This is basically due to the differential amplification in bipolar recordings that are able to selectively amplify the difference in the signal (from the muscle action potential) while supressing the common signal, i.e. the background noise. That is also the reason why the double-differential electrodes/amplifications are even more effective in improving the signal-to-noise ratio

Thanks for your answer but you do not understand my question.

Dieter is correct. Basically signal which is common to both electrodes is removed. This gets rid of the noise such as mains frequency and radio signals. This leaves you with the actual muscle signal. Monopolar electrode systems do not allow you to do this leaving you with more noise in your signal.

...then you might want to rephraise/edit your question in the way you want it understood!

Hello,

Bipolar recording system consists of two electrodes that are placed on the same muscle. If there is any noise, it affects the two electrodes in the same way. As a result, we can hope the noise effects to be reduced during differentiation. on the other hand monopolar electrode is a single electrode that measures voltage with respect to the ground. Considering the comments, we can easily realize that bipolar electrode would have higher SNR. Thus, I firmly agree with Alasdair and Dieter.

Bests,

Simply put. The two electrodes are inline with the muscle fibre or the electrical wave as the action potential traverses the fibre. The signal processing includes a single differential model - which is like a simple subtraction technique. I.e electrode A minus electrode B. the voltage from the action potential varies as it progresses along the muscle fibre so the electrical signal from the muscle fiber will be different when recorded from both electrodes at any particular time. Now consider the electrical noise from the mains power ( lights) in the testing room. Each electrode would record this wave form but essentially since the signal is the same distance from each electrode the signals are almost identical - hence the subtraction cancels out the distal signal. Noise is the signal you do not want - the electrical signal from the mains power is not what you want ...so bipolar single differential electrode configuration is the standard method. There are other branched configurations. Regards GTA

Mostafa, perhaps you are referring to the differences between a bipolar electrode configuration (where two electrodes are placed above the muscle of interest) versus a monopolar electrode configuration (where one electrode is placed above the muscle of interest and the reference electrode off the muscle of interest); there is often another ground electrode involved with both these configurations. In both monopolar and bipolar configurations, a differential biopotential amplifier is used (as noted by other respondents, this is needed to reduce the common mode noise such as power line interference)

If you look at Bullar et al. "Selective noninvasive electrode to study myoelectric signals" he refers to earlier work by Lindstrom. The bipolar configuration, as noted by some respondents, is essentially a differentiator (assumption is the electrodes are aligned with the muscle fibers). The bipolar configuration can be modeled mathematically as a monopolar measurement filtered with a transfer function of Hbipolar(f) = 2j*sin(pi*f*d/v) where d is the inter-electrode distance and v is the conduction velocity; for lower frequencies, the sin function looks like a differentiator.

A bipolar configuration, has a smaller pickup area relative to a monopolar measurement and also suppresses lower frequency components (look at the transfer function shape), which provides better temporal resolution. This helps improve SNR, as well as reduce crosstalk.

If you use a single diffential amplifier with a good CMRR (common mode rejection rate) it will drastically reduce all noise which are commonly recorded by the two electrodes. In monochannel configuration you measure the EMG against the ground-reference electrode (theorically at 0 voltage), and therefore the amplifier is not going to reduce the noise recorded in that single channel.

Hi all, I think I have a similar issue. I would like to record simultaneous EEG and facial EMG but I only have monopolar EEG amplifiers so I was wondering If I could record two monopolar EMG signals (with cephalic ground and reference) over the belly of the muscle and the subtract the activity of the two offline. Would that give me an EMG signal that is clean enough?

Thank in advance for your help!

Sorry, but from my point of view it is not so simple. The circuitry in differential amplification is such that the difference in the two signals is amplified but the common signal, i.e. the unwanted background noise, is suppressed. The quality of such amplifiers is given by the so-called common mode rejection ratio (CMRR). If you want to know more about it: The SENIAM publications (based on a former EU-granted project) specify the surface EMG technology and background...

Due to the inherent capacitive coupling our bodies have with the power lines, we are having a significant (nano-amps) line frequency current through our bodies. Thus, when we take a mono-electrode recording, we get a good 50 (or 60Hz) recording, and we behave like an antenna.

Now, if we take a simple op amp and use it as a diff amp, and again, you will not see much better. The reason is because there will be significance difference between the skin-electrode conductivity, and this leads to imbalance resistance. Next, build your own differential amp with three op-amps, and the result will not be good, because the op-amps are not balanced.

The next step is to take an instrumentation amp, and now you will see the difference. You will see that th eline noise has been cancelled because of the matching of the impedance.

Hope this helps,

Within the ASC system (Analog Signal Conditioning), composed by amplifiers and analog filters, the first stage of the front end amplifiers is constituted of an operational amplifier which usually shows a very high CMRR (Common Mode Rejection Rate). In other terms, the first stage rejects what is common between the to recording electrodes and potentiates what is different: that is, the noise, which is usually ugual at the input of the amplifier is practically eliminated (with a CMRR ove 130-140dB) whereas the usefull signal is correctly amplified.

Hi Maria Panasati, Dieter Rosenbaum is correct. You need a more sophisticated preamplifier for processing signals from the electrodes. Algebraic summation of the output won't work. If you do not have differentials amplifiers, it is better to stay with monopolar recording and adjust filter values.