This accounts for their positive then negative configuration, with the fast rise of the positive phase and slower return to baseline. The slowness of the latter part reflects the slowness of repolarization. The configuration of this positive sharp wave is typical. Theoretically, in motor or sensory nerve conduction velocity studies, latency could be accurately measured from the point where the potential first becomes positive. However, detectability of the onset of the positive phase varies with distance of the electrode from the nerve or muscle fibers, with the amplitude of the approaching potential and with technical factors in the recording system.
Most workers now use the peak where the wave first changes from positive going to negative going.
This position is at the end plate zone and is desirable because it eliminates the slow conduction velocity of muscle fibers from the latency. In these cases, latency is measured from the onset of negativity. Such an electrode position may be difficult or impossible to find in some muscles. No analogous location exists in nerve conduction velocity studies. Further thoughts on polarity, montages, and localization.