Waveform changes of compound muscle action potential (CMAP) with muscle length.

Changes in finger positions influence the waveform of CMAP recorded from the muscle that moves that finger. The present study suggested that the muscle length, dependent on the finger position, was a main factor affecting the waveform. On shortening the muscle length, the amplitude of CMAP increased with concomitant reduction in the duration, and on lengthening, the amplitude decreased with concomitant increase in the duration. These changes are considered due to changes in propagation velocities of muscle fibers dependent on the muscle length. In nerve conduction studies, it is important to carefully monitor the finger position to distinguish the waveform changes with muscle length from those due to nerve lesions.