DEVELOPMENT OF REACHING DURING THE FIRST YEAR - ROLE OF MOVEMENT SPEED

When infants first learn to reach at about 4 months, their hand paths are jerky and tortuous, but their reaches become smoother and straight er over the first year. Here the authors consider the role of the unde rlying limb dynamics, which scale with movement speed, on the developm ent of trajectory control. The authors observed 4 infants weekly and t hen biweekly from reach onset to 1 year. Improvements in trajectories were not linear, but showed plateaus and regressions in straightness a nd smoothness. When infants' nonreaching movements were fast, their re aches were also fast, and faster reaches were also less straight. This is consistent with an equilibrium trajectory form of control, where d evelopment involves the increasing ability to stabilize the trajectory against self-generated movement perturbations.