Electromyogram (EMG) is a test that evaluates electrical activity within skeletal muscle. Muscles emit signals that vary depending upon the state of the muscle fibers (e.g. relaxation and contraction). Various disease states can cause changes in these patterns that are detectable via EMG. EMG can also localize the cause of muscle weakness due to nerve impingement or damage.
EMG is performed by inserting a fine needle into a specific muscle and observing the response as the needle is inserted. Then the muscle is observed while at rest. Finally the patient is asked to contract the muscle, and the activity is again observed. The needle is relocated and the procedure repeated until a sufficient number of motor units have been observed.
EMG is useful in the diagnosis of many musculoskeletal and neurologic diseases, such as herniated disc, peripheral neuropathy.