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Increased gravitational force reveals the mechanical, resonant nature of physiological tremor
Author(s) -
Lakie M.,
Vernooij C. A.,
Osler C. J.,
Stevenson A. T.,
Scott J. P. R.,
Reynolds R. F.
Publication year - 2015
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jp270464
Subject(s) - fictitious force , inertia , physics , acceleration , centrifuge , stiffness , moment of inertia , mechanics , control theory (sociology) , acoustics , physical medicine and rehabilitation , classical mechanics , computer science , medicine , control (management) , artificial intelligence , nuclear physics , thermodynamics
Key points Physiological hand tremor has a clear peak between 6 and 12 Hz, which has been attributed to both neural and resonant causes. A reduction in tremor frequency produced by adding an inertial mass to the limb has usually been taken as a method to identify the resonant component. However, adding mass to a limb also inevitably increases the muscular force required to maintain the limb's position against gravity, so ambiguous results have been reported. Here we measure hand tremor at different levels of gravitational field strength using a human centrifuge, thereby increasing the required muscular force to preserve limb position without changing the limb's inertia. By comparing the effect of added mass (inertia + force) versus solely added force upon hand acceleration, we conclude that tremor frequency can be almost completely explained by a resonant mechanical system.Abstract Human physiological hand tremor has a resonant component. Proof of this is that its frequency can be modified by adding mass. However, adding mass also increases the load which must be supported. The necessary force requires muscular contraction which will change motor output and is likely to increase limb stiffness. The increased stiffness will partly offset the effect of the increased mass and this can lead to the erroneous conclusion that factors other than resonance are involved in determining tremor frequency. Using a human centrifuge to increase head‐to‐foot gravitational field strength, we were able to control for the increased effort by increasing force without changing mass. This revealed that the peak frequency of human hand tremor is 99% predictable on the basis of a resonant mechanism. We ask what, if anything, the peak frequency of physiological tremor can reveal about the operation of the nervous system.