The emergence of purposeful reaching: Developmental changes and constraints in hand and joint kinematics of early arm movements

Infants consistently reach for objects at three to five months of age. In the months before reaching, infants produce a variety of arm movements. The relationship between early arm movements and reaching is still unclear. The purpose of the present research was to determine changes in hand and joint kinematics of arm movements in the presence of a toy, in the weeks before reaching emerged. I conducted two different studies: a cross-sectional and a longitudinal study. During both the experiments, infants' arm movements were compared across two conditions: with and without a toy present. Kinematic analyses examined 3D hand, shoulder, and elbow motions. Infants showed multiple toy-oriented changes that were classified into three developmental phases. During the early phase infants scaled down their hand and joint motions in the presence of a toy and appeared to spend time visually exploring the toy. During the mid phase infants showed discontinuous and continuous toy-oriented changes. Discontinuous changes were limited to the mid phase and included an increase in movement number, speed, and smoothness. In general, infants scaled up their hand and joint motions and showed an "active period" wherein they explored their various joint motion combinations. Continuous changes existed across the mid and late phases and included an increase in hand's midline and forward positions and a decrease in distance between the hand and the toy. In addition, infants' ability to move closer to the toy was a result of toy-oriented changes in shoulder orientation and not the elbow or forearm. During the late phase infants moved closer to the toy and showed inconsistent or consistent toy contacts. The emergence of reaching showed increase in magnitude of continuous changes and additional increase in upward hand position and shoulder internal rotation orientation. These results suggest that there exists a systematic developmental trajectory for early arm movements. Early shoulder changes contribute to the emergence of reaching. Moreover, purposeful reaching does not emerge de novo but is shaped from early arm movements. Together, these findings provide a normative database for future studies to identify patterns of infants at risk for reaching delays.