 100 to 1000 mega-Ohms. Just like CMRR, the impedance decreases at higher frequencies.

Modern systems offer ‘switching amplifiers’. The unit provides a ‘head box’ with many input connections. The user can select the inputs in software to select any pair of inputs to make a recording.  This facility is used mainly for evoked potential studies where a small set of electrodes is used to create multiple channels of recordings.  These channels usually have a much lower CMRR.  Such channels may not be suitable for recording signals with high frequencies (e.g., needle EMG) or when the electrodes differ in their impedances (surface versus needle).  The best strategy for high quality recordings is to reduce the ambient noise and to ensure that the noise is not different on E1 and E2 electrodes.

Filters

Ideally, our measurement system should reproduce the signal of interest as exactly as possible while rejecting undesired signals.  In clinical practice, however, we typically obtain a mixture of signals and ‘noise’, where the latter refers to any signal that does not contain relevant information for our diagnostic procedure. The undesired signal components can result from ambient power line noise (50 Hz or 60 Hz) or movement artifacts, but can also include EMG signals in a frequency range that is outside the region of interest for a particular procedure.  For example, in recording single-fiber action potentials, the signal of interest is in the frequency range 0.5–5 kHz, where lower-frequency components (such as distant EMG potentials) can be safely suppressed.  While filtering refers to any process where irrelevant signals are suppressed, in most clinical situations filtering is limited to attenuation of particular frequency components in the signal. The name of a filter is then defined by the frequency values that the filter attenuates (low or high frequency filters) or passes (high-pass filter, bandpass filter).  The amount of attenuation depends on the ‘‘steepness” of the filter (expressed as the attenuation in dB/octave) and the cut-off frequency.  The cut-off frequency is defined as the frequency where the original signal’s amplitude is attenuated by 3 dB.  In today’s equipment, all filters (except for the anti-aliasing filter) are digital, which also allow filtering of frequency components with minimal phase distortion of the signals.  

Low-frequency filters (LFF; high pass filters):  Low-frequency (or, synonymously, high-pass) filters attenuate low-frequency components in the signal.  An increase in the low-frequency cut-off causes initial amplitude loss of slowly changing signals, waveform distortion, but more importantly it also decreases the latency to the peak of the waveform and can introduce artifacts (i.e. a tail of the motor unit action potential).  When recording motor unit potentials (MUP), the duration as well as the amplitude decreases when the cut-off is increased up to 500 Hz.  Using a 500 Hz cut-off the contribution from distant muscle fibres is  attenuated because of the soft tissue itself acts as a high-frequency filter.   Ideally, the low limiting frequency should be one decade (factor 10) lower than the lowest frequency of the signal, to make sure that a phase shift, if present at all, does not affect latencies.  Movement artifacts contain slow frequencies.  In some cases, the only way to remove this artifact is to increase the lower limiting frequency.  This is commonly done for surface EMG recording of movements (e.g., tremor recordings; gait) and may be necessary for motor evoked potential recordings to transcranial magnetic stimulation (TMS).  It is usually required if EMG traces are rectified before averaging, e.g., for averaged F waves.

High-frequency filters (HFF; low pass filters).  High-frequency filters attenuate high frequencies.  A decrease in the high-frequency cut-off reduces the amplitude and rise time.  If using a high-frequency cut-off that is too low, the system will not be able to record adequately the rise of the potential (containing the highest frequencies of the signal), and this may lower the amplitude, reduce the number of phases, and prolong the duration of the main peak component of the signal.  An inappropriate HFF can also affect the measurement of onset latency of a potential.  It will be prolonged because the abrupt decline from the baseline is missed and more time elapses before the beginning of the potential can be appreciated.

Band-pass filters:  Most filters used are band-pass filters, a combination of a high and low-frequency filter.

Notch filter:  A notch filter is a special type of band-stop filter.  In electrophysiology it is normally designed to reduce power line interference (50 Hz or 60 Hz). Ideally, it should not be used because most neurophysiological signals contain significant components at this frequency, and their use may hide interesting components.  In addition, the phase changes abruptly back and forth around the notch frequency, and this may distort the waveform. 

Signal averaging and noise reduction

The widespread adoption of signal averaging in the 1970s has greatly enhanced the precision of NCS, particularly those on sensory nerves, where the signal-to-noise ratio is lower than during motor conduction studies.  In clinical practice, signal averaging is indicated whenever there is a low signal-to-noise ratio, i.e., when the background ‘‘noise” obscures the potential to be recorded or the latencies of that potential.  In these recordings, the signal may be so small that the noise inherent in the recording obscures the potential.  Sensory nerve action potentials (SNAPs) may be recorded with surface electrodes (or near-nerve needle electrodes) but with both techniques the SNAP is often difficult to define in single sweeps; somatosensory evoked potentials (SSEPs) are always so.  Signal averaging is recommended routinely, even when the potential can be visualised readily, because it is critical that onset latency be defined accurately.  

Safety in NCS and EMG

Surface electrodes 

Surface electrodes are used as stimulating or recording electrodes for NCS, for recording surface EMG data, for serving as reference electrodes for monopolar needle EMG, and as a common reference or ‘‘ground” electrode.  In the past, surface electrodes generally consisted of small round or square reusable metal disks or metallic wire loops (the last for use in measuring sensory potentials from the digits), all employed in conjunction with a conductive electrode gel. However, to reduce the risk of infection and for reasons of convenience, this approach has been increasingly replaced with the use of self-adhesive, disposable electrodes.  These are generally silver-silver chloride electrodes, with an adhesive conductive gel overlying the electrode surface; they can often be used several times on a single patient before they need to be replaced, generally because the adhesive loses its efficacy.  Reusable, saline-saturated Velcro fabric band electrodes are also used to record sensory potentials from the digits.  For surface recordings of electrical impedance myography, both reusable metal and disposable carbon-based electrodes have been used.  Electrical impedance myography has been discussed in detail in a separate IFCN guidelines ‘‘Standards for Quantification of EMG and Neurography” (Stålberg et al., 2019).

A variety of surface stimulating electrodes are used for nerve conduction studies.  Most commonly these include metal ‘‘prong” electrodes used with a small amount of adhesive gel or saline soaked felt electrode pads or pledgets.  In either case, the electrodes are embedded within a handheld stimulator that can be easily repositioned to help identify the best region for stimulation.  Small metal disk or adhesive electrodes, described above for recording, can also be used for stimulation, especially if repeated recordings from a single nerve are desired over an extended period of time.  This is the case, for example, when performing studies of nerve excitability (including measurements of strength-duration time constant, threshold electrotonus and recovery cycle).  The details of excitability techniques are beyond the scope of the present recommendations and have been discussed in detail in a separate IFCN guidelines, ‘‘Measurement of axonal excitability”.

Although surface electrodes are widely used for NCS, conventional surface EMG methods consisting one bipolar signal from two electrodes cannot provide detailed information about motor unit morphology, including their distribution across the muscle endplate.  Therefore, special multi-channel high-density surface EMG (HD-sEMG) techniques have been developed.  HD-sEMG requires sophisticated instrumentation and signal analysis of measures from multiple closely spaced electrodes, overlying a restricted area of the muscle.  This enables measurement of both temporal and spatial EMG activity, thus providing different aspects of motor unit characteristics, such as muscle fiber conduction velocity measurements, motor unit number estimation and the evaluation of single motor unit characteristics.  HD-sEMG has been shown to be beneficial for the assessment of different clinical conditions including motor neuron disorders (Wood et al., 2001; Drost et al., 2004) and disorders (Huppertz et al., 1997) and pathological changes have been shown at the motor unit level in neurogenic and myopathic muscles.  However, HD-sEMG has not yet been incorporated into clinical practice as a diagnostic tool.  Recent research has also mostly focused on physiological studies (Sleutjes et al., 2018; Lapatki et al., 2019), sports medicine (Martinez-Valdes et al., 2017) and rehabilitation (Gallina et al., 2016) rather than the primary diagnosis of neuromuscular disorders.  Detection of the HD-sEMG signals requires high quality amplifiers with suitable specifications.  Several kinds of amplification chains have been described for different strategies.  As HDsEMG remains a research method, further details of these techniques are beyond the scope of the present report.  

Stimulation

For nerve conduction studies, repetitive nerve stimulation test, stimulated single fibre EMG, and nerve excitability testing, an electrical stimulus is delivered to the nerve.  Standard stimulation techniques use surface electrodes, whereas needle electrodes are used for nerve root stimulation or deep nerve stimulation (e.g., the sciatic nerve).  Usually two surface electrodes are placed over the nerve with an inter-electrode distance of 2–4 cm with the cathode (where the nerve is stimulated and membrane depolarisation occurs) placed closest to the recording electrode.  Inversion of the polarity of the stimulation electrode will affect the point of stimulation, and thereby the onset latency and nerve conduction velocity. With deep nerves, a short interelectrode distance should be avoided because a greater stimulus is required the deeper the nerve and, it is then more painful.  This may be an issue when stimulating the radial nerve in the spiral groove or the femoral nerve.  The duration of the stimulus is usually between 0.1 ms and 1.0 ms.  The activation time of axons varies with the duration of the stimulus, even when it is supramaximal, and the measured latency includes this time.  In routine nerve conduction studies, short-duration stimulus pulses are therefore preferred, and this also (1) minimizes patient discomfort, (2) restricts the site of stimulation which may spread with longer duration pulses, as well as high-intensity pulses, and (3) reduces the stimulus artifact.  In Hreflex studies, a longer stimulus duration (1.0 ms) is used to favor the activation of the large sensory fibres (Ia afferents) responsible for the reflex.  The stimulus can be applied as single pulses, paired pulses, or as trains with repetition frequencies between 1–2/s and 50/s for repetitive nerve stimulation test.  High-frequency stimulation (20–30 Hz for 1–2 s) has been used for detection of incremental responses in Lambert-Eaton myasthenic syndrome.  More recently a single shock before and after a brief maximum voluntary contraction is preferred to demonstrate increment because this reduces discomfort and is equally effective.  Two types of stimulators are used.  The constant voltage stimulator delivers a constant voltage (0 V to 400 V).  The disadvantage of this type is that the resultant current is not constant because of the fluctuations of the electrode-tissue impedance.  Constant current stimulators deliver a constant and stable current (0 mA–100 mA).  Stimulus current is independent of electrode/skin impedance as long as the stimulator is not overloaded.  Constant current stimulation is therefore preferred by most physicians.  Supramaximal stimulation is required in standard nerve conduction studies and repetitive nerve stimulation test to activate all axons at the same time for measurement of motor and sensory response amplitudes, and nerve conduction velocity.  Particularly when stimulating at proximal sites (e.g., Erb’s point) or in nerves with very high threshold due to pathology (demyelination, nerve hypertrophy), it may be difficult to reach a supramaximal level. The most proximal parts of peripheral nerves can be activated by TMS or high-voltage electrical stimulation over the spine, providing additional information to that from peripheral NCS  (Matsumoto et al., 2010). Conduction across proximal segments of peripheral nerves, plexuses and nerve roots is commonly tested using H-reflexes and F-waves.  An alternative technique using magnetic stimulation or high-voltage electric stimulation involves direct stimulation of the nerve roots, and this may be preferable.  Conversely too-high stimulus currents will result in both distal displacement of the stimulation site, leading to a shorter onset latency, and stimulation of nearby nerves. Avoiding a stimulus that is too high is important to ensure the most information with the least discomfort or pain.  However, this must be tempered by the fact that erroneous reports of conduction block occur when the stimulus was not really supramaximal, particularly when stimulating at proximal sites.  If a supramaximal stimulus causes discomfort that is regrettable but preferable to an erroneous conclusion.

Needles

The concentric needle consists of two electrodes, the first electrode is a wire electrode, typically platinum that is insulated and housed within a steel cannula acting as the second electrode.  The surface area of the wire electrode depends on the wire diameter and the bevel angle of the needle and is usually between 0.01 and 0.09 mm2, typically 0.07 mm2.  The differential recording is then achieved by measuring the voltage between the wire electrode (active electrode (E1)) and the entire cannula shaft (reference electrode (E2)).  The main spike component of the MUP is generated by approximately 2–12 fibres within a radius of about 0.5–1 mm around the tip of the needle.  More distant fibers contribute to the initial and late parts of the potential.  Due to the short distance between the electrodes, a great common mode voltage recorded by the active and reference electrodes is present leading to the elimination of much distant activity and providing a relatively sharp and self-contained MUP.

Monopolar needles for EMG recordings are usually constructed from a stainless-steel core that is coated with Teflon except for an exposed cone tip of 1–5 mm that acts as the active electrode.  The recording area is approximately 0.03–0.34 mm2.  The potential difference is measured between the exposed tip of the needle and a second reference electrode.  This reference electrode may be a needle placed subcutaneously or a surface electrode at some distance from the active electrode.  The reference electrode should be placed over an electrically silent area, such as a tendon or a bone.  Impedance mismatch between the active monopolar needle and a surface electrode can lead to a reduced common-mode signal and greater artifacts, including power line interference (50 or 60 Hz). Monopolar needles record larger amplitudes and greater duration than concentric needles, but the number of phases is comparable.  Monopolar needles can also be used as recording electrodes for sensory nerve action potentials in the near-nerve technique and as stimulation electrodes (e.g., stimulated single fiber EMG) (Kouyoumdjian and Stålberg, 2008).  In the former situation, the reference electrode is also usually a monopolar needle placed subcutaneously at a distance from the nerve.  Hollow core monopolar needles are also used for botulinum toxin injection, assisting with correct muscle localization prior to injection.

Single fiber EMG electrodes have an active region consisting of a platinum wire approximately 25 mm in diameter exposed on a side port of a steel cannula, with the cannula itself serving as the reference lead (similar to a concentric needle).  Within the pick-up range (a semicircle with a radius of 300 mm, pick-up area 0.0005 mm2) in healthy muscles there are usually no more than 2–3 fibers, allowing for pairs of fibers to be easily obtained.  Concentric needles can also be used to collect ‘‘single-fiber-like” data; this is achieved by increasing the cutoff frequency of the high pass filter so as to help distinguish individual spikes within the MUP.

Important Technical Factors Influencing NCS and EMG

Physiologic factors: 

Non-physiologic factors:

NCS and temperature

Age: 


Height:


Filters:

Impedance

Common mode rejection

Measures to reduce stimulus/shock artifact

How to reduce electrode impedance mismatch and 60 Hz interference

Gain/sensitivity and Sweep speed

Amplitude measurement during needle EMG

Duration of waveform measurement during needle EMG

Frequency of waveform measurement during needle EMG

Simple or polyphasic

Stable or unstable

Machine Settings

NCS: 

Motor study: 

Sensory study:

Needle EMG

Artifacts

It is important to consider a number of procedures to reduce the impact of artefact on the quality of the recording.