Frequently Asked Questions
How does one prepare the skin prior to recording with Delsys EMG sensors?
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.
How should Delsys EMG sensors be cleaned?
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.
How can you improve the quality of EMG signals?
View Signal Quality Monitor.
What factors affect EMG Signal Quality?
View Signal Quality Monitor.
How does one verify the quality of an EMG signal?
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
How reliable is the RF communication?
The antenna design, the communication protocol and the radio hardware have been optimized for trouble-free operation up to 40m in open office environments.
Communication Test:
16 sensors at 20 m for 10 minutes
| Sensor | Dropped Packets | Sesnor | Dropped Packets | Sensor | Dropped Packets | Sensor | Dropped Packets |
|---|---|---|---|---|---|---|---|
| 1 | 0 | 5 | 0 | 9 | 1 | 13 | 0 |
| 2 | 0 | 6 | 0 | 10 | 0 | 14 | 0 |
| 3 | 0 | 7 | 0 | 11 | 0 | 15 | 0 |
| 4 | 0 | 8 | 0 | 12 | 0 | 16 | 0 |
Total Packets Sent: 711,112
Total Dropped Packets: 1
Error Rate: < 0.0002 %
Trigno sensors have been meticulously designed to ensure trouble-free operation for distances up to 40m and recording sessions as long as 8 hours. A custom RF protocol operating within the 2.4GHz spectrum has been developed to ensure no data latency between sensors and a high level of transmission robustness. Our approach has minimized the potential for interference from commercially available products that make use of WiFi, Bluetooth, Zigbee and similar RF communication schemes. None-the-less it is advisable to remove any unnecessary 2.4GHz sources from the recording environment when possible. The Trigno System has the ability to track all data packets that are sent from the sensor to the receiving Base Station, so that communication quality at any point in time is known to the system. Multiple sets of operating channels are accessible to the user so that communication quality can be maximized in any given environment.
Technical Notes
Tutorials
- A Practicum on the Use of Surface EMG Signals in Movement Sciences
- Imaging the Behavior of Motor Units by Decomposition of the EMG Signal
- Electromyography. Encyclopedia of Medical Devices and Instrumentation
- Detection and Recording
- The Use of Surface Electromyography in Biomechanics
- Fundamental Concepts in EMG Signal Acquisition
