Equifinality and its violations in a redundant system: control with referent configurations in a multi-joint...

We tested a prediction that equifinality at the task level may be accompanied by violations of equifinality within the redundant set of elemental variables. Seated subjects grasped a handle, and occupied an initial arm configuration against a bias force produced by a robot. The robot applied a smooth, transient change in the force (perturbation). The subjects were instructed "not to intervene voluntarily" with hand deviations. After the robot force returned to the bias value, the hand returned close to the original position and orientation. Analysis of across-trials variance in the joint configuration space confirmed that most variance of the difference between the initial and final states was compatible with unchanged values of both hand position and orientation. These results provide direct support for the theoretical scheme that combines the ideas of synergies and control with referent configurations.