EMG (Electromyography) is a technique for measuring signals produced by skeletal muscles and nerves. EMG signals arise when a muscle takes action. These electrical muscle signals can be measured in a so-called EMG test, often by means of sticky electrodes on the skin or an electrode-needle. These electrodes are connected to an EMG machine through conductive wires. A pattern is created when the EMG-signals are received by the EMG machine.
The actual EMG pattern is much more comprehensive. This is not your problem. Machines and experts know what to do with it.
This pattern shows the muscle activity at the time of measurement. Knowledge about this muscle activity can be used for many things. For example in order to see whether a muscle functions properly. Or to see which muscle is causing a movement and which one is not. When you have symptoms of muscle weakness, for example, chances are that the doctor will perform such an EMG test on you. Measuring electromyography is also used for identification of neuromuscular diseases.
How to prepare for getting an EMG test
You don’t really have to do much for this. It’s good to ask your doctor what you should do in your specific situation. The following applies to everyone. During an EMG test, the body should be free of fatty creams as these suppress conductivity. Great ways for making sure this happens are not applying these creams or washing them off if they have been applied already. Sometimes, it is needed to warm up the arms or legs before an EMG test. As shown in the video, this can be done with a warm bath. Removing hair is often not necessary. The EMG test usually takes about 30 to 90 minutes, depending on the goal. Once the EMG-test has been performed, you will usually not directly receive the EMG-results. These are examined first.
This video gives an impression about and EMG test. I know nothing about this organisation, except that this video gives a clear image about an EMG test.
What happens in an EMG test?
EMG can be used for a conductive test. In this, an electric signal is sent by means of an electrical stimulator. The provided shocks are of low intensity, so no worries here. The EMG test can either be performed from the skin or from within the muscle. It is necessary to use at least 2 electrodes. This is because EMG signals show the voltage difference between two points. This approach makes it very easy to perform an EMG-test. Non-invasively and cheap. On the downside, this approach can only measure superficial muscles well and the amount of fat between the skin and the muscle influences the recorded signal. Furtherly, it can be hard to determine what muscle is causing the EMG-signal in some cases.
Needle EMG test
Testing from within the muscle can be performed more specific. This can be done by putting two needles into a muscle, or with one needle and a comparison electrode on the skin. This is somewhat less comfortable though, so electrodes on the skin are used when this is not necessary. Still, the level of pain is very low, so don’t let this in any way scare you. Also keep in mind that these tests serve a greater goal for you.
What is done with the EMG results?
A raw EMG-signal comes forth from this. A couple of tricks are performed on this signal, and then it can be read by a specialist. Or even by a not-so-specialist, as some machines can just tell you what the measured signal means.
This signal can then be used for determining whether the investigated person suffers from a disorder or not. Certain patterns represent certain disorders. These can either be muscle disorders, nerve disorders, plexus disorders, root disorders, motor neuron diseases, or disorders of the neuromuscular junction. A list of conditions that could cause abnormal results can be found on the Wikipedia page about electromyography.
Further possibilities with electrography
Many other forms of electrography exist. Some well-known examples are electrocardiography (ECG), concerning the activity of the heart, and electroencephalography (EEG) concerning the activity of the brain waves. You can check out the list of electrographic techniques on the Wikipedia page about sorts of electrography.
EMG combined with prosthetic devices, a BMI, or FES.
Electromyography offers another really interesting possibility. It offers the possibility to be combined with other technologies, such as prosthetic devices, a Brain Machine Interface, Functional Electrical Stimulation or TENS units. This is very useful when it is no longer possible to voluntarily perform movements. In this, the EMG signal of a (partly) functioning muscle can be used to activate a helping device. This helping device can either robotically finish the movement, or stimulate the other needed muscles to generate the complete movement. Especially in this last case, this technique can be used for training to relearn muscle movements. This could be very useful in many conditions. For spinal cord injuries, this can even be combined with epidural stimulation, another very promising technique.
If you’re interested in more information behind EMG and electrical stimulation
As mentioned, a muscle’s activity is measured in an EMG-test. This measured activity consists of MUAPs, short for motor unit action potentials. A motor unit is a combination of a motor neuron and the muscle fibres it innervates. An action potential is an extremely fast change in the electric current of a cell. This extremely fast change in a motor unit’s electric current is what causes muscle movement. Read on to find out what electrical stimulation is, how it is related to the above, and what it can be used for. You can also learn more about how electrical stimulation should be applied and when to avoid electrical stimulation.