We tested the hypothesis that the movement trajectory of the center of mass (CM) and the horizontal trajectory of the head are the primary focus of the brain's control of the task of standing up from sitting to reach an object. Both infants and older children were studied. Stability was estimated by intertrial variability of actual CM, head, and wrist movement trajectories. They were compared to each other and indirectly to the stability of body segment motions via comparison to head or wrist motion variability estimated from body segment motion variability. The results suggest that standing up from a seat is organized primarily around controlling global task parameters: trajectories of the body CM and, during the postliftoff phase of the task, the head horizontal trajectory, rather than individual movement components. Infants at the earliest age of independent performance exhibited the major effects.
The authors are with the Physical Therapy Department and Interdisciplinary Neuro-science Program, 307 McKinly Laboratory, University of Delaware, Newark, DE 19716.