Why do MUAPs from Long and Short Muscles have Similar Durations?

McGill KC, Lateva ZC. Why do MUAPs from long and short muscles have similar durations? Am Assoc Electrodiagnostic Med Ann Sci Mtg, 1997.

Reported values of normal MUAP duration from different muscles differ by only a few milliseconds, even though there is more spatial and temporal dispersion of single-fiber potentials in larger muscles than in smaller ones. To try to explain this we compared MUAPs from muscles of different sizes. We examined 120 monopolarly recorded MUAPs from brachial biceps (BB) and first dorsal interosseous (FDI) in five normal subjects. The MUAPs had sharp onsets marking the initiation of the potential at the endplate and distinct terminal deflections marking the end of propagation at the tendon. The final positive phases did not return to the baseline until several milliseconds after the terminal deflections. The durations from the initiations to the negative peaks averaged twice as long in BB as in FDI, while the durations from the negative peaks to the ends were comparable. The duration from the initiation to the negative peak meas ures the propagation time from the endplate to the electrode, and thus varies with muscle length. Computer simulations show that the tail of the final positive phase is a manifestation of the negative afterpotential of the intracellular action potential and does not depend on muscle length.