| Typically developing infants begin reaching between 3 and 5 months old. Infants produce thousands of arm movements during the months leading up to their first reaches. Empirical and theoretical evidence has suggested these movements are developmentally linked with purposeful reaching. Our previous work has focused on quantifying a range of spatial and temporal changes in arm movements throughout the pre-reaching period, which quantified the hand and joint changes separately. This present study continued the focus on spatial and temporal variables but with specific attention to joint coordination. The purpose of this current study was to use principal component analysis (PCA) to quantify the longitudinal changes in coordination between hand, shoulder, and elbow kinematics during arm movements over the pre-reaching period up to the onset of reaching in typically developing infants.; The results of this current study identified features of coordination that were consistent throughout the pre-reaching period, as well as features that changed. In terms of consistent features, the hand and both joints displayed relatively unique coordination structures that, in general, did not change over development. These invariant features may indicate important constraints on coordination that reduce the degrees of freedom that must be actively controlled, and provide a foundation from which infants build purposeful control. In terms of features that changed, the shoulder and elbow displayed both similarities and differences that were dependent on both the developmental phase and whether a toy was present or not. One important difference between joints was the similarity of developmental changes in coordination during spontaneous movements and movements with a toy present for the shoulder. Moreover, the results in general highlight the critical role of spontaneous arm movements in the emergence of purposeful reaching, and the need for a better understanding of these non reaching movements. |
