Picking a raspberry requires a continuously increasing pull-force. In order not to slip off, one has to increase grip-force simultaneously. Therefore, the condition pull-force = friction coefficient * grip-force has to be fulfilled. The task is performed with maximal efficiency when equality of both terms is achieved. Thus, pull-force and grip-force can be assumed to follow the same time-course.

In a motor task comparable to picking a raspberry, the biomechanical performance of 10 healthy subjects and 9 cerebellar patients has been studied. Each subject had to perform the task about 30 times using the dominant hand. For each trial, grip-force was derived by pull-force with a linear regression model. The median of all regressions R² values for each person was determined. For healthy subjects the median R² values ranged from 0.90 to 0.98, whereas for cerebellar patients the same model yields R² values ranging from 0.5 to 0.93.

These results indicate that pull-force and grip-force are not bound to the same extent and to the same time-course but decomposed, when cerebellar function is impaired. Thus, this analysis allows to detect decomposition of movement and to quantify the impairment.

The study was supported by the Else Kröner-Fresenius-Stiftung (A12/07) and the Friedrich-Baur-Foundation (0006/2003; 0005/2004, 0004/2005).