Amplitude changes in the 8-12, 20-25, and 40 Hz oscillations in finger tremor.

OBJECTIVE: The study examined the amplitude and frequency modulation of the 8-12, 20-25, and 40 Hz frequencies of tremor to determine the degree to which increments of load affect the amplitude of these neural rhythms. METHODS: Finger acceleration from the middle phalange and electromyographic (EMG) activity of the extensor digitorum communis (EDC) muscle were recorded on 10 normal adult subjects. Two experiments are reported that manipulated loads ranging from 0 to 40 and 0 to 200 g that were attached to the distal portion of the outstretched middle phalange. RESULTS: There were 8-12, 20-25, and 40 Hz oscillations in the EMG recording but only the 8-12 and 20-25 Hz rhythms were present in the tremor and tremor-EMG coherence. Adding load to the finger reliably decreased the 20-25 Hz band of acceleration, reduced the relative power within the 20-25 Hz EMG band, increased the relative power of the 40 Hz band, but had no effect on the relative power within the 8-12 Hz EMG frequency band. The tremor-EMG coherence in the 8-12 and 40 Hz regions was independent of load, but was markedly reduced with load in the 20-25 Hz band. CONCLUSIONS: The 8-12, 20-25, and 40 Hz neural rhythms of physiological tremor have a stable frequency consistent with central oscillations. There is an increase in the relative power of the 40 Hz EMG band with force, but only the amplitude of the 20-25 Hz band is modulated by mechanical-reflex feedback.