Pulling to stand: common trajectories and individual differences in development.

This longitudinal study of 27 infants examined the development of pulling-to-stand (PTS). In general, infants began PTS using a two-leg strategy and transitioned to a half-kneel strategy. As a group, infants showed no preference for either strategy at the onset of PTS, switching between strategies until half-kneeling became the dominant pattern about 1 month after the onset of PTS. Examination of individual developmental trajectories revealed variability in age at PTS onset, time between PTS onset and half-kneel strategy onset, duration of the two-leg strategy as the dominant pattern, time until the half-kneel strategy became the dominant pattern, shape of the transition between strategies (gradual vs. abrupt), and timing of PTS relative to onset of other motor milestones. We discuss variation in developmental trajectory in terms of adaptive behavior during the acquisition of new skills and as a process shaped by infants' unique experiences prior to and during the acquisition period.