Since the description by Galen in the 2nd Century, A.D., clinical neurology has acknowledged the existence of two types of tremor: that which occurs at rest and that occuring during the execution of movement. With the help of refined methods of analysis, E.M.G. and cinephotography, the authors have carried out a detailed clinical assessment in more than 400 patients.
The basic criterion used to define a tremor was the classical definition of Dejerine: “An involuntary, rhythmical and symmetrical movement about an axis of equilibrium.”
As a result of this study, the conclusion has been reached that there are two types of tremor: postural tremor and tremor of attitude. Both are present while the limb remains immobile, whether by wilful design or when at rest in a position of posture and subject only to the action of gravity.
During voluntary movement, tremor is not present. Irregular, asymmetrical and non-rhythmic oscillations may appear however — as in so-called intention tremor, of cerebellar origin — but this abnormal movement can hardly be called a real tremor. It is merely a manifestation of ataxia.
As a consequence of this study, it is suggested that further understanding of the basic mechanism of tremor can be reached by the investigation of the central neural structures which are involved in the physiology of posture and attitude.