Post - movement changes in the frequency and amplitude of 1 physiological tremor despite unchanged neural output

22 Active or passive movement causes a temporary reduction in muscle stiffness which 23 gradually returns to baseline levels when the muscle remains still. This effect, 24 termed muscle thixotropy, alters the mechanical properties of the joint around 25 which the muscle acts, reducing its resonant frequency. Since physiological tremor is 26 affected by joint mechanics, this suggests that prior movement may alter tremor 27 independently of neural output. To address this possibility, vertical acceleration of 28 the outstretched prone hand was recorded in eight healthy subjects, along with 29 electromyographic activity (EMG) of the Extensor digitorum communis muscle. A 30 series of voluntary wrist flexion/extension movements were performed every 20s, 31 interspersed by periods during which hand position was maintained. Time32 dependent changes in the amplitude and frequency of acceleration and EMG were 33 analysed using a continuous wavelet transform. Immediately following movement, 34 acceleration displayed a significant increase in wavelet power accompanied by a 35 reduction in peak frequency. During the post-movement period, power declined by 36 63% and frequency increased from 7.2 to 8.0Hz. These changes occurred with an 37 exponential time constant of 2-4s, consistent with a thixotropic mechanism. In 38 contrast to acceleration, EMG activity displayed no significant changes despite being 39 strongly related to acceleration during the movement itself. These results 40 demonstrate that prior movement transiently increases the amplitude and reduces 41 the frequency of physiological tremor, despite unchanging neural output. This effect 42 is best explained by a reduction in joint stiffness due to muscle thixotropy, 43 highlighting the importance of mechanical factors in the genesis of physiological 44 tremor. 45 46 Introduction 47 The origin of physiological tremor remains somewhat contentious. Neural 48 explanations include central oscillatory activity (Conway et al., 1995), motor unit 49 firing properties (Dietz et al., 1976;Elble and Randall, 1976) and reflex loops (Lippold, 5