Changes in object-oriented arm movements that precede the transition to goal-directed reaching in infancy.

The emergence of prehensile skills in infancy is one of the central issues in motor development. The objective of this longitudinal study was to quantitatively describe the changes in the object-oriented arm movements that precede the transition to goal-directed reaching movements in infancy. Arm kinematics in 10 full-term infants were recorded biweekly from the age of 10 to 28 weeks while objects were presented for prehension. The kinematics were analyzed across three progressive phases of object-oriented arm movements (early, before, and after onset of reaching movements). As infant age increased through the stage of object-oriented movements, the distinguishing feature was that there was a decrease in movement jerk (when normalized to a dimensionless quantity), which reflects the increasing ability to adaptively modulate arm movements. This change in the dynamic characteristics of the object-oriented arm movements precedes the onset of goal-directed reaching movements and is hypothesised to reflect a critical variable in the infant developmental process of learning to reach in prehension.