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Lateva ZC, McGill KC. Mapping of surface potential fields of thenar and hypothenar CMAPs. Am Assoc Electrodiagnostic Med Ann Sci Mtg, 1997.
The morphology of the compound muscle action potential (CMAP) is largely determined by muscle anatomy. To understand this relationship more fully, we mapped the potential field over the base of the thumb during the thenar CMAP and over the ulnar palm during the hypothenar CMAP. We recorded signals on a 2.5 by 2.5 mm grid using an array of needle electrodes that pricked the outer layers of skin by approximately 1 mm in two normal subjects. The maps clearly show the initiation of the action potential at the endplate, its propagation in both directions along the fibers, and its termination at both muscle/tendon junctions. Two distinct endplate zones are visible in the thenar maps, while only one is visible in the hypothenar maps. During the hypothenar CMAP there is a large standing wave which originates from the interosseous muscles. The maps elucidate the relation between muscle anatomy and CMAP morphology and help explain the influence of the sizes and locations of the active and reference electrodes in clinical CMAP recordings. The results also suggest that a "balanced reference," consisting of one electrode over each tendon, will reduce volume-conducted interference better than a single reference electrode.