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Frequently Asked Questions |
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1) How does one verify the quality of an EMG signal? |
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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
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2) How does one prepare the skin prior to recording with Delsys EMG sensors? |
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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.
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3) How should Delsys EMG sensors be cleaned? |
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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. |
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4) What is the difference between the DE-2.1 Single Differential Sensor and the DE-3.1 Double Differential Sensor? |
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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 |
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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
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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).
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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.
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. |
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5) Is the DE-3.1 sensor available for the Myomonitor ™ wearable EMG systems? |
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No, the DE-3.1 sensor is not available for Myomonitor EMG systems at this time. |
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6) Can I connect an EMG sensor not manufactured by Delsys to an EMG system manufactured by Delsys? |
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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. |
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7) Can I use Delsys Sensors with amplifiers not manufactured by Delsys? |
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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. |
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8) Can Delsys EMG Sensors measure skin impedance? |
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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. |
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1) Can I use Delsys Equipment inside MRI machines? |
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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.
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2) Can I use Delsys systems on animals? |
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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.
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| 3) The Bagnoli ™ Main Amplifier turns on but no signal is received. What are the possible causes? |
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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.
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| 4) Can I use stimulation equipment while recording with Delsys EMG equipment? |
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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.
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| 5) I have a PC and my own A/D card. Can I use this with Delsys equipment? |
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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.
Click here for a listing of Delsys software comparable A/D cards.>>
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| 6) How can I integrate signals other than EMG into Myomonitor® devices? |
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There are three methods available for integrating additional external signals to Myomonitor recordings.
1) The preferred method is to use a Delsys Biosignal Sensor, than can connect directly to any available channel Myomonitor. Contact Delsys for custom modifications and/or custom sensors.
2) For situations where an external data acquisition system is available, the Myomonitor Analog Output System can be used to recreate the analog signals and connect these to the acquisition system. This option is typically chosen when the external system and software supports EMG signals, and other dedicated equipment such as motion capture cameras and force plates are needed in the setup.
3) For situations when an external data acquisition system is available, but it cannot support additional analog EMG signals, then the Myomonitor device can synchronize with this A/D card by use of input and output triggers.
Please refer to the technical note TN-301 for in-depth details on Myomonitor data integration and synchronization.
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| 7) The Myomonitor screen is on but the device never boots. What is the cause? |
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The most likely cause is a wrong power supply. It is important that you only use the power supply supplied with the Myomonitor, since any other power supply may damage the device. The Myomonitor power supply is manufactured by "Phihong" and is MODEL PSA31U-090. This number is clearly listed on the power supply label.
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| 8) How is a 'Cold Reset' performed on the Myomonitor IV device? |
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 | Depress and Hold the ‘ON/OFF’ button on the Myomonitor. While holding this button in a depressed state, use the stylus to depress the ‘RESET’ button for at least 2 seconds. Release the ‘RESET’ button, and continue to hold the ‘ON/OFF’ button until the crosshairs for a screen calibration appear. This will perform a complete reboot of the Myomonitor device. |
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| 1) Which A/D cards will function with EMGworks®? |
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Delsys is constantly seeking to expand its offerings for our clients, while maintaining the highest quality standards. At this time National Instruments is the preferred A/D card manufacturer. Several A/D card families are supported as described by the Data Acquisition document. |
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Each installation of the EMGworks must be registered and unlocked by Delsys Inc. thru the issuance a registration number. Without registering, EMGworks will stop working after the 30 day evaluation period expires. Delsys Inc. will generate a unique code to unlock EMGworks for each computer installation. If EMGworks is reinstalled on your computer, or the computer is updated or replaced, a new unique registration code must be issued to you. You must provide Delsys with the “Computer Key” and “Computer ID” listed on the registration page in order to generate a new registration code. Click here to register your copy of EMGworks >>.
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2) The "LINE INTERFERENCE" and "HI LED" indicators turn on in EMGworks. What is the cause? |
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If the EMG system specified in the Hardware Configuration has clipping and line interference outputs, these lights will illuminate in EMGworks when the signal is being clipped or there is line interference in the signal. If the EMG system specified in the Hardware Configuration does not have clipping and line interference outputs, these lights can be disabled by choosing Options from the Tools menu and un-checking the Display Clipping and Line errors check box in the Options dialog box. The control signals for the software indicators come from the Bagnoli 4/8/16 Main Amplifiers. 
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3) How can I read EMGworks raw data files in Matlab? What is the data file structure? |
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4) I have performed a calculation on my data and the resulting plot is blank. How can I display the traces? |
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This situation commonly occurs if the result of the calculation is only 1 datum point. It is not possible for the EMGworks graph tools to plot only 1 datum point, since at least 2 points are required to draw a line. The single datum point can be easily viewed by using the “gridview” feature in the “view “menu. Note that versions of EMGworks 3.6 and higher will display the X and Y coordinates of the datum point on the plot area.
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5) I see varieties of files in the EMGworks workspace. Please explain the various types and their purpose. |
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Data Acquisition File Structure
Data Acquisition TESTS consists of one or more SETS, each of which can be executed once, or repeated multiple times. The repetition of a SET is referred to as a “REP”.
- Files terminating in “*.emt” are the TEST files; these describe all the parameters associated with a TEST.
- Files terminating in “*.emg” are raw data files acquired during a single “REP” (i.e. repetition) of a SET.
The minimum number of files generated during data acquisition is one file for the TEST parameters (*.emt) and one file for the one repetition of a data SET (*.emg). Multiple repetitions of a SET result in multiple files terminating with *.emg.
Example: A TEST consisting of two SETs, each repeated twice results in the following files:
“TEST.emt”
“Set1[Rep1].emg”
“Set1[Rep2].emg”
“Set2[Rep1].emg”
“Set2[Rep2].emg”
Data Analysis File Structure
All data analysis is performed within the context of a WORKSPACE. The WORKSPACE is a concept used to track all active data files and plots used during an Analysis session.
- Files terminating in “*.ewp” are WORKSPACE files.
- Files terminating in “*.emc” are data sets resulting from performing an EMGworks calculation. These files can be generated from raw data (*.emg files) or from other calculation outputs (*.emc files).
- Files terminating in “*.emp” are plot files. Their function is to graph either *.emg files or *.emc files.
Example: A simple workspace that contains an imported TEST, with a single calculation and a single plot would consist of the following files:
“Workspace.ewp”
“Calculation.emc”
“Plot.emp”
Note that the raw TEST data terminates with *.emt and *.emg files. These files are needed by the WORKSPACE, but are generated during Data Acquisition and may not necessarily reside in the same location as the WORKSPACE files.
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6) The error message “Inappropriate input array size” appears when exporting data from EMGworks to Matlab. How is this resolved? |
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Mathworks has acknowledged this as a bug in MATLAB 7.0 (R14) and MATLAB 7.0.1 (R14SP1). Their recommendation is presented below:
There are several known issues with the COM interface shipped with MATLAB 7.0 (R14) and MATLAB 7.0.1 (R14SP1). See the problem descriptions below for more information.
Fixes for all of the problems listed below have been addressed in an updated version of comcli.dll. You should replace the current version in your MATLAB installation with this updated version. Here are the required steps:
- If you have not already done so, upgrade to MATLAB 7.0.1 (R14SP1). The revised version of comcli.dll will work only with this version of MATLAB.
- Quit MATLAB.
- Rename the following file: $MATLAB\bin\win32\comcli.dll (where $MATLAB is the MATLAB root directory on your machine) to $MATLAB\bin\win32\comcli.dll.old.
- Download the new file: comcli.dll and place it in the same directory mentioned in step 3.
- Restart MATLAB
- After restarting MATLAB, issue the following command at the MATLAB prompt:
rehash toolboxcache.
Note that if you use MATLAB Compiler to deploy applications that use the COM Interface, you will have to package the revised comcli.dll separately. On the target machine, the revised comcli.dll should replace the original in $MCR_ROOT\bin\win32.
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7) Myomonitor automatic network configuration does not work on my PC. What are the possible reasons? |
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There are three possible reasons for this:
- The network is already configured. In this situation, you already can establish connectivity and there is no need to configure the adapter again.
- The Wireless Zero Configuration service is stopped. (You may also see error 1059.) In this case, go to Control Panel, choose Administrative Tools, choose Services, right click Wireless Zero Configuration, and choose Start.
- Configuration is disabled on the wireless adapter. In this case, double click the adapter icon on the taskbar (
 ), click Change Advanced Settings, and navigate to the Wireless Networks tab. Make sure the item labeled "Use Windows to configure my wireless network settings" is checked.
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8) How can I test my hardware connections outside of EMGworks? |
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You can test the board using the Test Panels in National Instruments Measurement and Automation (MAX).
- Open MAX. In the configuration tree, select your A/D card (under NI-DAQmx devices). Click the "Test Panels..." button.
- On the Analog Input tab, set the channel name to DevX/ai0, set the mode to On Demand, set the input configuration to RSE, and uncheck Auto-scale chart. Connect an EMG sensor to channel 1 on the amplifier and set the gain knob to 1k.
- Press Start. At this point you should see the data responding if you tap one of the silver bars with your finger. The signal should be flat when the bars are shorted.
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9) I have a data set in which some channels are EMG data and some channels are goniometer data. I would like to perform a RMS on the EMG data and leave the goniometer data untouched. |
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To do this, you can expand the data set in the workspace tree so that you can see the channels within it. Hold down Ctrl and select the channels on which you would like to perform a RMS by clicking them, then right click and choose Calculations->RMS. In the RMS window, the Applies To section will list these channels.
To plot the processed channels in conjunction with the unprocessed channels, expand the RMS calculation as well. Then Ctrl-click to select the processed channels from one data set and the unprocessed channels from the original data set. You can then right click and choose Plot. (This operation can also be done within the Plot->Create Plot wizard.) |
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10) I want to export my EMG data to Microsoft Excel, but Excel can only handle 65536 rows of data at a time. |
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Use the Subset calculation to split your data into smaller data series. Right click on the file in the EMGworks Analysis tree and choose Calculations->Subset.
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11) Why does the data sampling frequency change when I do a windowed calculation? |
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Given the subset of data within one window, the calculation will produce a single data point (for example, the RMS of the data in this window for a windowed RMS). This is repeated for each window within the data set. This reduction of data points means that the sampling frequency changes.
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12) How can I save my .emg or .emc data as a text file? |
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Select your data or calculation result in the workspace tree, then choose Tools->Export to CSV File. This will produce a comma-delimited text file which can be read by Excel and many other programs (including Notepad or other text editors).
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