Two functionally different synergies during arm reaching movements involving the trunk

Abstract

1. To address the problem of the coordination of a redundant number of degrees of freedom in motor control, we analyzed the influence of voluntary 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 velocity profile of the endpoint trajectory remained invariant regardless of trunk movements. Trunk motion preceded endpoint motion by approximately 175 ms, continued during endpoint movement to the target, and outlasted it by 200 ms. This sequence of trunk and arm movements was observed regardless of the direction of the endpoint trajectory (to or from the far target) or trunk movements (forward or backward). 4. Our data imply that reaching movements result from two control synergies: one coordinates 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 synergies may underlie a solution of the redundancy problem.

Coordinated Turn-and-Reach Movements. I. Anticipatory Compensation for Self-Generated Coriolis and Interaction Torques

“…In all conditions, the trunk began to move ϳ75 ms prior to the onset of finger motion and outlasted the duration of finger motion, consistent with findings from previous studies of reaching movements involving the trunk (Kaminski et al 1995;Ma and Feldman 1995;Pigeon et al 2000;Saling et al 1996). Nevertheless, the finger reached peak velocity relative to the trunk ϳ80 ms prior to the trunk reaching peak velocity.…”

Equifinality and its violations in a redundant system: multifinger accurate force production

“…Similar conclusions have been reached in earlier studies of trunk-assisted reaching (Adamovich et al 2001;Ma and Feldman 1995). In those studies, different trajectories in the joint configuration space were observed that maintained relative consistency of the endpoint trajectory depending on whether the trunk contributed to the movement or the trunk motion was mechanically blocked.…”

Variations of Tool and Task Characteristics Reveal That Tool-Use Postures Are Anticipated

“…In some tasks-for example, reaching with the hand to a target in front-one synergy has been found to control the trunk, independent of the motion of the hand, whereas the other synergy coordinates the motion around the arm joints to bring the hand to the target (cf. Kaminiski, 1995;Ma & Feldman, 1995;Saling et al, 1996;Wang & Stelmach, 1998). In other tasks-such as manipulation of accuracy constraints-the trunk was also shown to contribute to the focal movement (cf.…”

“…We expected the posture to be organized in two synergies: one synergy organizing the arm and the other organizing the trunk and legs (Ma & Feldman, 1995;Wang & Stelmach, 1998). To examine whether there was a synergy in the arm, we analyzed the relationship between shoulder angle and elbow angle.…”

“…Researchers who have focused on postural adjustments during reaching and grasping without a tool have postulated two postural synergies: (a) one synergy coordinating the relation between trunk and arm, in which trunk movement is functionally separate from the hand action; and (b) a synergy in the arm, which brings the hand to the target (Ma & Feldman, 1995;Wang & Stelmach, 1998). It appears that those two synergies also underlie reaching with a tool .…”