Muscle are in a state of partial contraction at rest. this is called muscle tone.
When doing EMG it is usually told at rest there will be no recording.
the question is whether EMG can record the tone at rest?
Hello Senthil Kumar, in our lab. we attempted placing an electrode on the quadriceps femoris muscle while the subject was at rest. Although signals were generated as evident from the baseline, the muscle contraction information embedded was not significant.
When a striated muscle is at rest, thus fully relaxed, no electrical activity is produced from either the motor axons or from the muscle fibres such that EMG recordings (wether surface or needle recording) are silent. To answer the question directly: EMG does not allow to record an activity pertaining to "muscle tone" at rest. When full rest is difficult to obtain in a subject tensed, a slight contraction of the antagonist induces a full EMG silence in the recorded muscle.
Muscles fully at rest are not activated by the motor neurons and there is therefore no EMG signal. Passive resistance may be produced by the non contractile elements but this is not tone. If the muscle is partially activated but in an isometric contraction (ie no shortening or lengthening) then that is tone and this does produce a signal. If a subject is stood passively ( at rest?) then muscles with generally slow twitch fibres that provide support for the skeleton eg soleus are tonically active and therefore will produce as signal
In needle recordings, muscle at rest shows silent , with some isolated motor unit, because absolute rest does not exist. Ther is alway some spontaneous and isolated activity. In surface recordings, this activity is practically zero. Nevertheless, baseline recording is useful to know light electrical activity present in skin but not corresponding to muscle work.
it would be very difficult, it seems no EMG signal can be recorded at rest.
Thank you all for answering. I want to understand your answer better so i am asking a question again.
lets take a patient in supine position and do needle EMG in Quadriceps muscle.
At rest according to you there will be no recording after putting the needle.
But we have been taught all muscles are in a state of partial contraction even at rest and no muscle is compeltely relaxed. if it is true my doubt is that whether the EMG will be able to record that partial contraction.
It depends on the sensitivity of your amplifier. If you amplify enough and the noise is low enough you can see electrical activity from muscles at rest.
Hi Senthil. Can I ask why you want these EMG signals? It would seem that you are looking for, at best, a very low level signal that might have very little physiological or functional significance.
Dear Senthil, dear all,
As I also receive your additional question, let me participate again to the answer.
This is because :
- I remember very well my surprise, and slight frustration…, when I first saw (was this in 1967 or 1968 ? ;-) that EMG did not translate “muscle tone” into any electrical signal during full rest; and mainly also
- to provide a reference, which is probably one of the first to discuss the matter and to include an information about the sensitivity of the recording :
“Adrian and Bronk (1929) found that in completely relaxed normal muscle, even at amplifications up to 2x106 there was no spontaneous electrical activity.” Licht S. 1961. Adrian, E.D. and Bronk, D.W. The discharge of impulses in motor nerve fibers. J. Physiol., 67:119, 1929.
Today we know that the smallest discrete electrical signal that the muscle generates is that of a single muscle fiber, namely the « fibrillation ». For needle EMG recording, a fibrillation is an electrical signal (in the order of 10 µV-200 µV). All EMG apparatus record fibrillations easily, on an every day basis…, since it realizes, with the « slow positive sharp waves » (a similar signal but mainly positive instead of the biphasic positive-negative shape of the fibrillation) the spontaneous signs of « denervation ». Voluntary activity is composed of « motor unit potentials » which are larger electrical signals (summation of many potentials of muscle fibers; in the order of 200µV to 5mV and sometimes larger in case of reinnervation). Using needle recording, both fibrillations and motor unit potentials are well observed on the EMG screen and heard from the EMG loudspeaker.
When a normal muscle is fully at rest, a condition usually easily obtained from most patients (see my previous note), both surface and needle recordings are fully silent. During this relaxed condition, neither “motor unit potentials” nor “fibrillations” are recorded. This is so from most striated muscles, and thus also for the quadriceps! Very slight voluntary activities will be recorded as motor unit potentials firing with a frequency of a few Hz. This frequency increases and new motor units appear into the recording as soon as contraction increases. But this is basic physiology! By the way, the paper from Adrian and Bronk discuss recordings from the quadriceps and it is free to download from PubMed. You may find further information and a better answer to your question. Best wishes. MRM
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