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Contribution of motor unit activity enhanced by acute fatigue to physiological tremor of finger.

The contribution of motor unit activity to a physiological tremor (hereafter called as tremor) in a middle finger is studied by both a power spectrum and a correlation analysis in which the correlation coefficient and the coherence spectrum are obtained when five kinds of loads, 0, 50, 100, 150, and 200 g, are added to the middle finger for two minutes in a loading experiment on twelve male subjects. A weight of 200 g is applied to the subjects for ten minutes in a fatigue experiment. Throughout both experiments, the middle finger remains stretched from the load of the weight. The tremor is measured by an accelerometer (MT-3T, Nihon Kohden, Japan) attached to the middle finger, and the surface electromyogram (EMG) is measured by bipolar electrodes placed on m. extensor digitorum communis. A power spectrum analysis is carried out on the tremor and EMG, and a correlation analysis is performed on the relationship between the tremor and the demodulated EMG. It is found in the loading experiment that when the weight on the finger increases, the amplitude of the tremor oscillation increases since the activity of the motor units of the muscle is enhanced by the phenomenon of recruitment. Two frequency components of the tremor spectra at 10 Hz and 25 Hz under a no load condition reflect the components of the activity of the motor units of the muscle because the tremor shows a significant correlation in the frequency zone of 10 Hz and 25 Hz with the demodulated EMG. The lower frequency component of the tremor spectrum at 10 Hz results in synchronized activity of the motor units, while the higher frequency at 25 Hz occurs from the stretch reflex loop via the motoneurons of the spinal cord. The shift of the higher frequency component to the lower frequency domain due to the load of the weight originates from the prolongation of the response time of the finger mechanical system because the lag time at the peak of the correlation coefficient increases with the load of the weight. It is found in the fatigue experiment that the amplitude of the tremor oscillation increases with the progress of fatigue. The increase is caused by the recruitment of the motor unit activity of the muscle holding the finger as well as by the synchronization of the firings of the motoneurons. The progress of the synchronization is verified by the fact that the mean power frequency (MPF) of the EMG spectrum decreases and the correlation between the tremor and the demodulated EMG increases with the progress of fatigue. The mechanisms of the increase of the amplitude of the tremor oscillation under the load of the weight to the finger and under the state of fatigue of the finger are elucidated by the analysis of the tremor and EMG.