Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, C10
The effect of prior movement on tremor frequency
R. F. Reynolds1, M. Lakie1
1. School of Sports and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom.
Physiological tremor frequency is influenced by the mechanical properties of limbs. Changes in the stiffness of a limb will alter its resonant frequency, and therefore may affect tremor[1,2]. Since muscle tissue contributes to limb stiffness, anything which alters its mechanical properties will affect tremor frequency. Prior movement is known to reduce passive stiffness of muscle, a property known as thixotropy. We therefore sought to determine the effect of prior movement on peak tremor frequency. Tremor of the outstretched pronated hand was measured in the vertical plane in four subjects (3 male, 1 female; aged 23-55; all gave informed consent) using a miniature accelerometer attached to the tip of the middle finger. Wrist extensor activity was also recorded with surface EMG. Participants performed forty 2s sinusoidal up/down movements of the wrist, interspersed by 18s periods during which the hand was maintained by the subject in the normal tremor recording attitude. Post-movement changes in EMG and acceleration frequency were assessed using a continuous wavelet transform. Immediately after wrist movement all participants displayed a drop in tremor frequency which progressively increased over the subsequent period (see figure 1). Peak frequency changed from 7 to 8 Hz over 12 seconds (F11,22=20, p=0.009). In contrast to hand acceleration, EMG activity did not change in size or frequency content. This is reflected by increased gain between EMG and acceleration at lower frequencies. These results show that prior movement transiently reduces peak tremor frequency. This effect occurs despite constant neural output, and so is likely to be due to a movement-induced reduction in muscle stiffness.
Where applicable, experiments conform with Society ethical requirements