We can look at EMG signals in both time and frequency domains to define bandwidth. The overall electrical signal varies according to muscle excitation.
Typical EMG signal is defined by muscle fiber population electrical activity picked up by surface and needle electrodes.
Surface and Needle electrodes pick up a population of electrically active muscle fibers. The closer the electrodes are the larger the signal will be.
The more active the area is, the more fibers will be evoked, and the larger the signal will be and louder the EMG audio.
The characteristic of muscle signals is defined by the capability of the muscle fibers to generate an electrical event.
Typical muscle signal characteristics span a bandwidth of 10-500Hz. EMG signal strengths below 10 and above 500Hz are very small.
One can say that measurements of activities outside of the peak range is considerably less descriptive, and although hearing anything requires very much more amplification there is less useful information here.
Recording EMG with Hypodermic Needle Electrodes
EMG may, or may not, be present when the hypodermic needle electrode is inserted. Here are a few possible scenarios:
- Needle insertion may result in transient insertional activity, resulting in a transient EMG signal that will disappear.
- Needle insertion may result in spontaneous continuous EMG
- Needle insertion may result in no spontaneous EMG signals. (If this is the case, confirmation of the muscle target must be confirmed and isolated, by patient initiated movement and/or specific manual manipulation to activate the muscle, using patient engagement and resistance to movement to initiate EMG signals)