Electrodiagnostic tests, nerve conduction velocity (NCV) and electromyography (EMG) evaluate the function of peripheral nerves that supply sensation and motor signals to the arms and legs. They demonstrate abnormalities in nerve and muscle function but do not identify the specific cause. EMG evaluate the electrical activity generated by muscles at rest and when contracting. NCV measures the speed of transmission of the electrical signals in a nerve. Both of these tests are useful in differentiating among neurologic disorders that cause arm and leg pain.
EMG involves inserting small needles into normal and muscles thought to be damaged. The amount of irritation and the amount of electricity generated by the muscle help determine the degree of injury to the muscle and the nerve that supplies it.
NCV determines the speed of signals as they travel down nerve in the arms or legs. Pads are placed of the far end of an arm or leg. Another pad is placed over the same nerve in a location further up the leg. An electrical signal is generated in the pad closer to the center of the body. The signal is detected in the distal pad and the speed of the signal is matched against normal speeds. Slower speeds suggest the nerve is being compressed in a location away from the spine.
Although the tests do not define a specific cause of dysfunction, the tests do identify the level of nerve damage. This helps direct therapy to the specific level that is damaged. This can help direct locations for injections or surgical decompression.