Motor unit action potential duration recorded by monopolar and concentric needle electrodes: Physiologic implications

Controversy exists regarding motor unit action potentials (MUAPs) recorded with monopolar v concentric needle electrodes. All investigations to date have used different instrumentation parameters combined with different motor unit potential populations to assess comparative durations for MUAPs. In this investigation, the same MUAP was analyzed for both monopolar and concentric needle electrodes with identical instrumentation parameters. Monopolar needle electrodes were found to record MUAPs with slightly longer durations, a result that reached statistical significance. Manual wideband high-resolution MUAP analysis demonstrated durations approaching 30 ms for both electrodes, which is different from the approximately 10 ms presently measured for both electrode types. A hypothesis was proposed whereby the total duration of current flow, which is directly proportional to muscle fiber length, is the primary determinant of MUAP duration. The physiologic implications of this hypothesis are discussed.