I personally have never encountered what you have described during my 25 years in the EMG field. I believe it would help if you can describe to us exactly what you experienced. From what muscle did you record from, and where were your electrodes? Can you provide diagrams of the waveforms?

Eleftherios S. Papathanasiou, PhD,

Clinical Neurophysiologist,

The Cyprus Institute of Neurology & Genetics,

Nicosia, Cyprus

Based on the phrasing of your question, I think you are referring to a sensory nerve action potential reaching the E2 (or reference or red) electrode. In those recordings, e.g. digital sensory nerve action potentials, the amplitude and duration are less if E1 (or active or black) and E2 electrodes are closer to each other. That is indeed due to use of a differential amplifier. When it comes to the CMAP, I agrre with Dr Howard's comments.

We need more information about your setup or technique and, if possible, one record to understand your question...

Thanks for the replies, I'll try to explain it further.

As an example, if I was to record 2 CMAPs from the biceps brachii using 2 different configurations:

1. Mono-polar configuration with E1 placed over the motor point and E2 over the bicep tendon.

2. Differential configuration with a bipolar electrode with an inter-electrode distance of 10mm, placed over the motor point.

Ground electrode placed on the elbow. Stimulation applied to the musculocutaneous nerve for both methods.

Would we expect the CMAP to be the same amplitude, duration and area for each method? Or would the CMAP be smaller in setup 2 as the inter-electrode distance is reduced?

I do not have a clear understanding of your setup or technique so the following may not apply. CMAP amplitude should not be affected (or minimally so) if E2 is over an inert surface rather than the same muscle from which you are recording

Dear Professor Piasecki,

Thank you for the clarification. Method 2 is definetely not the way to record from a long muscle like the biceps brachii. The same holds true for the tibialis anterior muscle, with the recording electrode over the upper prominent portion of the muscle, and the reference electrode lower nearer the ankle. Indeed, this an example of where the recording electrode should not be on the muscle if at all possible. I believe that the recording with method 2 will produce a lower amplitude response, although I have never tried it. Method 1 would be a method I would always use.

Agree with Prof. Papathanasiou. Method 2 will result is a smaller response.

Thank you for your clear description. Method 1 is the correct way to measure CMAP in such muscle. Method 2 may result in a smaller response.

Many thanks for your replies.

Could anyone point me to any work/references that explain why this occurs? Is it simply because of the duration of the CMAP; with a short inter electrode distance it will reach E2 before completely passing E1?

Agree with Dr. Howard and Dr. Papathanasiou: Method 2 will result in a smaller CMAP amplitude. Here is a thorough explanation: Daniel Dumitru, Anthony A. Amato, Machiel Zwarts. Electrodiagnostic Medicine. Hanley & Belfus; 2 edition (September 18, 2001), p. 27-53, especially p. 51

If you place a differential electrode setup over the motor point you should expect low amplitude signals or even no signals at all. The action potentials are radiating out in both directions from that point so one should subtract from the other! Further the monopolar setup you describe is not monopolar at all, it is bipolar with a large interelectrode distance.

You are over complicating this, move your pair of electrodes away from the motor point, over the belly of the muscle and your signals should be fine.

Perhaps a better explanation of method 1 would be:

Mono-polar configuration with E1 placed over the motor point and the reference over the bicep tendon. Amplified with a monopolar amplifier.

I understand your point on placing it over the motor point, however the location on the muscle doesn't make a great deal of difference. The signal is still much smaller using method 2 as detailed above. It's refreshing to know others agree.

Also, it's a point of interest. The more complicated the better.

But surely the point of EMG is not just producing something that looks like EMG but rather getting meaningful information? The belly tendon config will give larger signals as you won't be subtracting signals from deeper lying units. But could be more prone to cross talk and noise.

It is all about the question you are trying to answer. We generally use a 2cm interelectrode distance and never have a problem with signal quality.

The two electrodes can be considered as two points of sampling of the potential distribution, which changes according to both space and time. So, I think the problem is related to the inter-electrode distance. To avoid this problem, it is necessary to adapt the duration of the signal with the inter-electrode distance.