The Influence of the Reference Electrode on the Compound Muscle Action Potential of the Flexor Digiti Minimi Brevis

Abstract

When recording compound muscle action potentials, the active electrode is traditionally placed over the muscle and the reference over the corresponding tendon (belly-tendon recording). This placement assumes that the reference electrode is electrically inactive. We report on the influence of reference electrode placement on the compound muscle action potential recorded from the flexor digiti minimi brevis. We recorded compound muscle action potentials from the flexor digiti minimi brevis using belly electrodes, tendon electrodes, and an electrode on the contralateral medial malleolus from 11 healthy subjects. We measured compound muscle action potential amplitudes and onset latencies. Compound muscle action potentials recorded using electrodes on a tendon and the contralateral medial malleolus had higher amplitudes and shorter onset latencies than the compound muscle action potentials recorded with the belly-tendon montage. When recording compound muscle action potentials from flexor digiti minimi brevis, the influences of the tendon "reference" electrode on latency and amplitude are large. Electrodiagnositicians should consider the influences of the reference electrode when assessing the lateral plantar nerve. This study may also inform us about the influence of proximally innervated muscles on the compound muscle action potential recorded in the limbs.