The baseline, the time characteristics and the slow after-waves of the motor unit potential.

Abstract Seventy-five motor unit potentials (MUPs) from four healthy persons were recorded systematically from m. biceps brachii and m. interosseus dorsalis, from ten different muscles. Summation and digital techniques were applied. The total duration of the MUP measured in this way is on average twice as long as when measured by the analogue method. The duration of the first part of the MUP (from the first deviation to the summit of the negative rapid phase) can be measured far more accurately than the duration of the second part (from the negative peak to the last intersection point of baseline and after-wave) and is significantly shorter in the distal than in the proximal muscle. The best criterion for the baseline of the MUP is the horizontal part preceding the signal, but a delay of 10 msec often proved to be too short in the proximal muscle. All MUPs studied ended with either a slow positive or negative after-wave. It is shown that the slow after-waves, at least in part, originate in the RC circuits, probably as the result of the DC component of the MUPs. This means that determination of absolute MUP duration is impossible with the application of RC recording techniques.