FAQ

EMG

1 – What is EMG, and where does it come from?

EMG, short for Eletromyography, is a procedure used translate electrical signals created during muscle contraction into a signal that can be translated into a graph, sound, or numerical value for further interpretation. These electrical signals are detected and transmitted by electrodes that can be stuck onto the skin or inserted directly into a muscle via a needle. [1]

2 – What does an ideal EMG signal look like?

An EMG signal is usually a function of time and is describable in terms of its amplitude, frequency, and phase. However, this signal can be interrupted and distorted by noise upon traveling through different tissues. As such, an ideal signal is free of noise. Additionally, surface/skin electrodes may pick up signals from nearby muscles. Natural oil secretions and dead skin can interfere with a signal, and thus the skin must be properly prepped before electrode placement. There are two main types of electrodes – gelled and dry. Dry electrodes are heavier, more difficult to secure onto the skin, and have higher rates of noise. In contrast, gelled electrodes use electrolytic gel as a chemical interface and are vastly lighter than dry electrodes. Redox reactions take place between the metallic surface of the electrode and the gel, and allows for muscle currents to pass more freely. This results in less electrical noise compared to dry electrodes; gelled electrodes(primarily Ag-AgCl electrodes) are used in over 80% of surface EMG applications. [2][3]

3 – What is the voltage range of a typical EMG signal, before amplification?

Prior to amplification, a typical EMG signal’s voltage range is typically between a few microvolt and 2-3 microvolt. This charge is normally amplified by a factor ranging from 500-1000.  [4]

4/5 – How does EMG relate to muscle activation? And how does the EMG signal of a skeletal muscle relate to the amount of force exerted by it?

When an individual’s muscles contract, neurons in the muscle fire and create electrical signals. An EMG detects this charge, and produces a signal. The more effort put into muscle contractions, the more signals fire and the larger the charge, and vice versa.

6 – Describe an EMG instrumentation system from end-to-end, as it relates to your project (i.e., from the placement of the electrodes to the acquisition of the EMG signal in MATLAB).
For this project, EMG electrodes will be placed on each side of the jaw. These electrodes will be connected via a cable to the circuit board. From here, the signal flows to an amplifier. The amplifier is needed because the raw signals from the electrodes are far too small for reading, and must be scaled by a factor ranging between 500-1000. After the signal is scaled, it is sent to a DAQ to be digitized and collected. From here, the signal is sent to the computer to be processed in MATLAB. 

EOG	Figure 1: EMG instrumentation system

1 - What is EOG, and where does it come from?

Electro-oculography or EOG is a testing method to find the difference in electrical potential between the cornea and the ocular fundus. This potential is due to the retinal pigment epithelium (RPE), which has a charge that varies based upon levels of light.

2 - What does an ideal EOG signal look like?

The function should resemble a square function, which stabilizes when the user’s eyes are held in place, and moves rapidly when the eyes are in motion. The amplitude should be proportional to the electrical potential of the eye from the cornea to the ocular fundus.

 3 - What is the voltage range of a typical EOG signal, before amplification?

The voltage range is on the order of tens of microvolts.

4 - How does EOG relate to eye movement? What types of eye movement can be obtained from an EOG signal?

As the eye moves the potential difference across the electrode will change based on its proximity to the positively charged front of the eye compared to the negatively charged back of the eye. Horizontal motion can be determined from a electrode to the side of the eye, for the left eye, a negative EOG signal would represent a movement to the user’s right, and a positive signal would represent a movement to the user’s left, the opposite is true if the electrode is placed to the outside of the right eye. For determining vertical movement an electrode above the eye would record a negative EOG signal for movements downward, and a positive signal for movements upward, once again, the reverse is true for an electrode placed under the eye.

5 - Describe an EOG instrumentation system from end-to-end (i.e., from the placement of the electrodes to the acquisition of the EOG signal in MATLAB).

The electrodes are placed to the outside of the left eye and the top of the left eye and a reference electrode is placed on the forehead. The electrodes will be connected to a signal amplifier in order to increase the amplitude of the signal for use in MATLAB. The amplifier will then be connected to the data acquisition board, which will send the signal to MATLAB, which will be coded to make a graph of the function of Voltage over time.

Citations

[1] Mayo Clinic Staff. (2013, March 7). Electromyography (EMG) [Online]. Available: http://www.mayoclinic.com/health/emg/MY00107

[2] M.B.I. Raez, M.S. Hussain, F. Mohd-Yasin. (2006, March 23). Techniques of EMG signal analysis [Online]. Available: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1455479/            

[3] S. Day (Unknown). Important Factors in Surface EMG Measurement [Online]. Available: http://www.andrewsterian.com/courses/214/EMG_measurement_and_recording.pdf

[4] P. Konrad (2005, April). The ABC of EMG [Online]. Available: http://demotu.org/aulas/controle/ABCofEMG.pdf

[5] Bulling, A.; Ward, J.A.; Gellersen, H.; Troster, G., "Eye Movement Analysis for Activity Recognition Using Electrooculography," Pattern Analysis and Machine Intelligence, IEEE Transactions on , vol.33, no.4, pp.741,753, April 2011

[6] Bach, M.; Brigell, M.; Brown, M.; Marmor, M.; Vaegan; Zrenner, E., “ISCEV Standard for Clinical Electro-oculography (EOG)” Documenta Ophthalmologica. 2006 November; 113(3): 205–212.