Electromyography

A concentric needle electrode is inserted into muscle. The central wire is the active electrode and the outer casing the reference electrode. This records from an area of 300/jradius.

The potential difference between the two electrodes is amplified and displayed on an oscilloscope. An audio monitor enables the investigator to 'hear' the pattern of electrical activity.

Normal muscle at rest is electrically 'silent' with a resting potential of 90 mV; as the muscle gradually contracts, motor unit potentials appear . . . followed by the development of

\ an interference pattern „

200/iuV

20ms mtm

200juV

Abnormalities take the form of: Spontaneous activity in muscle when at rest. Abnormalities of the motor unit potential. Abnormalities of the interference pattern. Special phenomena, e.g. myotonia.

20ms

The recruitment of more and more motor units prevents identification of individual potentials

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