Motor-unit activity responsible for 8- to 12-Hz component of human physiological finger tremor

Tremor of the extended third digit and bipolar surface and needle electromyograms of the extensor digitorum were recorded from six healthy volunteers for the purpose of elucidating the motor-unit activity responsible for the 8- to 12-Hz component of physiological finger tremor. Tremor was measured with a force transducer during steady voluntary contractions of approximately 200-250 g. The surface EMGs were full-wave rectified and low-pass filtered (-3 dB at 21 Hz), producing the envelope of the surface EMG (the demodulated EMG). Spectral analyses of simultaneous tremor and demodulated EMG records were performed. In four of six subjects, a pronounced 8- to 12-Hz amplitude modulation in the surface EMG was present, and coherency analysis demonstrated that this modulation was strongly correlated with the well-known 8- to 12-Hz tremor. In two subjects this amplitude modulation and tremor were barely detectable, despite the sensitive recording and analysis techniques used in this study. Spectral analysis was performed on 43 motor-unit spike trains. Twenty-two spike trains, having mean firing frequencies in the range of 10-22 spikes/s, produced statistically significant spectral peaks at 8-12 Hz, in addition to the expected spectral peaks at the mean firing frequencies. Of the 22 8- to 12-Hz-producing motor units, 12 had mean firing frequencies in the range of 17-22 spikes/s and exhibited the greatest 8- to 12-Hz activities of all motor units recorded. These motor units displayed transient sequences of double discharges in which interspike intervals (ISIS) of approximately 8-30 ms alternated with ISIS of 60-90 ms, thus producing an 8- to 12-Hz spectral peak. Adjacent ISIS of these motor units were correlated in the range of -0.5 to -0.9. Coherency analyses demonstrated that the 8- to 12-Hz activities of these motor units were correlated with the 8- to 12-Hz finger tremor and surface EMG modulation. The remaining 10 8- to 12-Hz-producing motor units had mean firing frequencies in the range of 10-17 spike/s. Although these motor units did not display the intense double-discharge firing pattern of the more rapidly firing motor units, a tendency toward action potential grouping was present and resulted in 8- to 12-Hz spectral activities which were correlated with the tremor and surface EMG modulation. .. ..