Electromyography (EMG) and Nerve Conduction Velocity (NCV) are tests your doctor may order to measure muscle action potentials and nerve conductions. These tests are performed to determine if symptoms (e.g. weakness) are caused by a disorder of the muscle or the nerve supply. Both tests may be performed. These test can establish if a nerve is pinched, and give a numeric value to how severely it is pinched and where it is pinched.  The test can last anywhere from a half an hour to an hour. 

During the Nerve Conduction portion of the test, electrodes much like EKG patches are placed along the known course of the nerve.  The nerve is stimulated with a tiny electrical current at one point.  The nerve must then transmit the signal along its course, and an electrode placed further down the arm or leg captures the signal as it passes it.  A healthy nerve will transmit the signal faster and stronger than a sick nerve.

The EMG portion of the test measures the electrical activity in muscles.  Muscles normally receive constant electrical signals from healthy nerves, and in return "broadcast" their own healthy electrical signals.  During the EMG portion of the test, the doctor places acupuncture like needles into the muscles to record the electrical signal from the various muscles in the arm or leg.  If a muscle doesn't receive adequate signals from a sick nerve, it broadcasts signals, which show the muscle is confused.

Test Preparation and Performance for the EMG
There is usually no special preparation required. The patient may lie down or sit during the test. During the test one or more needle electrodes are inserted through the skin into a specific muscle or muscle group. Then the patient may be asked to contract the muscle (e.g. bending the knee).

The muscle contraction causes a waveform to be traced and detected on a computer. The computer displays a visual representation of the electrical variations on the screen. The muscle at rest is displayed as inactive (e.g. small wavy line). When the muscle is contracted the waveform spikes denoting electrical activity in the muscle.

Discomfort and Risks
Some discomfort may occur as the needle electrodes are inserted. The muscle(s) tested may feel sore after the EMG. The risks are minimal. There may be some minor bleeding and there is a small risk of infection where the needle electrodes are inserted.

From the Nerve Conduction Test and the EMG's the doctor can correlate which nerves are pinched and the seriousness of the condition.  This information can then be used to help formulate further treatment plans.

Electrodiagnostic studies should be considered an extension of the history and physical examination and not merely a substitute for a detailed neurologic and musculoskeletal examination.  Electrodiagnostic testing is usually not necessary in clear cut radiculopathy or in isolated low back symptoms.  Furthermore, these studies do not assess the smaller myelinated and unmyelinated nerve fibers which are typically responsible for pain transmission.