Learning a motor task involving obstacles by a multi-joint, redundant limb: two synergies within one movement

In this study, we investigated the hypothesis that the problem of motor redundancy could be solved using synergies representing rules for relative joint involvement to ensure a desired endpoint trajectory which may be context-dependent and may change with practice. Subjects practised a planar movement 'as fast as possible' from an initial to a final position, avoiding three round obstacles. The improvement in motor performance included a decrease in movement time and in the error scores. It was accompanied by the emergence of two distinct synergies. The first one involved elbow and shoulder movements and was used to move from the initial position to the first obstacle, and from obstacle to obstacle, whereas the second, involving the wrist, was used while going around the obstacles. The first synergy was seen before practice and showed an increase in joint coupling with practice. The second synergy was not seen prior to practice; it could not be explained by the pseudo-inverse transformation. We concluded that the central nervous system (CNS) has options for solving the redundancy problem, and that the solutions may be chosen based on such considerations as accuracy requirements, inertial properties of segments, and efficacy of particular joints to move the endpoint along a desired trajectory.