Effects of body build and arousal on newborn infant stepping.

Newborn stepping is widely believed to be a "primitive reflex" whose disappearance signals cortical maturation. Observations of normal newborns showed that the number of steps was directly related to generalized behavioral arousal. In less highly distressed infants, those who were relatively heavier for their length stepped less. These results challenge the view that stepping is reflexively released by the upright posture and support the hypothesis that the movements disappear because infants' muscle strength may not be sufficient to lift their increasingly heavy legs.