The timing of primary orthostatic tremor bursts has a task-specific plasticity

Primary orthostatic tremor is characterized by unsteadiness and shakiness o f the legs while standing. It is due to a remarkably strong and regular EMG modulation at similar to 16 Hz that is thought to be of CNS origin, Previo us studies have shown that the tremor frequency is the same in all involved muscles and that the time relation between bursts of activity in different muscles may be fixed (e.g. always co-contracting or always contracting in an alternating pattern). Here we have used frequency domain analysis of pos tural muscle EMG signals in five primary orthostatic tremor patients and in two normal controls to explore the nature of such fixed timing patterns. T he timing is found not to relate simply to the relative conduction times fo r passage of rhythmic bursts from a central oscillation to different muscle s. Indeed, although the timing pattern (expressed as phase) of the 16-Hz EM G bursts in different postural muscles remains constant while the subject a dopts a certain steady posture, it is different for different subjects and also changes when the same subject adopts a different posture. It seems unl ikely that such complex task-dependent timing relations of rhythmic postura l muscle activity are due to the primary pathology of primary orthostatic t remor. Instead, we suggest that the abnormally strong peripheral manifestat ion of a 16-Hz CNS oscillation merely unmasks normal central processes so t hat the timing patterns may provide a clue to the nature of postural motor control.