Training surface EMG when readings do not reflect the actual muscle tension

DeLee Lantz, Ph.D.

Over the years, I have been intrigued by surface electromygraphic (sEMG) readings that don’t appear to reflect the tension that I know is present in the client's muscles. If sEMG readings are low and/or stable, there is little way for a client to get the feedback for retraining. Here are some of the ways I’ve approached this problem:

1. Take off as much smoothing of the signal as possible by decreasing the time averaging. When spasms are present, but superimposed on lower baseline levels, the spasms are often "smoothed away" unless this is done. I’ve found this to be especially important in pelvic floor and abdominal feedback.

2. Try different band pass filter settings. Long ago, in doing TMJ work, I started routinely taking baselines with filter settings of both 100-200 microvolts and 0-1000 microvolts. (The latter with a 60 Hz notch filter, of course.) Invariably, I saw the abnormal activity in the broader band pass but not always in the narrower band pass. I then trained on whichever band pass showed the most EMG activity. Now I continue, with all my sEMG clients, to monitor at whatever band pass settings are available in the system I’m using. When sensors are placed on the trunk of the body and a broad band pass is used, heart rate artifact is picked up. However, this is actually quite helpful. If muscle tension is high, the artifact is obscured. As the client lowers tension, the heart rate signal emerges clearly and becomes another way to reinforce the training.

3. Look at the raw sEMG. This can be confusing, but once you’ve learned to identify artifact, it can be helpful in seeing otherwise undetected sEMG activity.

4. If your system has this capability, view the power spectra of the sEMG signal. This will enable you to see the frequency range where the greatest amplitude is.

5. Recognize that when the main source of the muscle activity is in deeper muscles or more distant muscles, the amount we pick up may be quite attenuated. The same is true when subcutaneous fat is greater than usual. Using a high gain setting and attending to smaller changes than we’re accustomed to, may take care of this.

6. Have the client use the muscle several times and then see if it recovers quickly back to baseline. Often, full recovery does not occur and we can see plainly the excess tension the muscle carries during normal activities throughout the day.

7. Watch the sEMG signal as the client talks about stressful subjects. A low reading may shoot up. This is often an eye-opener for the client and lets you see what that muscle does under stress, or even thinking or talking about stressful subjects.

If you’ve found other ways of understanding or dealing with this problem, I’d appreciate hearing from you. I will add them to this page.

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