View Full Version : EMG electrodes: sizes, shapes and frequency responses

Oyvind Stavdahl
04-10-1995, 10:00 PM
Hello, BIOMCH-L!

I'd like to add the following information - and question - to the
recent EMG electrode discussion/info. exchange:

We just bought a couple of preamplifier EMG electrodes (MYO 115 - EMG
Research Electrodes) from Liberty:

Liberty Mutual Research Center
Prosthetics Group
71 Frankland Road
Hopkinton, MA 01748
Phone: (508) 435-9061
Fax: (508) 435-8369
Orders: (800) 437-0024

and we're about to buy some more in near future if they turn out to be
OK. One of the reasons for our choice was the following publication:

Hogan, N. & Mann, R. W., Myoelectric Signal Processing: Optimal
Estimation Applied to Electromyography - Part II: Experimental
Demonstration of Optimal Myoprocessor Performance. IEEE Transactions on
Biomedical Engineering, Vol. BME-27, No. 7, July 1980.

Here, Hogan and Mann predict and demonstrate that the distance between
the electrodes (that is, a differential electrode pair) influences the
bandwidth of the differential signal, with reduced interelectrode
spacing resulting in a higher signal bandwidth (this will for certain
myoprocessor algorithms, such as root-mean-square, result in higher
signal-to-noise ratio). They also consider the size and shape of each
electrode, which is also shown to influence the spectrum of the
differential EMG observed. It seems to me that Liberty's engineers have
been reading Hogans article....

(A second reason for our choice was the small size of the
electrode/preamp. unit: 26.5x17.2x4.8 mm., similar to those used for
controlling the Boston elbow prosthesis.)

>From the recent discussion I learn that Andrew W. (Drew) Smith at U. of
Toronto uses Motion Control's electrodes, which I know have a
relatively large distance between it's electrodes, and the electrodes
themselves are also quite large. Furtermore, their ground electrode is
placed right between the differetial pair, as opposed to Liberty's (and
several other suppliers') which require an "extra" ground electrode
with arbitrary placement.

Can anyone provide expert opinions or personal experience on the

1. The importance and implications of placing the ground electrode
BETWEEN the differential pair. Does this influence the observed
differential EMG signal in any significant (positive and/or negative)
way? (The only positive result I can think of is a possibly improved
Common Mode Rejection.)

2. Larger electrodes cover a larger area and thus measure the EMG of a
larger number of motor units, while they also apparently low-pass
filter the signals (according to Hogan&Mann). Does anyone have opinions
or experience regarding the tradeoffs that have to be made with respect
to this? Does any formal or informal standard exist with respect to
electrode geometry? (What were your specific reasons for choosing the
Motion Control electrodes, Andrew?)

As usual, I will post a summary (edited) of the responces.

By the way, I have no commercial interests in either of the products
mentioned above.

Best regards,
Oyvind Stavdahl (Siv.ing., Dr.ing. student)

Dept. of Engineering Cybernetics Direct line: +47 73 59 43 77
O. Bragstads plass 8 Switchboard: +47 73 59 43 76
N-7034 TRONDHEIM Fax: +47 73 59 43 99
NORWAY Email: stavdahl@itk.unit.no