Effects of kinematics constraints on hand trajectory during whole-body lifting tasks

Trajectories of the hands and whole-body center of mass were studied during whole-body lifting tasks. The movements of different parts of the body wer e monitored with the ELITE system. Subjects were instructed to lift to shou lder height an object placed at one of two distances (5-45 cm) before them on the floor. The lifts were performed both with and without kinematics con straints (i.e. to produce a straight hand trajectory while lifting, and to lift without any instructions, respectively). Hand trajectories were roughl y straight when performed under the constrained condition, but curved when performed without instruction. Hand velocity curves showed bell-shaped prof iles. In both groups, body centers of mass (whole-body, upper and lower par t) were calculated and their trajectories showed invariant sagittal displac ements. These results support the idea that movement contributes to postura l control and, reciprocally, that whole-body center of mass is a robust and controlled variable which plays an important role in hand trajectory forma tion. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.