Frequently Asked Questions

EMG Sensors

The most effective way to verify the quality of an EMG signal is to first establish the noise baseline of the system.  Delsys EMG systems exhibit 5uV pk-pk noise baseline with the sensor inputs connected to Reference.  Once affixed to the skin, the noise baseline is between 5 and 10 uV pk-pk, depending on the impedance characteristics of the skin. This low baseline is observable only when the skin has been carefully cleaned and the muscle is completely relaxed.  A soon as the muscle fibers underneath the EMG sensor become activated, individual action potentials can be discerned, appearing at amplitudes as low as 20uV and as high as 2mV.  By maintaining a constant contraction level, an estimate of the EMG signal amplitude can be obtained.  A signal-to-noise ratio can be computed by calculating the Root-Mean Square (RMS) of the detected signal and dividing it by the RMS noise baseline of the system.  The formal signal-to-noise measurement is expressed in decibels, and is calculated as the logarithmic ratio of the RMS signal amplitude to the RMS noise amplitude.  The full scale signal-to-noise ratio of our EMG equipment is calculated as follows:

  • SNR = 20log(10mV/5uV)
  • SNR = 65dB

SNR-1

Quality EMG signals can only be obtained with proper application of the sensors. EMG signals originate from the movement of very small charged ions in the muscle cell membranes. The skin barrier poses an impediment to the detection of these electric fields from its surface. The impact of the skin in attenuating and possibly distorting these signals can be minimized by ensuring that it is free from extraneous matter which can include hair, oils and dry dermis.

  1. Remove excessive hair that may occlude a muscle site.
  2. A brisk wipe using an alcohol swab is effective in removing surface oils and other contaminants. The use of excessive amounts of alcohol will be detrimental as this will cause the skin to dry.
  3. If dry skin cells are causing difficulties, these can be easily dislodged by dabbing the surface with medical grade tape. Dry skin cells will adhere to the tape and be dislodged when the tape is removed. Ensure that no adhesive residue remains on the skin by wiping the areas with an alcohol swab.
  4. In cases when skin surface is persistently dry, a very small amount of ionic soap or saline solution can be used to line the EMG sensor contacts. This solution will add electrolytes to the sensor-skin interface, facilitating the necessary ionic exchanges that must occur. Excessive amounts of ionic soap will be detrimental as this could short out the sensor inputs or interfere with the adhesive features of the sensor interfaces. Liquinox (R) brand name soap is an example of an ionic surfactant that can be used for this purpose. This hospital grade detergent should be diluted with a ratio of 50 to 1.
  5. Proper sensor application requires the use of the Delsys Sensor Interface which is specifically designed to promote strong skin adhesion, to minimize movement artifacts, and to manage the build-up of surface sweat over the coarse of long-duration and/or vigorous activities. Using sensor attachments or approaches other than this one will most likely result in decreased signal quality.

Delsys sensors can be cleaned with a isopropyl alcohol or a mild detergent solution. Gas sterilization methods are also acceptable. Sensors should not be fully submersed in liquids for any period of time. It is recommended to clean Delsys Sensor before and after each use.

The standard single differential sensor has 2 muscle site contacts, 1mm in diameter and separated by 10 mm. This sensor performs a differential measurement of the voltages appearing under the contacts as determined by the reference potential (at the Reference Electrode). This type of measurement is shown in Figure 1 below, where the output of the sensor is the difference between the voltages at contact “V1” and contact “V2”.

DE-2.1 EMG Sensor
DE-2.1 EMG Sensor

SD Amplitude

Figure 1: The DE-2.1 Differential EMG sensor performance a subtraction of the voltages detected by the 2 contact bars.

The Double Differential Sensor contains three contacts, each separated by 10 mm. The sensor performs a two-stage subtraction: the first stage establishes the voltage between contact “V1” and contact “V2” as well as the voltage between contact “V2” and contact “V3” (Figure 2). The second stage then performs the subtraction between these differences.

DE-3.1 EMG Sensor DE-3.1 EMG Sensor

DD Amplitude

Figure 2: The DE-3.1 Double Differential Sensor performs a two stage subtraction: the first stage establishes the differential voltages at the input, the second stage removes those components of the signals that are common.

The second differential subtraction will remove those signals which are common to all sensor contacts, while propagating those signals that exhibit potential differences across the contacts. EMG signals originating from muscles that are not immediately below the surface of the skin will have a larger latency than those immediately below the surface. These will appear in a similar fashion to all bars, and will thus be subtracted from the final sensor measurement (Figure 3).

DE-3.1 contact illustration

DE-3.1 and DE-2.1 comparisson for crosstalk

Figure 3 : Removing EMG crosstalk- the signals originating from deep muscles, depicted by location “C” disperse as they travel to the skin surface and are detected by all sensor contacts. The signals originating from the fibers immediately below the skin surface (depicted by locations “A” and “B”), are only detected by the contact-pair directly above. The signal components originating from location “C” are common to all bars, and are removed in the double differential subtraction, while those components from locations “A” and “B ” are preserved. The figure on the right side demonstrates the effectiveness of the DE-3.1 sensor in removing EMG crosstalk from flexor and extensor activity.

The DE-3.1 sensor is recommended for those situations where EMG muscle crosstalk is considered to be problematic. Note that care must be taken to ensure proper and well-balanced skin contact with the sensor. In general, managing the two-point contact of the DE-2.1 sensor over curved surfaces is more natural than the three-point contact of the DE-3.1 (see Figure 4). Note also that the effective contact area of the DE-2.1 sensor is 10x10mm, while the effective contact area of the DE-3.1 sensor is 10x20mm. For those situation where EMG crosstalk is not a significant consideration, the user may benefit from the user-friendly characteristics of the DE-2.1 sensor.

DE-2.1 Muscle Site

DE-3.1 Muscle Site

Figure 4: Care must be taken when applying the DE-3.1 sensor to ensure that all three sensor inputs maintain event distributed contact on the skin throughout the recording.

No, the DE-3.1 sensor is not available for Myomonitor EMG systems at this time.

No, Delsys EMG systems are designed to exclusively use Delsys sensors. Connecting any other sensor to Delsys equipment constitutes a violation of the warranty, may damage the equipment, and may result in bodily injury or harm.

No, Delsys sensors are designed to detect minuscule electrical currents appearing on the surface of the skin as a result of muscle contractions. The measurement of skin impedance requires the injection of a known current on the skin, so that the response can be characterized. Delsys electrodes are not designed to inject current in to the skin.

Skin impedance measurements are very difficult to make, and have a very high variability. Once known, this value alone carries little significance. The more informative parameter used to gauge EMG signal quality is the Signal-to-Noise Ratio. Delsys EMG systems have a noise baseline of 5uV (pk-pk). This extremely low noise baseline allows individual motor unit action potentials to be identified from minimal contractions when sensors are properly placed on the skin.