The influence of the reference electrode on CMAP configuration

The ulnar, hypothenar compound muscle action potential often shows a double-peaked configuration in the negative phase component while the median, thenar potential has a simple dome shape. To investigate the origin of these differences we evaluated the activity at the belly and tendon electrode locations by referencing those sites to an electrode on the contralateral hand. The tendon sites are not electrically inactive. The ulnar tendon electrode contributes a large amplitude potential which corresponds to the second peak of the ulnar belly-tendon potential. The median tendon electrode contributes only a minimal component to the negative phase of the belly-tendon potential. The distribution of such potentials throughout the hand is evaluated and possible mechanisms for the presence of a tendon potential as well as the differences between ulnar and median sites are discussed.