Analysis of kinematically redundant reaching movements using the equilibrium-point hypothesis

Six subjects performed a planar reaching arm movement to a target while unp redictable perturbations were applied to the endpoint; the perturbations co nsisted of pulling springs having different stiffness. Two conditions were applied; in the first, subjects had to reach for the target despite the per turbation, in the second condition. the subjects were asked to not correct the motion as a perturbation was applied. We analyzed the kinematics profil es of the three arm segments and, by means of inverse dynamics, calculated the joint torques. The framework of the equilibrium-point (EP) hypothesis, the lambda. model, allowed the reconstruction of the control variables, the "equilibrium trajectories", in the "do not correct" condition for the wris t and the elbow joints as well as for the end point final position, while f or the other condition, the reconstruction was less reliable. The findings support and extend to a multiple-joint planar movement, the paradigm of the EP hypothesis along with the "do not correct Instruction.