Proceedings of The Physiological Society

Physiology 2012 (Edinburgh) (2012) Proc Physiol Soc 27, C80

Oral Communications

The role of resonance in physiological finger tremor

M. Lakie1, C. A. Vernooij1, R. F. Reynolds1

1. School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom.

  • Fig 1. Frequency spectra of the acceleration. Left panel is obtained from a white noise mechanical input, right panel is obtained from a white noise electrical stimulation input. Lighter grey colours represent an increased RMS of the input, shaded areas represent the standard error

Human postural physiological finger tremor has two main frequency components with variable preponderance. One is at a broad frequency of 20 - 25 Hz and is sometimes attributed to mechanical resonance. The other is at a lower frequency around 8-12 Hz and is usually attributed to a central oscillator. However, it has been known for some time that the resonant frequency of the finger cannot be expressed by a single number as it depends on the vigour with which it is oscillated and also on its history of movement (Lakie and Robson 1988). We have investigated finger resonance using mechanical inputs and muscle stimulation so that no voluntary activity is required. The hand and forearm of fifteen comfortably seated subjects (24.73 ± 10.12 yr, 8 male) was fully supported at waist height. With ethical permission and informed consent, induced flexion-extension movements of the splinted middle finger were recorded by a miniature accelerometer and a retro-reflective laser rangefinder. In one condition, mechanical inputs were applied to the finger by a miniature servomotor and titanium linkage. Any induced EMG was recorded from the extensor digitorum communis (EDC) muscle. In a second condition, per-cutaneous electrical stimulation was applied to the EDC muscle. In both cases the input was in the form of broad band random noise. The RMS size of the broad band noise was systematically altered from very small (producing a barely visible movement of the finger) to moderate (producing movements of the finger of several degrees). The acceleration and phase results showed that both forms of random excitation produced a clear resonance in finger acceleration. Also, although not identical, both forms of excitation revealed striking non-linearity in the resonance of the finger. With mechanical input the frequency peak was at 23 Hz for the smallest excitation and at 9 Hz for the largest (fig 1). We suggest that both components of finger tremor are likely to be due to resonance, with the high frequency predominating when the tremor is small and the low frequency predominating when the tremor is large. The change in resonance may result from a progressive change in a single resonant system or it may result from a switch from one mode of oscillation to a second mode.

Where applicable, experiments conform with Society ethical requirements