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George E. Carvell, PhD, PT   University of Pittsburgh
 
PERIPHERAL NERVE DYSFUNCTION VII: ELECTROMYOGRAPHY
 Clinical Electrophysiology of the Peripheral Nervous System (PNS) typically includes Nerve Conduction and Electromyographic (EMG) Studies. An EMG Needle Exam uses indwelling electrodes to record extracellular skeletal muscle activity. Activity is assessed upon needle entry (insertion activity), at rest (no muscle contraction), and during submaximal and finally maximal volitional isometric contraction. The EMG needle exam is used to disclose the location and severity of suspected lower motor neuron involvement or to rule out an upper motor neuron syndrome or other causes of weakness. Muscles may be selected according to peripheral nerve or root level innervation. Both extremity and back muscles may be examined. Some muscles innervated by cranial nerves may be examined by experienced electromyographers. The examiner looks (at an oscilloscope trace) and listens (to an audio monitor of EMG) for electrical signs of PNS denervation and/or reinnervation.
The EMG needle exam of an intact muscle shows a brief period of insertional activity (discharge of muscle fibers as the needle advances). Normal insertional activity does not outlast the actual movement of the needle. This is followed by electrical silence at rest (flat baseline when no contraction is taking place). There should be no signs of denervation. At submaximal muscle contraction, a number of different motor unit potentials (MUPs) should
be seen, with increasing rates of discharge as the contraction builds (fig xx-x). At maximal contraction,
the screen should be filled with activity; individual MUPs overlap & can no longer be separated (full interference pattern).
Ben.  fig 13-23 p 414
GMOMM  2001
MUPs