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 straighter over the first year. Here the authors consider the role of the underlying limb dynamics, which scale with movement speed, on the development of trajectory control. The authors observed 4 infants weekly and then biweekly from reach onset to 1 year. Improvements in trajectories were not linear, but showed plateaus and regressions in straightness and smoothness. When infants' nonreaching movements were fast, their reaches were also fast, and faster reaches were also less straight. This is consistent with an equilibrium trajectory form of control, where development involves the increasing ability to stabilize the trajectory against self-generated movement perturbations.