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EMG is an abbreviation for electromyography. This is an electrical study of nerve and muscle function. While MRI scans can give us images of nerves and thereby display the anatomy, they yield no information on the function of nerves. This is the role of an EMG. In this way, you can think of the EMG as the neurologist’s equivalent of the cardiologist’s EKG. A chest x-ray will tell you what the heart looks like but the EKG will provide more information as to how the heart is functioning.

An EMG actually consists of two portions—the NCV (nerve conduction velocity) and the EMG (needle electromyogram). In most cases, both components are essential for a proper evaluation. A typical EMG study will require 30 minutes per arm or leg.

To perform the NCV, surface electrodes are typically employed. A stimulating electrode is placed over a nerve at one site and a recording electrode is placed over a nerve at a second site. A brief electric shock is provided at the stimulating site and the equipment records the time taken for the electrical impulse to travel from the stimulating site to the recording site. Various stimulus sites can be chosen to test various areas along the path of a nerve. The difference in time taken from a stimulus at point A and a stimulus at point B to arrive at a recording site can then be divided by the distance between point A and point B to provide a conduction velocity for that segment of nerve. In this way, the NCV is like using a voltammeter to locate a short in a wire. A decrease in conduction velocity in a specific segment along the course of a nerve indicates an area of damage to that nerve. This portion of the test is therefore often used to localize the area of damage. When this damage is due to compression, that localization aids the surgeon in identifying the precise location for the incision to relieve that compression. For example, the median nerve can be compressed at the wrist (carpal tunnel syndrome), in the mid-forearm (anterior interosseous syndrome), in the upper forearm below the elbow (pronator teres syndrome), and in the arm above the elbow (ligament of Struther’s entrapment). Clearly, knowing the precise location of entrapment is critical to relieving that entrapment and the corresponding symptoms.

NCV studies are performed for both sensory and motor nerves. In the arms, they are used to study the brachial plexus, median nerve, ulnar nerve and radial nerve. In the legs, they are used to study motor function for the lumbosacral plexus, the femoral nerve and the sciatic nerve and its branches—the tibial nerve and peroneal nerve. NCV studies also used to evaluate sensory function for the sural nerve.

The EMG is performed using a needle electrode to record electrical activity from the muscles chosen for testing. Muscles are tested both at rest and with contraction. All muscles have a normal level of resting electrical activity and a normal pattern of voluntary motor unit activity on contraction. When the nerve supply to a muscle is damaged, that muscle becomes hyperirritable and its pattern of electrical activity changes both at rest and with contraction. These changes are important not only in helping to determine the location of the damage but also in assessing the severity of that damage.