Postural and resting tremor in the upper limb
Objective: Tremor from multiple segments of the upper limb was recorded under postural and resting conditions. The aims of this study were to examine the nature of tremor within a single limb segment, intra- and inter-limb co-ordination of tremor, and the influence of cardiac mechanical events on physiological tremor. Methods: Tremor was recorded from eight healthy adult subjects during a postural pointing task where the level of support for the upper limb segments was successively increased. The dynamics of tremor within a single segment were examined using power spectral, ApEn and amplitude analyses. Inter-segment tremor relations were determined using coherence and Cross-correlation analyses. Results: Single segment analysis demonstrated that each (unsupported) limb segment contained two major frequency peaks (at 1-4 Hz and 8-12 Hz). Both peaks were still evident in the distal segments when the proximal segments were supported. External support of the more proximal limb segments also resulted in decreased finger tremor, but these changes were not simply additive over segments within a limb or equal across fingers. There were significant relations between adjacent proximal and distal limb segment pairs but no correlations between contralateral limb segments or between heart rate and limb tremor. Conclusions: These findings imply that: the low frequency component (1-4 Hz) of physiological tremor in the hand and finger could not be attributed to passive transmission of oscillations from the upper arm and forearm; and the contribution of proximal segments on tremor in the index finger tremor could not be predicted from mechanical principles alone. The minimization of finger tremor involved compensatory coupling of segments of the upper arm with particular emphasis upon active control of the wrist joint.
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