2 FUNCTIONALLY DIFFERENT SYNERGIES DURING ARM REACHING MOVEMENTS INVOLVING THE TRUNK

1. To address the problem of the coordination of a redundant number of degrees of freedom in motor control, we analyzed the influence of vol untary trunk movements on the arm endpoint trajectory during reaching. 2. Subjects made fast noncorrected planar movements of the right arm from a near to a far target located in the ipsilateral work space at a 45 degrees angle to the sagittal midline of the trunk. These reaching movements were combined with a forward or a backward sagittal motion of the trunk. 3. The direction, positional error, curvature, and veloc ity profile of the endpoint trajectory remained invariant regardless o f trunk movements. Trunk motion preceded endpoint motion by similar to 175 ms, continued during endpoint movement to the target, and outlast ed it by 200 ms. This sequence of trunk and arm movements was observed regardless of the direction of the endpoint trajectory (to or from th e far target) or trunk movements (forward or backward). 4. Our data im ply that reaching movements result from two control synergies: one coo rdinates trunk and arm movements leaving the position of the endpoint unchanged, and the other produces interjoint coordination shifting the arm endpoint to the target. The use of functionally different synergi es may underlie a solution of the redundancy problem.