Abstract
Cruising represents an important transition from quadrupedal to bipedal locomotion. While cruising, infants discover ways to control their trunks in upright. We predicted that the movement patterns infants demonstrated when learning to control the trunk in order to cruise and walk independently would reflect the process Bernstein [cf. N.B. Bernstein, The Coordination and Regulation of Movements, Pergamon Press, Oxford, England, 1967] proposed for adults' skill acquisition. In addition, we expected that infants would demonstrate a precursor poorly controlled exploratory phase, what we termed the “wobble phase”. We examined longitudinal changes in movement control of the thorax and pelvis from the onset of cruising through the onset of walking. Our findings suggest that learning to cruise involves an initial wobble phase followed by a gradual reduction in the wobble, or number of movement corrections. Finally a relative plateau period of control emerged with sporadic variability in the movement patterns of the thorax and pelvis. We suggest that the fact that infants did not show a phase in which they froze degrees of freedom was due to a lack of the minimum level of control one needs in order to do this. By the end of cruising, although functionally quite skillful, infants continued to demonstrate multiple and varied patterns of control. Infants limit the degrees of freedom just enough to successfully accomplish the task. We argue that variability remains due to: the minimal negative consequences of trunk inconsistency in this context given the available arm support; the trunk acts as a flexible interface for the, as yet, unstable coupling between the arms and legs; and the low number of constraints in relation to the multiple solutions that the task affords. Despite of the remaining variability, it appears that the control of the trunk acquired during cruising enables infants to shift into walking independently.
Published Version
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