A limb can be regarded as a pendulum which is able to swing rhythmically, that is, to oscillate1. These oscillations automatically take on the resonant frequency of the limb which is involved.

The resonant frequency is dependent on the limb's mechanical properties:

For example, wrist tremor and finger tremor possess different resonant frequencies, i.e. 8–12 Hz and 17–30 Hz respectively.

Any mechanical disturbance can activate an oscillation. In the hands, the most direct mechanical influence comes from the forearm muscles. Tremor measured in normal subjects during muscle activation mainly emerges from an amplification of the muscles’ effect on the hand at its resonant frequency. A pure resonant phenomenon does not produce pathologic tremors since its amplitude is typically quite low1. However, this low amplitude oscillation may lead to rhythmic stimulation of muscle receptors and can therefore activate segmental (spinal) or long (e.g., transcortical) reflex loops which can greatly enhance this oscillation. Since limb mechanics (and possibly reflex loops) play a role in these oscillations, they are termed “mechanical-reflex oscillations” 1.



1. Raethjen J, Deuschl G. Handbook of Essential Tremor and Other Tremor Disorders. (Lyons KE, Pahwa R, eds.). Taylor & Francis; 2005