Electromyography (EMG)

What is EMG?

Electromyography (EMG) is a diagnostic procedure that measures the electrical activity in skeletal muscles or the muscles responsible for motor activity.

How EMG works?

Muscles contract through the propagation of electrochemical signals, first through the nerves, and then through the muscle fibers. These signals are measured as a change in potential, or in voltage, using either surface electrodes that stick to the skin or small needle electrodes that are inserted through the skin into the muscle. Electrical activity, measured as the difference in voltage between the electrodes, is displayed in real time in the form of waves on an oscilloscope, and may also be heard through a speaker.

After the electrodes are in place, the patient is usually asked to contract, or flex, the muscle being tested. The electrical signal captured by the electrodes appears on the oscilloscope as a wave. The wave’s size and shape provide information about the muscle’s ability to respond when stimulated by the nerves. A more forceful contraction will create a larger wave on the oscilloscope.

EMG is similar to the procedure for a nerve conduction study (NCS), which measures electrical impulses through the nerves. EMG and NCS may be done at the same time to assess a patient for both muscle and nerve dysfunction.

Preparing for EMG

Before the procedure, notify the clinical team of any current prescriptions and implants, especially pacemakers. Avoid rubbing lotions and oils into the skin in the days leading up to the procedure. On the day of the EMG, wear light clothes that permit access to muscles being assessed.

An EMG typically lasts 30 to 90 minutes and can be performed in an outpatient clinic. The patient will be askee to remove any metal clothing, jewelry or accessories. Once electrodes are inserted, the patient will be asked to flex and relax muscles in different intervals and to different extents.

How are EMG results analyzed?

A neurologist will analyze the electrical activity recorded while contracting different muscles to look for any weakness. This will appear as a lower amplitude waveform on the oscilloscope. EMG can be used to distinguish between muscular dystrophies and muscle weakness caused by a nerve disease.

Risks linked to EMG

Few risks are associated with EMG. If needle electrodes are used, there is a small chance of minor bruising, bleeding, infection or nerve injury. If the patient experiences pain during the procedure, the neurologist should be notified immediately, as this can be uncomfortable and interfere with the results.

If chest muscles are being assessed, there is a very small risk of needle electrodes piercing the lung, potentially causing the lung to collapse. This is extremely rare, and techniques can be used to further minimize the risk.


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