Motor unit action potential duration and muscle length

Motor unit action potential (MUAP) components are investigated by means of single fiber computer simulations and clinical measurements. The single fiber simulations have essentially full bandwidth without noise, whereas the clinical measurements were made with a 3-10,000-Hz bandwidth utilizing approximately 1000 averages to reduce noise optimally. These parameters allow the recording of a MUAP's complete 'physiologic' duration including its very slow onset and termination. The simulation results demonstrate a constant waveform onset regardless of the electrode's recording location along the fiber. A far-field potential is initiated when the action potential encounters the muscle fiber's termination. The simulated waveform's and clinically recorded MUAP's near-field component extends between the potential's onset and its corresponding far-field potential's onset. This near-field component's duration should vary with fiber length, and this prediction is clinically confirmed by measuring three different muscle lengths. The far-field potential reveals a constant duration, independent of fiber length, and appears to be associated with the muscle fiber's intracellular action potential duration. A more complete understanding of the components contributing to MUAP duration should provide a more fundamental basis for quantitative clinical MUAP duration measurements.