Contrary to earlier recommendations it is now universally agreed that, in order to maximise the amplitude of the detected signal, the electrodes should not be placed either side of and at the same distance from a motor point, which usually coincides with the innervation zone. Differential amplifiers subtract the signal detected by one electrode from that detected by the other.
Thus, locating electrodes either side of a motor point will lead to the cancellation of symmetrical action potentials that are travelling in opposite directions from the neuromuscular junction and that reach the electrodes at approximately the same time. This is illustrated by using the concept of single MAP detected by a pair of electrodes from a linear array. However, if both electrodes are placed to one side of a motor point the signal is not cancelled to the same extent, as one electrode detects the MAP slightly earlier than the other.
Recommended guidelines for locating electrode placement sites over specific muscles also continue to be published. Virtually all major, superficial muscles are now covered by such guidelines, which typically locate electrodes about a point that is a specific distance along a line measured between two anatomical landmarks.
Following the location of an appropriate site, it is universally agreed that, if possible, the electrodes should be oriented along a line that is parallel to the direction of the underlying muscle fibres.
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