MULTIJOINT LIMBS PERMIT A FLEXIBLE RESPONSE TO UNPREDICTABLE EVENTS

The human arm is kinematically redundant, which may allow flexibility in the execution of reaching movements, We have compared reaching move ments with and without kinematic redundancy to unpredictable double-st ep targets. Subjects sat in front of a digitising tablet and were able to view an are of four targets reflected in the mirror as virtual ima ges in the plane of the tablet. They were instructed to move, from a c entral starting point, in as straight a line as possible to a target. In one-third of trials, the target light switched to one of its neighb ours during the movement. Subjects made 60 movements using shoulder, e lbow and wrist and then another 60 movements in which only shoulder an d elbow movement were allowed. By restraining the wrist, the limb was made non-redundant. The path length was calculated for each movement. In single-step trials, there was no significant difference between pat h lengths performed with and without wrist restraint. As expected ther e was a significant increase in path length during double-step trials. Moreover this increase was significantly greater when the wrist was r estrained. The variability across both single- and double-step movemen ts was significantly less while the wrist was restrained. Importantly the performance time of the movements did not alter significantly for single-step, double-step or restrained movements. These results sugges t that the nervous system exploits the intrinsic redundancy of the lim b when controlling voluntary movements and is therefore more effective at reprogramming movements to double-step targets.