ADAPTIVE DYNAMICS OF THE LEG MOVEMENT PATTERNS OF HUMAN INFANTS .3. AGE-RELATED DIFFERENCES IN LIMB CONTROL

In this article, the development of the increasingly differentiated co ntrol of the joints necessary to transform the spontaneous leg movemen ts of early infancy into adaptive and functional actions is described. The hypothesis-that increasing joint control requires the capability for disassociation of joint action, the active modulation of joint sti ffness, and a transition from proximal to distal control of the joints -is proposed. Kinematic and kinetic analyses of the vertical kicks of infants 2 weeks, 3 months, and 7 months of age (as well as a comparati ve group of adults) indicated increasing joint independence as well as phase-dependent dent and joint-dependent control modifications. The k icks of the younger infants were dominated by a proximal control strat egy and minimal adjustments of the limb energetics during the flexion and extension phases of the kick. By 7 months of age, much larger modu lations of the kick phases were observed as well as increasing evidenc e of distal control. These results revealed kinematic and kinetic patt erns of emerging limb control between 2 weeks and 7 months of age.