Muscle vibration alters the trajectories of voluntary movements in cerebellar disorders — a method of counteracting impaired movement accuracy?

Abstract

The alterations of voluntary wrist extension movements (30 degree target amplitude, one second movement time) induced by the application of vibration (125 Hz, 0.75 mm peak-to-peak displacement) to a flexor muscle, an antagonist being passively lengthened by active extensor contraction, have been studied in 12 patients with cerebellar dysfunction and an equal number of age-matched healthy subjects. Cerebellar patients generally perform wrist movements inaccu rately : thus, such movements are suitable for investigating the abnormalities of the rate and range (dysmetria) of their movements. Vibration elicited a significant reduction in the end-position of the relatively slow movements investigated, and a consequent undershooting of target trajectories in both patients and control subjects (p < 0.01, Wilcoxon test). The extent of vibration-induced undershooting was similar in the two groups. Nonvibrated extension movements were produced in both groups by progressive increases in extensor (prime- mover) electromyographic (EMG) activity with little antagonist co-contraction. Vibration-induced undershooting resulted principally from a sustained reduction in extensor activity. These findings support two main conclusions. First, the dysmetria shown by our patients cannot be simply attributed to the failure of a proprioceptive feedback loop. Secondly, the provision of vibratory stimulators, controlled by goniometers sensing joint angle, is a potentially viable approach to counteracting cerebellar dysmetria, e.g. by limiting excessive movement in certain situations.