Dynamic torque during a precision grip task comparable to picking a raspberry

Numerous studies have shown torque control to be an important factor in grip-force control. This study introduces a novel task which allows quantification of the dynamics of torque development while increasing grip forces during a task comparable to picking a raspberry. The performance of this task was analysed in two healthy subjects and two cerebellar patients. Individual grip forces and finger positions on a grip rod were analysed using a recently developed technique [Kutz DF, Woelfel A, Timmann D, Kolb FP. Detection of changes in grip forces on a sliding object. J Neurosci Methods 2007;166:250–8]. Levers and torques were derived from grip forces and geometric properties of the grip rod. The analysis of this task performance provides evidence that healthy subjects are able to minimise torque despite increasing grip force, whereas the cerebellar patients tested increased torque disproportionately with increasing grip forces, whereby these high torques were due primarily to the patients’ inability to optimise individual finger positions on the rod. Patients tried to compensate their ataxia-based insecurity by employing higher grip forces, resulting in disproportionately higher torques and increased instability, whereupon they again increased grip force, thus establishing a vicious circle. The analysis of this task suggests that effective rehabilitation strategies must be aimed at interrupting this circle.