Umair Gaines
Electromyography (EMG) is a technique used to evaluate and record electrical activity in skeletal muscles. EMGs are performed by inserting a small needle with an electrode through the skin and into the muscle, which then detects the electric potential generated by muscle cells when they are electrically or neurologically activated. The procedure can be used to diagnose injuries and conditions that affect muscles and the nerves that control them, such as carpal tunnel syndrome and muscular dystrophy.
| Characteristics | Values |
|---|---|
| Name of technique | Electromyography (EMG) |
| Purpose | To evaluate and record the electrical activity produced by skeletal muscles |
| Instrument used | Electromyograph |
| Output | Electromyogram |
| Detection | Detects the electric potential generated by muscle cells when these cells are electrically or neurologically activated |
| Analysis | Used to detect abnormalities, activation level, recruitment order, or to analyze the biomechanics of human or animal movement |
| Types | Surface EMG, Intramuscular EMG, Needle EMG |
| Usage | Used by neurologists, physiotherapists, kinesiologists, biomedical engineers |
| Applications | Help diagnose injuries and conditions that affect muscles and nerves, e.g., carpal tunnel syndrome, muscular dystrophy |
| Procedure | Small needles (electrodes) inserted through the skin into the muscle; electrical activity displayed on an oscilloscope and heard through an audio amplifier |
| Duration | Typically 30-60 minutes |
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What You'll Learn
Needle EMG
Needle electromyography (EMG) is a technique used to record and analyse electrical signals from individual muscle fibres, both at rest and during voluntary contraction. It involves inserting a needle electrode into a muscle to record electrical signals, which are then amplified and interpreted to determine the function of the muscle fibres and motor units.
EMG is a diagnostic test that evaluates the health and function of skeletal muscles and the nerves that control them. It is often used by neurologists to help diagnose injuries and conditions such as carpal tunnel syndrome and muscular dystrophy. The procedure can be performed in an outpatient setting or during a hospital stay, depending on the patient's situation.
During a needle EMG test, a small needle with an electrode is inserted through the skin and into the muscle. The needle is then moved to multiple spots within the muscle to evaluate both insertional activity and resting activity. The patient may be asked to relax and then use their muscles in certain ways, such as lifting or flexing a limb. The test typically lasts one to two minutes per muscle, and the patient may experience slight discomfort or pain during the procedure.
There are different types of needle electrodes used in EMG testing, such as monopolar and concentric needle electrodes. Monopolar needles are typically insulated and stiff, with only the tip exposed. They are designed to puncture and "divide" the skin and muscle tissue rather than "slice", which can minimise pain and trauma. Concentric needle electrodes, on the other hand, have a fine wire embedded in a layer of insulation that fills the barrel of a hypodermic needle, with an exposed shaft that serves as the reference electrode.
While needle EMG is generally considered a safe technique, there are potential risks to be aware of, including pain, bleeding, and pneumothorax. Additionally, in patients with gross oedema and taut skin, skin puncture by needle electrodes may result in chronic weeping of serous fluid, which could increase the risk of bacterial infections such as cellulitis. Therefore, it is important for physicians to carefully weigh the potential risks and benefits before proceeding with the procedure.
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Surface EMG
During a Surface EMG test, the provider will locate the muscle(s) to be tested and place the surface electrodes on the skin above the muscle. The electrodes will detect the electrical activity generated by the muscle cells when they are activated. The signals can then be analyzed to detect abnormalities, activation levels, or recruitment order.
However, Surface EMG has some limitations. It can only measure superficial muscles, and it is difficult to narrow down the signal to a single muscle due to muscle crosstalk. Additionally, the accuracy of Surface EMG signals is affected by the amount of adipose tissue and skin compliance, with lower body fat and more compliant skin resulting in more accurate recordings.
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Intramuscular EMG
Intramuscular electromyography (EMG) is a technique used to evaluate and record the electrical activity produced by skeletal muscles. It involves inserting needles or wires into the muscles to detect electrical activity. This technique is often used to diagnose injuries and conditions affecting muscles and the nerves that control them, such as carpal tunnel syndrome and muscular dystrophy.
During an intramuscular EMG, a small needle with an electrode is inserted through the skin and into the muscle. This can be done with a monopolar or concentric needle electrode. The needle is then moved to multiple spots within the relaxed muscle to evaluate both insertional activity and resting activity. Normal muscles will exhibit a brief burst of muscle fibre activation when stimulated by needle movement, usually lasting less than 100ms.
The procedure can vary depending on the reason for the test and the specific muscles and nerves being assessed. In most cases, the patient will be asked to sit or lie down, and the provider will locate the muscle(s) to be tested. The patient may feel slight discomfort or pain during the insertion of the needles. The provider will then ask the patient to relax and use their muscles in certain ways, such as lifting or flexing a limb.
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Concentric needle electrode
To record electrical activity in muscles, a technique called electromyography (EMG) is used. EMG is a diagnostic test that evaluates the health and function of muscles and the nerves that control them. It involves inserting a small needle with an electrode through the skin and into the muscle.
One type of needle electrode used in EMG is the concentric needle electrode. This type of electrode has a more complex design compared to other needle electrodes. It consists of a fine wire embedded in a layer of insulation inside a hypodermic needle. The shaft of the hypodermic needle is exposed and serves as the reference electrode, while the exposed tip of the fine wire is the active electrode. The needle tip is designed to optimise low penetration force while still obtaining signals from a sufficient number of muscle fibres.
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Nerve conduction study
A nerve conduction study (NCS) is a diagnostic test that evaluates the function of peripheral nerves—the nerves that lie outside of your brain and spinal cord (central nervous system). An NCS can help detect the presence and extent of peripheral nerve damage, as well as the severity and prognosis of related conditions.
An NCS is often performed alongside an EMG (electromyography) test, which measures how much electrical activity a muscle contraction produces. An NCS, on the other hand, assesses the flow of electrical current through a peripheral nerve before it reaches a muscle. It can also detect the flow of electricity through sensory nerves. Performing these tests together helps healthcare providers determine whether symptoms are the result of a muscle disorder or a nerve issue.
Before a nerve conduction study, it is important to maintain normal body temperature, as low body temperature can slow nerve conduction. Creams, lotions, oils, and perfumes should also be avoided for a day or two before the test, as they can affect accuracy. If you have a pacemaker, cardiac defibrillator, or other implanted device, it is important to inform your healthcare provider, as special steps may need to be taken.
During the test, you will sit or lie down. A healthcare provider will attach electrodes to the surface of your skin above the nerves being assessed. They will then deliver a small electrical impulse, which will feel like a shock, to send a signal to the muscle the nerve controls. The speed of the electrical impulse is then calculated by measuring the distance between electrodes and the time it takes for the impulse to travel between them. This process is repeated for each nerve being tested.
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Frequently asked questions
Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles.
EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyograph detects the electric potential generated by muscle cells when they are electrically or neurologically activated.
During the test, small needles (also called electrodes) are inserted through the skin into the muscle. The electrical activity picked up by the electrodes is then displayed on an oscilloscope (a monitor that displays electrical activity in the form of waves).
An EMG can help diagnose several injuries or diseases that affect motor nerves and muscles. It can help determine the presence, location and extent of these injuries and diseases. Categories of conditions an EMG can help diagnose include carpal tunnel syndrome, peripheral neuropathy, muscular dystrophy, ALS, and herniated discs.
There are two main types of EMG: surface EMG and intramuscular EMG. Surface EMG is a non-medical procedure used to assess muscle activation and can only measure superficial muscles. Intramuscular EMG involves inserting a needle electrode into the muscle and can be used to record electrical activity in deeper muscles.
