Disorders in timing and force of finger opening in overarm throws made by cerebellar subjects

Although there is agreement that an important sign of cerebellar dysfunction is disorder in timing of movement, it appears that authors who study different behaviors mean different things when they use the term "timing," and that the underlying mechanisms are likely to be different. For overarm throwing, skilled throwers can time ball release with a precision of less than 7 ms, whereas cerebellar subjects show a large variability of 50 ms or more in this timing. Furthermore, cerebellar patients show a larger variability in the amplitude of finger opening which could either reflect a disorder in force, or result indirectly from the increased variability in timing. To determine whether timing and force of finger opening were dependent variables, the time of ball release was plotted against the amplitude of finger opening. In control subjects these two parameters were related, with early (mistimed) throws having smaller finger amplitudes. However, in cerebellar subjects the increased variability in finger amplitude could not be accounted for by the increased variability in timing. Similarly, the increased timing windows could not be explained by disorder in force at the fingers. It is concluded that the abnormal finger opening that occurs in cerebellar patients when making overarm throws results from increased variability in both the timing and force of finger extension. Whether the increased variability in timing is a disorder in triggering finger opening at the right moment, or is due to a failure to combine finger opening appropriately with the hand trajectory, remains to be determined.