The second basic mechanism of tremor is a transmission of oscillatory activity from the central nervous system to muscles in the periphery. The rhythmic activity of muscles then leads to tremor. In contrast to mechanical-reflex oscillations, central oscillations occur at a centrally determined frequency and are independent of the limbs’ mechanics. This crucial difference between the two basic mechanisms can be used to distinguish them from one another.

For example, limb mechanics can easily be influenced by loading the limb being studied, just as weighting slows down a clock pendulum. Increasing weight leads to a decrease in resonant frequency, and therefore:

• The tremor frequency should become lower with additional weight in the case of mechanical-reflex oscillations.  Loading the limb in the setting of a purely mechanical and reflex enhanced physiological tremor will result in a reduction of the tremor frequency. Normal subjects with physiologic tremor have a decrease of more than 1 Hz in their tremor frequency with loading.
• By contrast, the tremor frequency should remain unchanged in the case of central oscillations.  Loading the limb in ET and PD does not affect tremor frequency, which remains constant indicating that ET arises from a central oscillator.  See also Physiology of Essential Tremor

Note that the mechanical loading approach to distinguish the two types of oscillations has its limitations. The greater the involvement of reflex pathways in mechanical-reflex tremors, the lesser the role that limb mechanics play in the frequency of tremor. If reflex pathways are important, then the frequency is more dependent upon reflex arc length, which differs between distal and proximal muscles, and therefore the frequency will also differ between different muscle groups. ET is characterized by uniform frequencies throughout the body, a further argument for a central oscillator involved in the generation of ET.