The Complete Frequency Spectrum of Physiological Tremor Can Be Recreated by Broad-band Mechanical or Electrical Drive. Running Title: Physiological Tremor Can Be Recreated by Broad-band Drive Aix-marseille Université Et Cnrs

16 Two frequency peaks of variable preponderance have been reported for human physiological 17 finger tremor. The high frequency peak (20-25 Hz, seen only in postural tremor) is generally 18 attributed to mechanical resonance, whereas the lower frequency peak (8-12 Hz, seen both in 19 postural and kinetic tremor) is usually attributed to synchronous central or reflexive neural 20 drive. Here, we determine whether mechanical resonance could generate both peaks. In 21 relaxed subjects an artificial finger tremor was evoked by random mechanical perturbations of 22 the middle finger or random electrical muscular stimulation of the finger extensor muscle. 23 The high and the low frequencies observed in physiological tremor could both be created by 24 either type of artificial input at appropriate input intensity. Resonance, inferred from cross25 spectral gain and phase, occurred at both frequencies. To determine any neural contribution, 26 we compared truly passive subjects to those who exhibited some EMG activity in the finger 27 extensor; artificially created tremor spectra were almost identical between groups. We also 28 applied electrical stimuli to two clinically deafferented subjects lacking stretch reflexes. They 29 exhibited the same artificial tremor spectrum as control subjects. These results suggest that 30 both typical physiological finger tremor frequencies can be reproduced by random artificial 31 input; neither requires synchronised neural input. We therefore suggest that mechanical 32 resonance could generate both dominant frequency peaks characteristic of physiological 33 finger tremor. The inverse relationship between the input intensity and the resulting tremor 34 frequency can be explained by a movement-dependent reduction in muscle stiffness, a 35 conjecture we support using a simple computational model. 36 37