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The Journal of Neurophysiology Vol. 82 No. 2 August 1999, pp. 664-675
Copyright ©1999 by the American Physiological Society
1Division of Neuroscience and Biomedical
Systems,
Halliday, David M.,
Bernard A. Conway,
Simon F. Farmer, and
Jay R. Rosenberg.
Load-Independent Contributions From Motor-Unit Synchronization to
Human Physiological Tremor. J. Neurophysiol. 82: 664-675, 1999. This study describes two
load-independent rhythmic contributions from motor-unit synchronization
to normal physiological tremor, which occur in the frequency ranges
1-12 Hz and 15-30 Hz. In common with previous studies, we use
increased inertial loading to identify load-independent components of
physiological tremor. The data consist of simultaneous recordings of
tremor acceleration from the third finger, a surface electromyogram
(EMG), and the discharges of pairs of single motor units from the
extensor digitorum communis (EDC) muscle, collected from 13 subjects,
and divided into 2 data sets: 106 records with the finger unloaded and
84 records with added mass from 5 to 40 g. Frequency domain
analysis of motor-unit data from individual subjects reveals the
presence of two distinct frequency bands in motor-unit synchronization,
1-12 Hz and 15-30 Hz. A novel Fourier-based population analysis
demonstrates that the same two rhythmic components are present in
motor-unit synchronization across both data sets. These frequency
components are not related to motor-unit firing rates. The same
frequency bands are present in the correlation between motor-unit
activity and tremor and between surface EMG activity and tremor,
despite a significant alteration in the characteristics of the tremor
with increased inertial loading. A multivariate analysis demonstrates
conclusively that motor-unit synchronization is the source of these
contributions to normal physiological tremor. The population analysis
suggests that single motor-unit discharges can predict an average of
10% of the total tremor signal in these two frequency bands. Rectified surface EMG can predict an average of 20% of the tremor; therefore within our population of recordings, the two components of motor-unit synchronization account for an average of 20% of the total tremor signal, in the frequency ranges 1-12 Hz and 15-30 Hz. Our results demonstrate that normal physiological tremor is a complex signal containing information relating to motor-unit synchronization in
different frequency bands, and lead to a revised
definition of normal physiological tremor during low force postural
contractions, which is based on using both the tremor spectra and the
correlation between motor-unit activity and tremor to characterize the
load-dependent and the load-independent components of tremor. In
addition, both physiological tremor and rectified EMG emerge as
powerful predictors of the frequency components of motor-unit synchronization.
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