Frequently Asked Questions
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
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.
- Remove excessive hair that may occlude a muscle site.
- 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.
- 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.
- 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.
- 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.
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.
No. All Delsys EMG Systems are classified as medical devices, and as such must conform to rigorous medical standards. Delsys sensors are critical components of these EMG Systems. Separating the EMG sensor from the rest of the System constitutes a violation of the Quality Assurance procedures established to ensure that compliance is maintained. Safety requirements such as isolation voltages and leakage currents, as well as signal quality performance specifications cannot be guaranteed by Delsys when the sensors are operated in unknown environments, with untested systems.
Delsys EMG systems are not approved for use in MRI machines. MRI machines make use of very strong magnetic fields that may induce unwanted currents in metallic objects they traverse. The sensitive electronic components used in Delsys EMG systems will be highly susceptive to these magnetic fields. More importantly, these unwanted current could result in patient harm or equipment damage. In general, metallic objects inside the MRI chamber will cause distortions of the resulting images. All conductive materials will be subject to these deleterious effects, some more than others. Ferrous objects exasperate these difficulties since these will also generate real magnetic forces when subjected to magnetic fields. Any research that introduces metallic objects in the MRI chamber should discuss the application with the MRI manufacture, since subtle safety and performance matters must be considered.
Yes, many studies and applications have successfully employed Delsys EMG sensors and systems on animals. Be sure correctly identify appropriate muscle sites and remove all fur/hair prior to applying the sensors. Wipe the site carefully with isopropyl alcohol to remove all oils and dry dermis. Apply the sensor with the arrow on top parallel to the muscle fibers of interest, and use the Delsys Adhesive Interface to affix the sensor to the skin. In highly dynamic situation it may be necessary to wrap the EMG sensor with additional tape or elastic bandages.
The most likely cause is a problem with the Input Module cable. Check the cable carefully, making certain that it is not damaged or excessively twisted. Additionally check the cable connectors to ensure that the contacts and connector shell are not damaged. If the connector is forced to mate upside-down, the shell will become damaged and protective fuses in the Main Amplifier will expire. In this case the cable will need replacement and the Bagnoli Main Amplifier will need to be returned to Delsys for repair. The bottom connector shows a deformed shell, indicated by the yellow arrows. This damage is attributed to a forced connection with upside-down orientation. Contrast this to the undamaged top connector shell.
Delsys devices are not designed to be used with stimulation devices.
The fundamental performance issue is the amplitude of the detected stimulus as recorded by the DE-2.1 sensor. An amplitude at the recording site that exceeds +/-5mV will saturate the system. This may cause the artifact tail to extend for some time before the amplifiers recover. If the latency of your evoked response is short, the information of interest may be masked in the artifact tail.
The sensors are powered with +/-5V. An input signal below this value may cause saturation but should not cause damage. Inputs above +/-5V should be presented with caution, as voltages exceeding this range may damage sensitive components.
The true amplitude of the detected artifact is generally unknown apriori, since the stimulus undergoes a transformation as it traverses the body as a volume-conducted wave. This artifact is often quite large and may mask the evoked signal of interest. Latency between artifact and evoked response is generally maximized so that the effect of the artifact tail on the signal is diminished.
All Delsys products that produce an analog signal output can be connected to any A/D card that can support +/-5V analog signals and is controlled by appropriate software.
However, when using Delsys software (EMGworks), it is only possible to use the specific A/D cards that have been tested and approved. Please refer to the software section of our web site for a listing of current compatible A/D cards.