POSTURAL CONTROL DURING REACHING IN YOUNG INFANTS - A DYNAMIC-SYSTEMSAPPROACH

We conceptualize the coordinated development of posture and reaching w ithin Schoner's (Ecological Psychology, 7:291-314, 1995) dynamic model of coupled levels of control: load, timing, and goal. In particular, the goal of postural stability must be maintained during a reach. Usin g longitudinal data from four infants followed from 3 weeks to 1 year, we show that coordination of the head with upper and lower arm activi ty is critical for successful reaching. First, infants acquire stable head control several weeks before reaching onset. Furthermore, reachin g onset is characterized by a reorganization of muscle patterns to inc lude more trapezius and deltoid activity, serving to stabilize the hea d and shoulder and provide a stable base from which to reach. We argue that initially, the system is working on postural stability and reach ing as goals. Infants secondarily select appropriate muscle patterns t o achieve those goals depending, in part, on their individual body siz es, body proportions and energy levels. Motor development proceeds as a continual dialogue between the nervous system, body, and environment . (C) 1998 Published by Elsevier Science Ltd. All rights reserved.