[Double discharges of motor units and tremor strength]

The relationship between the double-discharges (DDs) of single motor units (MUs) and the tremor strength was studied in 7 patients with parkinsonism and 3 with essential tremor. The potentials of single MUs and the force of the first dorsal interosseus muscle were recorded during slight isometric contractions. The rate of DDs and their interval duration were correlated with the global tremor strength and the amplitudes of the single tremor beats. With increasing tremor strength, the rate of DDs increased and the duration of their intervals decreased. It is concluded that DDs act as tremor amplifiers. This effect is probably accounted for by a more than linear summation of two consecutive MU contractions with short interval which is known from animal experiments.