The influence of the reference electrode on CMAP configuration: leg nerve observations and an alternative reference site.

Peroneal and tibial compound motor action potentials (CMAP) recorded using the standard belly-tendon montage have different configurations. The peroneal CMAP is a smooth dome shape, while the tibial CMAP has a slow-rising initial component followed by a higher amplitude negative peak. To evaluate possible causes of these differences we investigated the individual activity recordable at the belly and tendon electrodes by using a referential montage with the opposite foot as the reference. This type recording shows that the peroneal belly site produces most of the nerve CMAP, whereas the tendon site generates most of the high tibial CMAP. Some features and technical problems of referential CMAP recording using an opposite limb reference are shown. An alternative method using an ipsilateral distal leg reference site is described. A montage which separately records the activity at the belly or tendon electrodes may provide new insight into mechanisms of commonly observed nerve conduction phenomena.