The physiology differs in the many forms of human tremor. Tremors may derive from mechanical oscillations, mechanical reflex oscillations, normal central oscillators, and pathologic central oscillators. Methods of studying tremor include accelerometry and electromyography (EMG). An excellent method consists of accelerometry and EMG combined with spectral analysis and weighting of the body part, which allows separation of tremors coming from mechanical reflex and central oscillators. Physiologic tremor is a mechanical tremor with a possible contribution of the normal 8–12 Hz central oscillator; exaggerated physiologic tremor is a mechanical reflex tremor. Essential tremor (ET) comes from a central oscillator that can be easily influenced with sensory input. The classic rest tremor of Parkinson's disease (PD) comes from a central oscillator that seems less easily influenced with sensory input but can be affected by transcranial magnetic stimulation. Other tremors with central oscillators are palatal tremor and orthostatic tremor. Other tremors whose physiology involves central loops includes cerebellar tremor and cortical tremor. Neuropathic tremors may be a result of delays in peripheral loops, but central oscillators play a role in some.