The onset of voluntary reactive movement is temporally influenced by the central oscillation in action tremor by multiple sclerosis

The influence of oscillatory activity in the motor system on the generation of voluntary movement has been previously studied by revealing temporal coupling between voluntary movements and associated physiological or pathological tremor. The present study aims to investigate whether there is any temporal correlation between the onset of a rapid reactive movement and the action tremor at the wrist in patients with multiple sclerosis (MS). In 13 MS patients, their reactive wrist movements and tremor were simultaneously recorded during a visually cued simple reaction time task. Significant correlation was found between the tremor-related and non-tremor-related measurements of the wrist movement. The onset of reactive movement was unevenly distributed over the tremor cycle peaking at 177 degrees in the direction opposite to the reactive movement, suggesting a temporal coupling between the reactive movements and tremor. No significant difference in reaction time was found between voluntary flexion and extension movements, and no significant differences in the mean values or the standard deviations of the reaction time between the movements in-phase and out-of-phase with tremor were detected, suggesting that entrainment of the spinal motor neurons is not influenced by tremor activity. In conclusion, in MS action tremor, the timing of the initiation of a rapid voluntary movement may be influenced by the pathological oscillator at a supra-spinal level.