Motor unit action potential components and physiologic duration

Motor unit action potentials (MUAPs) recorded from the same motor unit at t wo distances along the biceps brachii muscle with monopolar needle electrod es at high amplifier gains (20 mu V/division) and averaged 2000-3000 times reveal total potential durations of 39.6 +/- 4.6 ms. In addition, the termi nal segment for each of these two MUAPs contained a late far-field potentia l with a mean duration of 23.8 +/- 4.1 ms. Computer simulations of MUAPs su ggest that this long-duration positive far-field mirrors the true morpholog y of the intracellular action potential (IAP), which is monophasic positive , possessing a terminal repolarization phase approaching 30 ms. This invest igation suggests that the MUAP's physiologic duration is directly proportio nal to the muscle fiber length and the IAP's duration, which becomes manife st as a positive far-field potential when the IAP encounters the musculoten dinous junction and slowly dissipates. The leading/trailing dipole model is used to explain qualitatively this study's quantitative clinical and compu ter simulation findings. (C) 1999 John Wiley & Sons, Inc.