|Abstracts||Human Sensorimotor Control Laboratory|
Abstract: We recorded reaching movements from nine infants longitudinally from the onset of reaching (5th postnatal month) up to the age of 3 years. Here we analyze hand and proximal joint trajectories and examine the emerging temporal coordination between arm segments. The present investigation seeks a) to determine when infants acquire consistent, adult-like patterns of multi-joint coordination within that three-year period, and b) to relate their hand trajectory formation to underlying patterns of proximal joint motion (shoulder, elbow). Our results show: First, most kinematic parameters do not assume adult-like levels before the age of 2 years. At this time, 75% of the trials reveal a single peaked velocity profile of the hand. Between the second and third year of life "improvements" of hand- or joint-related movement units are only marginal. Second, infant motor systems strive to obtain velocity patterns with as few force reversals as possible (uni- or bimodal) at all three limb segments. Third, the formation of a consistent interjoint synergy between shoulder and elbow motion is not achieved within the first year of life. Stable patterns of temporal coordination across arm segments begin to emerge at 12 to 15 months of age and continue to develop up to the third year. In summary, the development toward adult forms of multi-joint coordination in goal-directed reaching requires more time than previously assumed. Although infants reliably grasp for objects within their workspace three to four months after the onset of reaching, stereotypic kinematic motor patterns are not expressed before the second year of life.