ELECTROMYOGRAPH (EMG)

Electrical activity of nerves and muscles can be measured to demonstrate abnormalities of the neuromuscular system arising from disease or injury. The action of nerves and muscle is essentially electrical by which information is transmitted along nerves as a series of electrical discharges carrying information in the pulse repetition frequency, which may be in the range of 1 to 100 pulses/s. Contraction of muscle fibres is also associated with an electrical discharge which can be detected by measuring electrodes or brought about by electrical stimulation.

Most electromyography is performed using needle electrodes, normally in the form of hollow needles with one or more wires down the centre appearing as small electrodes at the tip. These needles can be placed within nerve trunks, or muscles to record the discharges arising from a number of fibres close to the tip. Such electrodes may detect signals up to about 500 [u]V with frequency content extending from 10 Hz to 5 kHz. Since these frequencies are in the audible range, it is common to present them through a loudspeaker after amplification. The sound of the EMG assists the operator to position the electrodes and to study the effects of various manoeuvres, such as voluntary contraction of muscle.

Many EMG tests involve the use of stimulators to induce discharges in a nerve trunk, and detect the response by surface electrodes over a muscle served by that nerve. In this case the signals may be as large as 2 mV, and may be presented audibly or for recording on a high-speed chart recorder. Such evoked response tests might be for determining the nerve conduction time, or for assessing the performance of the neuromuscular control system. There are many different disorders of the nervous system and EMG examination has to be tailored to the particular requirements of the individual patient. Thus, these tests are normally carried out by a specialist in electromyography in the neurology department.

EMG equipment consists of recording electrodes, preamplifiers, which are normally placed very close to the patient to avoid pick-up of electrical interference, amplifiers to provide the correct gain, calibration and frequency characteristics, a display system (usually a CRT), a range of integrators and averagers partly to achieve some data compression (chart records may be very long and difficult to read), and a recording medium, which is often a photographic (fibre-optic) system.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details