Muscle loading as a method to isolate the underlying tremor components in essential tremor and Parkinson's disease

Tremor is clinically evaluated and classified on the basis of its response to limb posture (resting, postural, and kinetic tremor), but the mechanisms underlying this powerful influence remain unclear and no satisfactory method exists to identify or quantify underlying tremor subtypes. Postural change is closely linked to changes in gravitational load. We therefore assessed the effect of changes in muscle load on essential tremor (ET) and parkinsonian tremor (PT) independently of postural change. A motor accurately delivered a series of constant (0.2-1.2 Nm) flexion and extension torques about the affected wrist while subjects maintained a constant wrist angle by isometrically contracting wrist flexors or extensors against the applied loads. Linear regression of tremulous electromyogram (EMG) spectral peak amplitude against the applied loads estimated the magnitudes of the load-dependent (LDT) and load-independent (LIT) tremor components. The amplitude of ET was linearly related to increase in gravitational load. It thus contained a large LDT component and a small or absent LIT component. Muscle loading revealed significant LDT and LIT components in PT. LIT was dominant at zero load (classic rest tremor) but both components were present during loading (classic postural tremor). Muscle loading more clearly identifies tremor subtypes than postural effects alone. The method could be applied in clinical and pathophysiological studies.