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Tremor is an approximately rhythmic, roughly sinusoidal involuntary movement. Despite more than a century of clinical and laboratory investigations, no tremor is understood completely. Common and uncommon forms of tremor are reviewed in this chapter, and two basic questions are addressed for each form: what is the source of oscillation and why does the oscillation occur.

Physiological tremor is barely visible to the unaided eye and is symptomatic only during activities that require extreme precision. Physiological tremor consists of two distinct oscillations: mechanical reflex (MR) and 8–12 Hz, which are superimposed upon a background of irregular fluctuations in muscle force and limb displacement.1,2 These background irregularities have a frequency of 0–15 Hz and are produced by motor units that fire near their threshold.3 The low-pass filtering property of skeletal muscle attenuates the amplitude of these irregularities at frequencies above 3–5 Hz.4

The MR component of physiological tremor is much larger than the 8–12-Hz component and is exhibited by everyone. The MR component is a passive mechanical oscillation that is produced by the underdamped inertial, viscous, and elastic properties of the limbs and other body parts. Participation of the stretch reflex is evident only when physiological tremor is enhanced by fatigue, anxiety, or drugs.5,6 The mechanical attributes of most body parts are such that damped oscillations occur in response to pulsatile perturbations. The frequency of these MR oscillations ω is determined largely by the inertia I and stiffness K of the body part, according to the formula image. Consequently, normal elbow tremor has a frequency, 3–5 Hz, which is lower than the 8–12-Hz frequency of wrist tremor because the forearm has much greater inertia than the hand.7 Similarly, the finger has even less inertia, so the frequency of metacarpophalangeal joint tremor is 17–30 Hz.8 Voluntary co-contraction of muscles about a joint produces a slight increase in tremor frequency due to the increased joint stiffness. Conversely, relaxation of the joint causes the frequency of MR tremor to fall.

The mechanical properties of the body are not sufficient to cause tremor. One or more sources of mechanical energy are required to force or perturb a limb into oscillation at a frequency determined by limb inertia and stiffness. Voluntary muscle contraction contains irregularities in subtetanic motor-unit firing that perturb the limb continuously and randomly. The ejection of blood at cardiac systole provides additional perturbations of the limbs. Such cardioballistics account for nearly all of physiological tremor at rest but only a fraction of physiological postural and kinetic tremor.2,9

Under normal circumstances, somatosensory receptors (e.g., muscle spindles) respond to the mechanical oscillations of physiological tremor, but the response is usually too weak to entrain motoneurons at the frequency of tremor. Consequently, the power spectrum of rectified-filtered electromyography (EMG) is essentially flat during normal steady muscle contraction (Fig. 27–1). The stretch-reflex response to oscillation increases during ...

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