Lightly damped hand oscillations: acceleration-related feedback and system damping

Acceleration oscillations (physiological tremor or die-away oscillations) of the out-stretched hand and surface electromyograms (EMGs) from wrist extensors were detected from each of nine normal subjects. Each lightly damped oscillation (DO) was initiated by a downward tap to the dorsum of the hand. Auto- and cross-spectral analysis (29) of amplitude-demodulated EMG and hand acceleration records gave values of root mean square (rms) amplitude, coherence, and EMG-DO phase. EMG-DO phase was also estimated from average epochs. Average damping ratios were calculated for different DO peaks for DOs between 3.5 and 8.25 Hz. All records were obtained with 90-s rest between runs.