Mechanistic components of normal hand tremor

Abstract

Hand tremor, the extensor digitorum communis electromyogram (EMG), and the electrocardiogram (EEG) were simultaneously recorded from six healthy adult volunteers. The single 7–12 c/sec peak in the hand tremor autospectrum was shown to consist of 3 components. The first and largest component decreased in frequency with mass loading of the hand and thus behaved as a mechanical system of mass, compliance and viscosity. This component exhibited no convincing or consistent statistical correlation with the extensor digitorum EMG except when the tremor amplitude was unusually large, as during prolonged extension of the hand. In contrast, the second hand tremor component did not decrease in frequency with mass loading but was strongly correlated with an 8–12 c/sec amplitude modulation in the EMG. This amplitude modulation is known to be produced by synchronous 8–12 c/sec activities in neighboring extensor digitorum motor units. The third component was produced by cardioballistics but comprised only 10% or less of the total tremor signal in five of our six subjects. In contrast to hand tremor, the 8–12 c/sec component of finger tremor is a single phenomenon produced by the synchronous 8–12 c/sec motor unit activity of the extensor digitorum. However, when finger tremor was recorded without precluding wrist movement, the 8–12 c/sec finger tremor was heavily contaminated with the mechanical component of hand tremor. We suggest that such contamination has been the cause of much of the conflicting data in the tremor literature.