Mechanical factors in human tremor frequency.

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Abstract
Acceleration measurements of hand and finger tremor contain reproducible peaks in their power (variance) spectra. Muscle tension and moment of inertia of the hand and of the finger were systematically varied and the corresponding frequencies of the spectral peaks were observed. When average muscle tension was held constant, as monitored by a modified electromyogram signal adding increments of mass to the finger lowered the 25 cycle/sec spectral peak. The 9 cycle/sec peak did not change. Adding to the whole hand lowered the frequency of the 9 cycle/sec spectral peak. The relationship between tremor frequency and added mass is consistent with that for a second-order underdamped system. A mechanical model was formulated by which the natural frequency can be computed in terms of the mass and dimensions of a cylinder similar to the finger, the mass of an added weight, and the spring constants and angle of attachment of a pair of springs. When the relationship between added mass and transformed tremor frequency was extrapolated, the predicted mass of the finger correlated well with the mass estimated by volume displacement.

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