The development toward stereotypic arm kinematics during reaching in the first 3 years of life.
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.
Konczak
J
J
Department of Neurology, University of Tübingen, Germany. Konczak@uni-duesseldorf.de
Dichgans
J
J
D000375:Aging / Q000502:physiology; D001132:Arm / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D002675:Child, Preschool; D005260:Female; D006225:Hand / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D006801:Humans; D007223:Infant; D008297:Male; D009068:Movement / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology; D013239:Stereotyped Behavior / Q000502:physiology*