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 multijoint coordination within that 3-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 2nd and 3rd 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 1st year of life. Stable patterns of temporal coordination across arm segments begin to emerge at 12-15 months of age and continue to develop up to the 3rd year. In summary, the development toward adult forms of multijoint coordination in goal-directed reaching requires more time than previously assumed. Although infants reliably grasp for objects within their workspace 3-4 months after the onset of reaching, stereotypic kinematic motor patterns are not expressed before the 2nd year of life.
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Acknowledgements This study would have been impossible to conduct without the continuing cooperation of our infants and their families over the years. We sincerely thank them. We also thank Maike Borutta for her invaluable help in collecting and analyzing the data. Our gratitude is extended to two anonymous reviewers for their criticisms and thoughtful input to an earlier version of this manuscript. This work was supported by SFB 307/A3 from Deutsche Forschungsgemeinschaft (German Science Foundation).
- Human infant
- Motor control
- Multijoint movement