The effects of weak perturbations on rapid finger oscillations.

Movement control can be position based or amplitude based; in the first case the frame of reference is fixed, while in the second case the origin is at the start of each movement so that in series of movements errors are propagated. In movements aimed at a target the kind of control is task dependent, though the majority of findings suggest a stronger weight of amplitude-based control. In rapid finger oscillations which do not require visuo-motor transformations, control turned out to be position based as revealed by a lack of propagation of small shifts of angular position induced by brief mechanical perturbations; the only effect of the perturbations in addition to the immediate mechanical effect on movement amplitude was enhanced myoelectric activity in both agonists and antagonists which showed up with some delay. The finding of position-based control points to the functional importance of the mechanical characteristics of peripheral musculo-skeletal systems in rapid limb oscillations.