The impact of paired motor unit discharges on tremor

We studied paired discharges (PDs) of single motor units (MUs) of the first dorsal interosseus muscle in parkinsonian and essential tremor. The number of PDs increased with increasing tremor strength and the duration of their intervals shortened. The amplitudes of the single tremor beats showed a significant negative correlation to the duration of the intervals of the preceding PDs. It is concluded that PDs play an important role as tremor amplifiers. The basic mechanism is probably the more than linear summation of two twitch contractions with short interval which is known from animal experiments.