text: To perform intramuscular EMG, typically either a monopolar or concentric needle electrode  is inserted through the skin into the muscle tissue.  The needle is then moved to multiple spots within a relaxed muscle to evaluate both insertional activity and resting activity in the muscle.  Normal muscles exhibit a brief burst of muscle fiber activation when stimulated by needle movement, but this rarely lasts more than 100ms.  The two most common pathologic types of resting activity in muscle are fasciculation and fibrillation potentials.   A fasciculation potential is an involuntary activation of a motor unit within the muscle, sometimes visible with the naked eye as a muscle twitch or by surface electrodes.  Fibrillations, however, are detected only by needle EMG, and represent the isolated activation of individual muscle fibers, usually as the result of nerve or muscle disease.  Often, fibrillations are triggered by needle movement (insertional activity) and persist for several seconds or more after the movement ceases.
  After assessing resting and insertional activity, the electromyographer assess the activity of muscle during voluntary contraction.  The shape, size, and frequency of the resulting electrical signals are judged. Then the electrode is retracted a few millimetres, and again the activity is analyzed. This is repeated, sometimes  until data on 10–20 motor units have been collected in order to draw conclusions about motor unit function.  Each electrode track gives only a very local picture of the activity of the whole muscle. Because skeletal muscles differ in the inner structure, the electrode has to be placed at various locations to obtain an accurate study. For the interpretation of EMG study is important to evaluate parameters of tested muscle motor units. This process may well be partially automated using appropriate software.